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PLC Remote I/O - Mitsubishi PLC
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Articles Entry:LKong Source:Network Published Time:2011-7-16
PLC Remote I/O - Mitsubishi PLC

So far we have assumed that a PLC consists of a processor unit anda collection of I/O cards mounted in local racks. Early PLCs did tend tobe 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 multicore cables. It will also make commissioning and faultfinding rather difficult, as signals can only be monitored effectively ata point possibly some distance from the device being tested.In all bar the smallest and cheapest systems, PLC manufacturerstherefore provide the ability to mount I/O racks remote from theprocessor, and link these racks with simple (and cheap) screened singlepair or fibre optic cable. Racks can then be mounted up to severalkilometres away from the processor.

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Identification of plant signals
There are many benefits from this. It obviously reduces cable costs asracks can be laid out local to the plant devices and only short multicorecable runs are needed. The long runs will only need the communicationcables (which are cheap and only have a few cores to terminate at eachend) and hardwire safety signals.Less obviously, remote I/O allows complete units to be built, wired toa built-in rack, and tested offsite prior to delivery and installation. Thepulpit in Figure contains three remote racks, and connects to thecontrolling PLC mounted in a substation about 500m away, viaa remote I/O cable, plus a few power supplies and hardwire safetysignals.
This allowed the pulpit to be built and tested before it arrivedon site. Similar ideas can be applied to any plant with I/O that needs tobe connected to a PLC.If remote I/O is used, provision should be made for a program terminalto be connected local to each rack. It negates most of the benefits if thedesigner can only monitor the operation from a central control roomseveral hundred metres from the plant. Fortunately, manufacturershave recognized this and most allow programming terminals to beconnected to the processor via similar screened twin cable.

 

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