What are the advantages and disadvantages of using hardwired vs. software-based safety interlocks with PLCs?

A more precise question would be when to employ software interlocks versus hardware interlocks. If the interlock is critical and a failure could have significant impacts such as equipment damage or safety hazards, it is advisable to prioritize hardware interlocks over PLC-based software interlocks. This is because PLCs may fail to respond reliably, given their reliance on CPUs which can encounter errors or malfunctions. For all other routine interlocking needs, PLCs are preferred, offering flexibility and numerous other advantages.

When dealing with critical interlocks, it is advisable to opt for hardwired interlock systems. Hardwired interlocks require specialized electrical expertise for troubleshooting, whereas software-based interlocks necessitate the involvement of a skilled PLC programmer.

Physical safeties that have an extra set of dry contacts that, when the safety trips, closes and will send a signal to the software that will alert the HMI/operator which safety caused the stoppage.

For critical applications it is best to have both hardwired and software safety interlocks, hardware interlocks can be traced with one requiring specialist software knowledge

IMHO, any hardware safety interlock should be completely INDEPENDENT of the PLC. They should perform their duty complexly, and independently of the PLC. You can use a set of contacts to let the PLC know the status of your Fire Alarm, High Pressure Status, etc. But when the device operates to shut something down, it should isolate outside of the PLC.

Also should note that software interlocks and safety are generally easier for someone to bypass. Extra control features are required to prevent this such as user access control for forces.

Redundancy is important. Many safeties are hard wired and have an a connection to a PLC input as well, like E-stops and auxiliary contacts in motor control circuits and overload contacts. Software interlocks should definitely be fail safe in case the PLC fails, or is dropped out of the run mode. Proper programming techniques, like using a safety summary rung in ladder, or using appropriate messages on an HMI, can assist in troubleshooting when software interlocks are used.

Most nightmare scenarios are caused by someone resetting a software safety while another person is troubleshooting the equipment. Always shut the power off at the equipment!

One thing to keep in mind. I always use PNP signals for safety reasons. I once had a system that was delivered to our company. It was wired NPN which means that it was looking for an active low. A safety interlock signal wire had been crushed and shorted to ground during shipping. The system saw the signal as low so it thought all was good. Luckily we found the malfunction and corrected it before the operator was injured. Although they did get a scare and would not return to the machine.

Hardwired advantage is you can physically check it and you become very confident that a break in the wire will put equipment in a safe situation unless if wiring or logical I/O type is wrongly configured. Consider using safety relays as well. Disadvantage is it is not as flexible and cannot be rapidly deployed when there is a need to change it. Software-based advantages are it is very flexible and scaleable if there are more equipment added to it or if there are newly discovered hazardous situations that can be easily be resolved by software patch or revisions. Disadvantage is for the end user, the implementations may be hidden and cannot be understood clearly so end users cannot provide their own suggestions effectively enough to the OEM.

Along the line that Steve Betram mentioned I saw one system which had safety pressure mat. Whoever build the system used two channels signal HIGH to indicate when somebody was standing on the pressure mat. This meant that if somebody simply unplugged the terminal block from the 1734-IB8S then the machine would run with somebody standing on the safety mat. Or, if as we found it if somebody switched the left and right terminal blocks the same issue would happen. The pressure mats were the only inputs being used on the 1734-IB8S.

? Hardwired interlocks are generally preferred in critical safety applications where reliability and immediate response are paramount, and where the system is unlikely to need frequent modifications. ? Software-based interlocks are ideal in environments where flexibility, integration, and scalability are important, and where the potential risks of software faults can be effectively managed.