TABLE OF CONTENTS When there are many inputs or outputs located considerable distances away from the PLC, while it would be possible to run cables from each such device to the PLC a more economic solution is to use input/output modules in the vicinity of the inputs and outputs and use just a single core cable to connect each, over the long distances, to the PLC instead of the multicore cable that would be needed without such distant I/O modules (Figure 4.17).
In some situations a number of PLCs may be linked together with a master PLC unit sending and receiving input/output data from the other units (Figure 4.18). The distant PLCs do not contain the control program since all the control processing is carried out by the master PLC.
The cables used for communicating data between remote input/output modules and a central PLC, remote PLCs and the master PLC are typically twisted-pair cabling, often routed through grounded steel conduit in order to reduce electrical noise . Coaxial cable enables higher data rates to be transmitted and does not require the shielding of steel conduit. Fibre-optic cabling has the advantage of resistance to noise, small size and flexibility and is now becoming more widely used.
Serial communication is when data is transmitted one bit at a time (Figure 4.19(a)). Thus if an 8-bit word is...
