Drives and Controls Magazine
Spare PLC I/Os 'can enhance safety cheaply'
Published:  30 March, 2017

ABB has revealed a cost-effective way of using spare I/O channels on its AC500 PLCs to achieve higher levels of functional safety, without needing extra safety I/O modules or larger cabinets.

In a new report*, ABB says that the technique could be useful for applications such as cranes, lifts and woodworking machines, that often operate at the lowest safety level – SIL 1 or PL c. PLCs in these applications typically have up to 32 digital I/O channels, yet often not all are used. By adding a safety CPU to the PLC, and testing the spare I/O channels dynamically, functional safety is enhanced instantly, says ABB. This can be up to 30% cheaper than building in safety from the outset. The safety CPU – such as ABB’s AC500-S – is used for safety processing and for diagnostics for the standard I/O modules.

The technique could also be attractive in countries that do not use the ISO 13849-1 or IEC 62061 functional safety standards. The report suggests that machine-builders in these regions could increase the safety of their machines without significant investment, opening up new export opportunities.

“Digital safety input channels should always be used, even for functional safety applications up to PL c (ISO 13849-1),” explains ABB product manager, Yauheni Veryha. “However, using standard digital I/O channels is still an option if they are available and their re-use for functional safety functions up to PL c is needed to satisfy specific customer requirements. This could be due to limited space in the control cabinet or too high costs to add digital safety I/O modules.”

ABB says its report proves the concept of using standard digital I/O in functional safety applications. Its safety analysis confirms that digital standard I/O channels on an AC500 can be used in safety applications up to PL c. However, the implementation, verification and validation of the approach in accordance with ISO 13849-1 and/or IEC 62061, is application-specific, because standard (non-safety) AC500 modules are involved in executing the safety functions.

Industrial cranes are one application where the technique for upgrading the safety functions of a standard PLC could be useful

Responsibility for implementing, verifying and validating the proposed approach is the end-user’s responsibility, ABB points out.

In some cases, standard I/O modules can be used in safety applications up to PL d and SIL CL 2. However, a more detailed functional safety analysis is needed in such cases.

* Usage of AC500 Digital Standard I/Os in Functional Safety Applications up to PL c (ISO 13849-1). Free copies are available from