Courtesy of: Delta Automation, Inc.

The default setting for the remote I/O hold up time (HUT) on a Modicon PLC system, is 300 milliseconds. This determines the amount of time that the discrete outputs associated with a particular drop will remain in the "on" state once communication with the processor is lost. This loss can be due to a power loss at the drop or a cabling problem for example. With the default setting of 300, the discreet outputs will de-energize after 300 milliseconds (0.3 seconds).

Analog modules behave differently. The analog modules options are: go to zero, hold last value, or go to a preset value. These options are all chosen in the traffic cop (I/O map) area of the configuration screen. This default time setting is usually adequate for most stand alone systems.

The purpose of a redundant PLC system is to allow the application or process to continue to operate in the event of a PLC processor type failure. This is accomplished by "moving" the I/O points from the control of one processor to another if a failure should occur. This switch-over takes a finite amount of time, during which the I/O is not communicating to the processor. Typically, on Modicon systems it can take several scans of the processor to complete the switch-over. On large applications, at 0.3 to 1.4 milliseconds per K of logic, this can be longer than the default setting of 300 milliseconds. If the switch-over time takes longer than the hold-up time, the I/O will have lost communications with the "failed" processor and began its timing sequence and will ultimately drop out or de-energize. This would cause an application or process to stop.

In these types of systems Delta Automation, Inc. suggests that the time be set to 1500 milliseconds (1.5 seconds). This setting is accessed on the traffic cop or I/O map screen associated with each drop. There are several items to be very careful of when changing this value. First, the value that you enter is already in hundreds of milliseconds, for example 300 milliseconds is entered as just "3", and 1500 milliseconds is just "15". Secondly, the PLC software will allow you to enter an enormous value.

This can be extremely dangerous as the I/O will remain on and active even with the processor turned off! With NO CONTROL!
It is advised that the time never be set greater than 15 (1500 milliseconds).

Also, in some versions of the PLC programming software, you cannot just highlight the "3" and replace it with a "15". If you attempt to do this you will get an error message. A work-around is to enter a "5" after the "3", then type over the "3" with a "1" to get a "15".