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G.1 Introduction
As with other forms of data acquisition hardware, stand-alone logger/controllers are designed to measure and record real world signals, as well as to act on these signals to provide control of a system or process. In addition, stand-alone logger/controllers have many features that distinguish their operation and use from other data acquisition hardware, such as plug-in boards and distributed I/O.
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This section looks at the hardware and software configurations of stand-alone logger/controllers, the system configurations for which they can be used, and the features that allow these devices to meet specific requirements in the field of data acquisition and control.
G.2 Methods of operation
Stand-alone logger/controllers are intelligent devices, capable of performing complex data acquisition and control functions, as well as making decisions based on current system or process conditions. To do this they must first be programmed, typically by a sequence of ASCII-based commands formatted by the host PC, which are interpreted and executed by the device so that it knows what actions to take at any point in time.
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Once programmed, the stand-alone device can continue to operate, taking sensor measurements, logging the data to memory and performing control functions, even when the host computer is not connected or functional. From an operational point of view, it's this important feature that distinguishes stand-alone logger/controllers from other data acquisition hardware, such as plug-in boards and distributed I/O.
Two methods of programming - the stand-alone logger/controller, and uploading logged data to the host PC, are available, either by the RS-232 serial communications interface or by using portable and re-usable memory cards.
This flexibility allows stand-alone logger/controllers to be operated in a number of ways, depending on the required location, volume of data to be stored and availability of power:
•Stand-alone operation with periodic data recovery (and programming where required) using memory cards or a portable laptop PC
•On line to a host PC with periodic uploading of data (and programming where required)
•On line to a host PC via modem, with periodic uploading of data (and programming where required), initiated by either the host PC or the remote device Where an application requires many more sensors than can be provided by a single stand-alone controller, and these are distributed over a large area, a distributed logger/controller network may be required. Each mode of operation, employing only a single logger/controller, is also applicable when more devices are connected as part of a distributed network.
G.2.1 Programming and logging data using PCMCIA cards
The credit card size portable memory card provides a reliable media for transporting data and programs, but requires a memory card interface connected to the RS-232 serial port of the computer. This is shown in Figure G.1.
Figure G.1: Using memory cards to program and log data from a stand-alone
logger/controller (Computer Memory Card Interface PCMCIA Card Remote Data Logger
Stand-alone logger / controller; Thermocouples Strain; gauges Relays)
Programming the operation of a logger/controller and recovering data using the PCMCIA card is especially useful when the logger/controller is remotely located and /or not connected to a host PC. Even when connected to a host PC, the storage capacity of the logger/controller can be greatly increased by leaving the PCMCIA card permanently inserted in the device. This is its intended use since it also increases system reliability because data is logged directly to a semi-permanent storage medium.
G.2.2 Stand-alone operation
As their name implies stand-alone logger/controllers are specifically suited to be operated independently of the host PC. This makes them especially useful where the device must be located in a remote and /or particularly harsh environment, or where they are unable to be continuously connected to a host PC, either directly or via modem.
Special applications, such as temperature monitoring of a refrigerated truck, or weather reporting at a remote weather station, make use of a logger/controller in a stand-alone configuration. In these real life applications the stand-alone device can be either programmed, in the office or with a portable laptop, then left to operate, powered from a local power supply. Data stored in the device's memory can be periodically uploaded using a portable laptop PC or memory cards.
When operating as a stand-alone device there are several important considerations.
Where it's necessary to power the unit from a battery supply, irrespective of whether the battery supply is rechargeable, battery power is not unlimited. This requires that the batteries be either recharged, or replaced where they are not rechargeable. Another important factor is that stand-alone units have a limited amount of memory. The greater the number of channels and the faster the sampling rate on each channel, the greater the number of samples that will be taken in a given time period. In time, the memory will become full. Care must be taken to keep the sampling rate of each channel to the minimum necessary, while still obtaining the information required. The memory capacity of a device can be greatly increased by leaving a higher capacity memory card in the device and logging data directly to the memory card.
