Optimize the PLC architecture

This post explores the impact that the I/O distribution and density of the modules can have on the cost of hardware, that is how we can optimize the PLC architecture according to the requirements of each project.

We have used the tool “IO_Builder” that allows us to make different distributions of signals and calculate its cost very quickly. In the examples we have used real hardware prices of a known PLC manufacturer.

We will see that the cost of the PLC is not always reduced using modules with higher amount of channels, in fact this is only true when there are only single I/Os (logic 1oo1) and no restrictions to distribute the signals. We will see this with an example.

The picture shows the first example for a PLC with 197 I/Os including the spares required by the customer. We have compared 3 different architectures, the first two with 32-channel digital modules and the other with 16-channel. For the analog modules we have considered 8 channels in all three cases.

The second difference is the criteria used to distribute the signals. In the first two cases the channels of the triple (logic 2oo3) and double (redundant IOs with logic 1oo2/2oo2) signals must be in separate racks, and in the third case is sufficient to place them in different modules. To increase availability, only in the third case, we have considered that the power supply and communication modules are redundant.

What we want is to ensure that the failure of a module only affects to a channel of triple and double signals. 


 We see that the highest price is when we use 32-channel digital modules because signals excess is very high (signals we don’t need). The shown prices only include the periphery (I/Os, racks, power supplies and communication modules).

The price reduction in the third case is significant and in many cases this architecture is valid (the channels must be located in different modules but they can be in the same rack).

We have made a second example without redundant or 2oo3 signals (only with single I/Os). In the first two cases we have distributed the I/Os by areas of the plant and in different racks, and in the last two there are not restrictions.

The result is shown in the image.


The main conclusion that can be drawn from the analysis is as follows:

The way to distribute the I/Os in the racks and modules of a PLC/DCS clearly impacts on the cost of the architecture and, in many cases, significantly.

If you are a System Integrator we recommend you to do this analysis from the bid phase as these price differences can decide the award in your favour or against you. On the other hand, you can give more value to your proposal by offering customer other options with an I/O distribution to provide more “availability” and an easier maintenance of the Plant. This type of analysis used to be complicated and required a long time, but now are extremely simple with a tool like “IO_Builder” that is designed specifically for this.

In the above examples we have always used the same model of PLC. IO_Builder is even more useful when comparing different models of PLCs, from the same manufacturer or different. We will see this in a next post.