PLC – Remote I/O
So far we have assumed that a PLC consists of a processor unit an data collection of I/O cards mounted in local racks. Early PLCs did tend to be arranged like this, but in a large and scattered plant with this arrangement ,all signals have to be brought back to some central point inexpensive multi core cables. It will also make commissioning and faultfinding rather difficult, as signals can only be monitored effectively data point possibly some distance from the device being tested. In all bar the smallest and cheapest systems, PLC manufacturers therefore provide the ability to mount I/O racks remote from the processor, and link these racks with simple (and cheap) screened single pair or fiber optic cable. Racks can then be mounted up to several kilometers away from the processor.
There are many benefits from this. It obviously reduces cable costs as racks can be laid out local to the plant devices and only short multi core cable runs are needed. The long runs will only need the communication cables (which are cheap and only have a few cores to terminate at each end) and hardwire safety signals. Less obviously, remote I/O allows complete units to be built, wired to a built-in rack, and tested offsite prior to delivery and installation. The pulpit in Figure contains three remote racks, and connects to the controlling PLC mounted in a substation about 500m away, via a remote I/O cable, plus a few power supplies and hardwire safety signals.
This allowed the pulpit to be built and tested before it arrived on site. Similar ideas can be applied to any plant with I/O that needs to be connected to a PLC. If remote I/O is used, provision should be made for a program terminal to be connected local to each rack. It negates most of the benefits if the designer can only monitor the operation from a central control room several hundred meters from the plant. Fortunately, manufacturers have recognized this and most allow programming terminals to be connected to the processor via similar screened twin cable.