G.2.3 Direct connection to the host PC
The most common system configuration, and one which provides the highest system reliability, is a direct connection to the host PC via the RS-232 communications interface as shown in Figure G.2. This setup allows frequent uploading of data, constant monitoring of alarm conditions and on-line system control. it's most likely implemented in industrial plants or factories, where critical processes must be constantly monitored and controlled.
The maximum distance that the logger/controller can be located away from the host PC is dependent on the baud rate of the communications interface. When a single logger/controller is directly connected to the host PC, it can be configured to return data as soon as it's available..
Figure G.2: A direct connection to a stand-alone logger/controller via an
RS-232 serial interface
Where an application requires more than one logger/controller, and each unit is distributed over a large physical area, e.g., in an industrial plant or factory, the logger/controllers can be configured as part of a distributed RS-485 multidrop network. A single unit, deemed to be the host unit or local unit, can be connected directly to the host computer via the RS-232 serial interface, as shown in Figure G.3, thus avoiding any requirement for an RS-232 to RS-485 serial interface card.
Figure G.3 Distributed logger/controller network
An advantage of this implementation is that other host PCs, printers or terminals can be connected to the RS-232 ports of other logger/controllers further increasing system reliability. This system configuration is shown in Figure G.4.
Figure G.4 Distributed logger/controller network with additional host
How frequently logged data is uploaded depends firstly on how critical the immediate analysis of data is to the system or process being controlled, and secondly on how much memory is available and how quickly it will become full.
How quickly the memory will become full is important for two reasons. During failure of the host PC or communications interface, there must be enough memory to allow data logging to continue without loss of data. In addition, a device connected to the host PC via a multi-drop network can only return data when requested by the host PC. Where a large number of units are connected to the host, the memory of each unit must be large enough to allow data logging to continue without loss of data, until the next time the host requests a data upload.
Aside from this specific limitation, it's good practice to recover data as often as possible since any sensor errors, power supply failures or problems with the unit itself will be detected early, thereby increasing system reliability. In addition, frequent data recovery will help to minimize the chance that data may be lost due to device failures such as battery-backed memory failure.
G.2.4 Remote connection to the host PC
Another useful configuration is the connection of remote logger/controllers to the host PC using modems via either a telephone network or radio communications. In large factories or industrial plants, where one or more devices are distributed over a wide physical area, the closest logger/controller to the host PC may be too far away or too greatly affected by noise to allow connection to a host PC via the RS-232 communications interface. In such applications, the use of radio communications is a practical solution. When radio communications take place between the host PC and the distributed network, all communications must go through the logger/controller to which the remote radio modem is connected. This is shown in Figure G.5.
Figure G.5 Remote connections to a logger/controller network via radio communications
In many applications, the stand-alone logger/controllers are not contained within the same factory or industrial plant, but located at a distance beyond the capabilities of radio modem communications. An example of this would be a remote electricity sub-station used to monitor alarm conditions, provide on-line voltage, current, and power readings to a central control room. Communications between the host PC and the remote units via the telephone network is shown in Figure G.6. A dedicated phone line allows frequent up-loading of data to the host PC, constant monitoring of alarms and on-line system control, where required.
Figure G.6 A remote connection to a logger/controller network via the telephone
network
G.3 Stand-alone logger/controller hardware
The important features that give stand-alone logger/controllers the power and flexibility to operate, either as stand-alone devices, or as part of a distributed network, fundamentally lie in their relatively complex hardware structure. The simplified hardware schematic of a typical stand-alone logger/controller is shown in Figure G.7.
Figure G.7 Simplified hardware schematic of a stand-alone logger/controller
The following hardware components discussed in this section are:
•Microprocessor
•Memory
•Real time clock
•Universal asynchronous receiver and transmitter (UART)
•Counter/timer circuitry
•Input multiplexer and elector
•Power supply
•Power management circuitry
•Analog and digital I/O circuitry
G.3.1 Microprocessors
At the heart of the stand-alone system is the microprocessor or microcontroller. In conjunction with the embedded software (firmware), it provides the control and functionality of the system.
