Input and Output Modules

One problem that occurs in robotic applications is that an input circuit or output circuit will have a fault and need to be replaced while the robot is in production. Older robots used IC chips on an input/output (I/O) board for the interface for all of the I/O signals. When an input went bad, the entire I/O board needed to be replaced. This was complex and expensive because one board had all of the inputs and outputs for the entire robot. It was also difficult to troubleshoot some of the ICs because they didn't have indicators to tell which one was faulty.

Newer robots have integrated I/O modules that are similar to those used in PLCs. The advantage of the I/O modules is that they have a status indicator for each circuit to indicate when they are operating correctly, and they usually limit the number of circuits to four or eight per module. The modules are mounted in racks like PLC modules so that the microprocessor in the robot can access all of the signals through the back plane of the rack. Since the I/O circuits are mounted on a module, the field wiring can be mounted to a field wiring arm that plugs into the module. The field wiring arm allows the technician to disconnect all wiring to each module by unplugging the field wiring arm on the faulty module and plugging it into the new module when it's placed in the I/O rack. Hence, a technician can change a module in less than 5 minutes on a modern robot, which limits the problem of downtime. It's also easy to stock one or more of each module that the robot uses, since they are much less expensive than the earlier I/O boards. ill. 1 shows the location of the I/O rack in a robot. The I/O rack is located near the top of the cabinet just to the right of the servo amplifiers. This diagram is similar to the one shown in this figure except the I/O rack and the processor for the robot are the only parts of the diagram that are identified.

Above: ill.1: Location of I/O modules in the robot cabinet. The I/O modules are mounted near the processor for the robot.