PLC module sizing

We want to do some thinking on how to make the PLC module sizing as to the density of the cards, i.e., about what’s the optimal number of channels, and we see that in many cases is more important than it looks like. At first glance all tend to think that the more channels the module has the better it is, especially in medium-large size architectures.

In some previous post we have shown some examples:

Optimize PLC architecture: here we compare several architectures of 197 I/Os, using the same PLC model, with 32 and 16 channels cards.

BMS Safety PLC: it is a real application comparing architectures from different PLC manufacturers and different I/O cards.

Surely the casuistry is huge. In any case, the cost of the PLC is very important and therefore we must always try to look for the architecture that best suits the project in question (for example, if the customer is requesting 144 digital inputs (120 + 20% of spares) we would have 16 spares more (that we don’t need) when using 32-channels modules, and none with 16-channels modules).

The analysis should be complete, i.e., the complete I/O architectures are compared, the costs are calculated, the distribution of the racks in the control cabinets is considered and the physical space they occupy as well.. The first three can make very quickly by using “IO_Builder“, and for the analysis of physical space we may use “Cabinet Layout“.

When architecture is simple and we have no restrictions to distribute the signals this analysis is not important, but it can be essential in more complex architectures.

In what cases we should analyze the PLC module sizing?

1-When you have to distribute the I/Os based on the location of the instruments and equipment in the Plant (if the PLC/DCS controls different units or sub units with more or less critical equipment, if there is duplication of equipment or packages, for example primary and back-up pumps, etc.)

2-When there are redundant I/Os (1oo2/2oo2) and/or with 2oo3 logic.

3-When there are technical reasons in the wiring of the control cabinet (if there are I/Os at different voltages that we should wired separated or we have special modules, for example, intrinsically safe, or have standard modules and “fail safe” in the same cabinet, etc.)

plc module sizing

4-When we compare the cost of different models of PLCs.


5-When we use different families of I/Os in the same PLC (e.g. in the case of Rockwell PLCs: ControlLogix 1756 and Flex IO modules).

Generally we don’t pay much attention to this issue of PLC module sizing, but there are many upgradable architectures in this regard, both technically and economically.

In future posts we will show you some more examples.