It is important to clarify the distinction between microprocessors and microcontrollers.
A microprocessor is just the central processing unit (CPU) part of a computer, without the memory, input/output circuitry, and peripherals needed for a complete system. The Intel 8088 and 80286 chips are microprocessors. All other chips in the PC are there to add features not found within the microprocessor chip itself. However, when a microprocessor is combined with on-chip I/O, memory and peripheral functions, the combination is called a microcontroller.
The microcontroller is probably the most popular choice for stand-alone systems, as it provides the necessary peripheral functions on chip. The advantages of microcontrollers include reduced cost, a reduction in chip count and hence reduction in printed circuit board 'real estate'.
G.3.2 Memory
Non-volatile memory, used for the storage of sensor measurements and control parameters, is an important feature of a stand-alone system. Typically, random access memory (RAM) is used for data storage and requires some form of battery backup to maintain the contents during power loss.
Manufacturers of stand-alone controllers are now incorporating memory card readers that allow measurement data to be stored directly onto memory cards. The memory card can subsequently be removed and the data transferred to a host computer.
ROM / EPROM
The embedded operating system or firmware of a stand-alone device is stored in either read only memory (ROM) or erasable programmable read only memory (EPROM).
Once-only programmable ROM technology is typically used in systems where high volume manufacturing is involved.
EPROM technology is therefore more popular in low to medium volume stand-alone systems, as it allows manufacturers to modify firmware, incorporating new features or enhancements, without committing to the volume requirements of ROM technology. To allow easy installation and replacement of the ROM or EPROM chip during the lifespan of the device, these chips are usually mounted on the circuit board via a socket.
RAM
Random access memory (RAM) is generally used in stand-alone systems for the storage of measurement data, control, and system parameters. The two most popular types of RAM technology are static and dynamic. Dynamic RAM requires periodic updating or refreshing, whereas static RAM does not require refreshing. However, the advantage of dynamic RAM over static RAM is a far greater memory capacity for a given area of silicon.
Dynamic RAM is suitable for a personal computer used in an office environment where memory capacity is an important requirement. However, in a stand-alone system the advantage of static RAM lies in its ability to maintain the data contents, using a backup battery, in the absence of main power. This can be achieved with relative ease, because static RAM does not require refreshing, even in standby mode.
EEPROM / FlashPROM
Electrically erasable programmable read only memory (EEPROM) is a non-volatile memory technology, generally used for the storage of limited configuration data and control parameters. The moderate memory capacities and slow write cycle of EEPROM (typically 10 milliseconds) limit its application.
Flash programmable read only memory (FlashPROM) is also a non-volatile memory technology, and is used for both mass data and program storage. FlashPROM is available in memory capacities ranging from 32 Kbytes to 2 Mbytes. The much shorter write cycle of FlashPROM is achieved at the expense of having to erase data on the chip in fixed-size blocks rather than a byte at a time.
Memory Card
Similar to RAM, plug-in memory cards are also used in stand-alone systems for the storage of measurement and control data. Although there are a number of memory card manufacturers, the Personal Computer Memory Card International Association (PCMCIA), USB Flash and similar standard cards, have become very popular for use with notebook computers.
PCMCIA memory cards are available in ROM, one-time programmable ROM, static RAM, UV EPROM, flashPROM and EEPROM technologies. The static RAM PCMCIA cards are the obvious choice for data storage in stand-alone systems, and are currently available in memory capacities ranging from 64 Kbytes to in excess of 8 Mbytes.
An important advantage of memory cards in a stand-alone system is their ability to be removed and replaced with another blank card in the field, providing a convenient data transfer mechanism. Additionally, memory cards allow the user to purchase and install only the memory capacity required for a particular application.
G.3.3 Real time clock
The real time clock (RTC) is an important part of any stand-alone system. It not only provides the necessary date and time information, but also provides periodic and alarm functions for triggering the reading of sensors and controlling outputs under program control.
The RTC will be connected to the associated power management circuitry, allowing the system to remain in standby mode conserving power, until the RTC periodic event or alarm event wakes the system up. The control software is then able to read and record sensor data and manipulate control outputs, before returning the system to the low power standby mode (sleep mode).
In a typical stand-alone data acquisition application, sensor readings are taken at periodic intervals, allowing the system to return to the standby mode conserving power, during these periods of inactivity. e.g., sensor readings may only be required once every 500 milliseconds. The RTC would, therefore, be programmed for an alarm wake-up event every 500 milliseconds (see low power mode). The system activity could be reduced to approximately 10 milliseconds in every 500 milliseconds, providing a substantial power reduction, which is very important for battery-operated systems.
G.3.4 Universal asynchronous receiver/transmitter (UART)
The start, stop, and parity bits used for checking data integrity in asynchronous transmission are physically generated by the universal asynchronous receiver/transmitter (UART), located between the microprocessor bus and the line driver, which interfaces to the actual communications link.
The main purpose of the UART is to look after all the routine housekeeping matters associated with interfacing between the parallel bus to the microprocessor and the serial communications link to the host computer.
When transmitting, the UART performs a number of functions. These are to:
•Set the correct baud rate for transmission
•Interface to the microprocessor data bus and accept characters one at a time
•Generate a start bit for each character
•Add the data bits in a serial stream
•Calculate and add the parity bit to the data stream
•Terminate the serial group with the required stop bit(s)
•Advise the microprocessor it's ready for the next character
The receiving circuitry of the UART performs the following functions. These are to:
•Set the correct baud rate for receipt of data
•Synchronize the incoming data with the start bit
•Read the data bits in a serial stream
•Read the parity bits and confirm the parity
•Read the stop bits
•Transfer the character as a parallel data onto the microprocessor data bus
•Interface the handshaking lines
•Observe and report any errors associated with the character frame received.
Typical errors that the UART can detect are:
•Receiver overruns - bytes received faster than they are read
•Parity errors - mismatch between parity bits and character frame
•Framing error - all bits in the frame are zero or a break condition is reported A break condition occurs when the transmitter that holds the data line is in a spaced (or positive voltage) state for a period of time longer, than that required for a complete character. This is a method of getting the receiving UART to react immediately and perform some other task.
G.3.5 Power supply
Due to the nature of their operation and the purposes for which they can be used, stand-alone devices have a variety of power sources:
•Low voltage AC (9-15 V AC)
•Low voltage regulated DC (11-17 V DC)
•9 V alkaline battery (6-10 V)
•6 V gel cell battery (5.6-8 V)
A simplified power supply schematic of a typical power supply circuit is shown in Figure G.8.
Figure G.8 Simplified power supply schematic
When both an internal non-rechargeable alkaline battery and an external AC or DC power supply are connected, the output of the regulator is increased to a voltage greater than the alkaline battery voltage (i.e. 10 V), so that power is drawn from the external supply and not from the internal battery. In this situation, the in-line diode connected to the alkaline + terminal prevents charging of the alkaline battery.
It is not recommended that both internal and external batteries be connected. If two batteries are required, it's better if the external battery is 12V and connected as external DC power.
Extreme care should be taken to ensure that external batteries are connected with the correct polarity otherwise, damage might occur. In addition, when an external DC supply is grounded it must be a negative (-) ground. AC supplies should never be grounded.
Battery Charging
Where an internal gel cell rechargeable battery is connected, an external AC or DC power supply can provide temperature compensated charging with voltage set by the output of the switch mode regulator and charging current limited by the 0.22 ohm charging resistor.
Sealed gel cell batteries may also be charged via a 12 V solar panel connected to the AC/DC power input terminals. The size of the solar panel required depends on the hours of full sunlight that can be expected. As a rule, one day in seven should be regarded as a charge day and the charge must be able to fully replenish the batteries on that day.
Battery Life
The maximum battery life that can be achieved depends on:
•How often the input channels are scanned
•The number of analog channels and how many are connected to sensors
•The number of digital channels and how many are driving outputs
•Sensor excitation power draw
•Complexity of any calculations A precise calculation of the battery life is extremely involved, however manufacturers can provide battery life charts, based on the number of channels and the time between each scan of all the channels.
G.3.6 Power management circuitry
All microprocessor systems need some supervisory functions that are analog rather than digital in nature. For a typical system, these functions include power reset, battery backup switching for RAM and real time clock, and the watch dog timer (WDT).
Reset Circuitry
The reset circuit ensures that the microprocessor is in the reset state in the absence of power. Embedded systems that may need to function in hostile environments require advanced reset circuitry that provides voltage threshold detection, independent of the rate of rise of the system power.
Battery Backup
The battery backup circuit ensures that the RAM and real time clock components receive a constant source of power. It also ensures that the RAM and real time clock are write-protected and in the low power mode, when the main power supply fails.
If the voltage from the main power supply falls below a preset level, then the battery backup circuit switches the RAM and real time clock power source to a supplementary battery supply. Additionally, the battery backup circuit ensures that the RAM and real time clock are write-protected, and in the low power sleep mode.
Low-Power Mode
Two power states, wake and sleep (standby), ensure that the minimum amount of power will be used when the unit is not required to perform any data acquisition function. e.g., the device will wake up when:
•RTC periodic interrupt signals a scan of the input channels is due
•A memory card is inserted
•Characters are received at the communications port
•A key is pressed (where fitted) When an internal or external battery is being powered from an AC or DC supply then the low power sleep mode is not required to be entered.
Watch Dog Timer
The watch dog timer (WDT) circuit is intended to detect software-processing errors.
During normal operation the software is responsible for periodically resetting the WDT.
Failure to reset the WDT on a periodical basis indicates that the software is no longer executing the intended sequence, and accordingly the WDT initiates a system reset.
In essence, the WDT is a fail-safe mechanism that resets the system if for some reason it 'runs off the rails'. Although the WDT would seem a perfect fail safe mechanism for static or electrical noise induced problems, there is still the possibility of the software erroneously entering a loop, which continuously resets the WDT and hence never initiates a system reset.
G.3.7 Analog inputs and digital I/O
Analog Input Circuitry
Logger/controllers typically have multiple analog input channels. A special feature of these devices is that each channel can be configured for operation with a variety of sensors and signals. The simplified schematic of a typical input channel is shown in Figure G.9.
Figure G.9 Simplified schematic of analog input channels
The versatility, that allows each channel to be configured for a wide range of sensors, different excitation requirements and either differential or single-ended input terminations, is provided by the analog signal selector. The configuration of each channel is provided by software commands that are interpreted by the logger/controller to switch the analog signal selector to the required settings.
Sensor excitation is typically provided in the form of a low level constant current source (250 µA), for measuring resistance, a higher level constant current source for RTD and Wheatstone bridge measurements or a voltage source (usually unregulated) via an internal resistance, useful for powering some sensors.
Input termination resistors, typically of 1 M §Ù can be switched into the circuit to provide a return path for instrumentation amplifier bias currents. Where the termination resistors are not switched into the circuit, the input impedance seen by the sensor is of the order of 100 Meg-ohm.
Digital I/O Channels
Logger/controllers typically have multiple dual-purpose digital I/O channels that share the same terminations and act as both digital inputs and outputs. This is shown in Figure G.10.
Figure G.10 Schematic of digital I/O channel
Digital inputs have a high impedance input resistance and are buffered to protect the more sensitive CMOS digital interface circuitry from damage from current surges. A 30V zener diode provides input over-voltage protection by limiting the incoming voltage to below the transient voltage threshold of the input buffer.
The most commonly implemented form of digital output available on stand-alone loggers/controllers is the open collector output configuration capable of sinking 200 mA at 30 V. In this configuration, the zener also acts as a voltage limiter when the channel is used as an open collector output.
The schematic of a typical digital counter channel is shown in Figure G.11. Counter input channels are provided with a Schmitt trigger input buffer with threshold voltage set to 2 volts. This prevents spurious noise below the threshold value causing a count transition. The capacitor at the input to the Schmitt trigger input buffer provides filtering but limits the count rate to approximately 1 kHz (=1/RC). When it's removed, the count rate can be as high as 500 kHz.
Figure G.11 Schematic of digital counter channel
G.3.8 Expansion modules
Expansion modules provide increased localized channel capacity for a data acquisition system using stand-alone logger/controllers. Expansion connectors provide an extension of the internal data and control bus lines of the logger/controller. When connected, additional analog input channels, digital I/O channels, and counter channels, on the expansion module, are treated as if they were part of the logger/controller to which they are attached. This is shown in Figure G.12.
Figure G.12 Connection of expansion modules
G.4 Communications hardware interface
Communications interface standards define the electrical and mechanical details that allow communication equipment from different manufacturers to be connected together and to function efficiently. Two standards are commonly employed for communications between PCs and stand-alone or distributed logger/controllers:
•RS-232 standard
•RS-485 standard
G.4.1 RS-232 interface
As the RS-232 communications interface is standard on most IBM PCs and compatibles (i.e. COM1 and COM2 ports), stand-alone devices first used this interface for communication to the PC. The RS-232 interface is discussed in detail in Section 6, however some of the most basic setup parameters for stand-alone logger/controllers are discussed below.
Comms Port Parameters
Usually, the comms port parameters (i.e. start bits, data bits, stop bits) are fixed. The only user-setable parameter is the baud rate, which is typically set by dip switches on the device. Commonly used baud rates are 300, 1200, 2400, 4800, 9600 and 19200. The optimum baud rate setting is a compromise between the speed necessary to transmit the necessary amount of data over the communications interface, and the speed required so that data can be transferred without error over the distance that the host PC is located from the local stand-alone logger/controller.
Communications Port Connections
Stand-alone loggers/controllers are typically equipped with a DB-9 connector. The pin allocations commonly used with the DB-25 and DB-9 connectors for the RS-232C interface are not quite the same and often provide a trap to the beginner.
Figure G.13 shows the standard connection between both the DB-25 and DB-9 connector of the host PC and the DB-9 connector of a stand-alone logger controller.
Figure G.13 Communication cable connections to IBM host PC
Great care should be taken when connecting stand-alone devices to the host PC. While the standard pin configuration is likely to be adhered to by manufacturers at the PC's communications interface, it's possible (and often likely) that the data receive and transmit lines on remote stand-alone systems are on different pins of the DB-9 connector.
It is therefore wise, to consult the manufacturers' data sheets.
Data handshaking is not usually employed on stand-alone logger/controllers as their use often leads to communication problems. Instead, the handshaking lines are connected at the host PC, as required by the communications software, and left unconnected at the logger/controller interface.
G.4.2 RS-485 standard
With a growing need for a distributed logger/controller network, RS-485 interfaces have been added to the hardware. The RS-485 interface typically operates as a balanced two wire, half duplex and un-terminated network (see Section 6). However, the protocols used for communications on the network are often proprietary, with different manufacturers using undisclosed protocols, and error detection/correction methods, between devices. This does not alter the fact that communications to devices on the RS-485 network still occur via a single logger/controller, known as the local device, which communicates to the PC via the RS-232 interface.
An RS-485 repeater is used where more than 32 stand-alone devices are required on one network. A further 32 devices may be connected for each repeater used.
G.4.3 Communication bottlenecks and system performance
When logger/controllers are constantly logging data to memory, data gathered can be sent to the host PC via the communications interface at any convenient time before the memory becomes full. This allows great flexibility in obtaining the data from a stand-alone device or a network of devices. However, when operating in real time, that is, data is continuously returned to the host PC from a single stand-alone logger/controller or a network of logger/controllers, an important consideration is 'can the volume of data obtained, be transmitted over the serial communications link?' This depends on a number of factors:
•Baud rate
•The number of channels being scanned
•How often the channels are scanned
•Whether the device is stand-alone or part of a distributed network stand-alone logger/controller
Consider first, a stand-alone logger/controller connected to the host PC via the RS-232 interface operating at 9600 baud. As we have seen previously, data sent over the communications interface is sent in a 10-bit frame consisting of 1 start bit, 8 data bits and 1 stop bit. The time to transmit each byte of data at 9600 baud is 1.042 ms (t = 10 bits/ 9600).
Therefore, to transmit the maximum amount of data at the required baud rate, the maximum time between each data byte, being ready to be sent, is 1.042 ms.
Consider a logger/controller that is scanning 10 channels. If, for each channel, seven bytes of data are sent (on average), plus there are another ten bytes for each scan of the input channels, then the total number of bytes to be sent for each channel scan is 80 bytes.
The maximum time each channel scan could take is 83.36 ms (80 bytes × 1.042 ms).
Therefore, all channels could be scanned at the maximum rate of approximately 12 Hz (1/83.36 ms).
This calculation assumes that there are no hardware factors, such as multiplexer settling time, input amplifier-settling time etc, which limit this rate even further. Irrespective of the performance limitations imposed by the stand-alone communications interface, logger/controllers are not designed for high-speed data measurement.
Distributed logger / controller network
When operating as part of a distributed network, the considerations that determine the performance of the system are different. Despite RS-485 being an extremely reliable interface, even at high speed, the potential speed at which the network can operate is limited by a number of factors:
•Each device in the system has a unique address and must be polled by the host computer for information.
•Only one device can be polled at any time.
•As the RS-485 network is half-duplex, the host PC must wait for a response to each request for data before polling the next device.
•There is an inherent delay, or turnaround time, in responding to a host request irrespective of whether one byte or one hundred bytes of data are returned in the response. This is because the device must interpret, process, then act on the command received before returning its response.
•Where the RS-485 network is operating much faster than the RS-232 interface (i.e. more than twice as fast) the potential speed at which the data can be sent to the host PC is limited by the baud rate of the RS-232 interface. Where this is not the case, the baud rate of the RS-485 network is the limiting factor.
It has been shown in the example for a stand-alone device, that a device scanning 10 input channels and returning 80 bytes of data to the host PC for each scan, would take 83.36 ms to transmit the data. If the time required to transmit a ten-byte poll command is 10.42 ms, and assuming a turnaround time of 1 sec, the total time to obtain the data from a single device is 1093.78 ms. The time taken for 10 devices operating in the same manner would be approximately 11 sec. Clearly, the system would not be able to operate in real time unless each device scanned its input channels and returned data to the host once every 11 sec.
Where the channels of one or more of the devices must be scanned at a faster rate, the data should be logged to memory and returned at a more convenient time.
G.4.4 Using Ethernet to connect data loggers
Data loggers have traditionally used RS-485 as a type of networking system. RS-485 works very well for multi-dropping up to 32 data loggers. As requirements have expanded in the plant environment, we have seen a need to connect data loggers to expanding networks of systems. This has seen the rise of data loggers being connected via existing Ethernet networks. The advantages are obvious. One of the main problems with connecting data loggers on an RS-495 network is the limited range of access to the system. By having access to the data loggers over the Ethernet, the user can view and even change data anywhere the network is connected. This brings rise to the use of data loggers using the Internet. Intra- and inter-networking of data loggers also raises security issues. How safe is the data? Will someone that does not have authorization be able to access the hardware? These problems and their solutions will be the subject of much discussion when it comes to connecting Ethernet to data loggers.
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