Encoder Backup Logic

We have issues with our encoders backing up on 1746-HSCE causing timing problems when restart. We use the Z which resets ok on next product, but may miss critical signals on the first product after restart machine.

Installing next another machine with 1756-HSC, was looking for some known good "Anti-Backup" plc logic for either a SLC, PLC5, or Controllogix that I could use. Anyone ever come across samples of this type of logic?

If the state of the encoder can change while powered off and it is vital to knoww the true state on restart then you probably need absolute encoders. The range of the encoder system must be large enough that any normal variation while off can be detected.

Another solution, and one that I use, is to just plain ignore the actual counts from the encoder.

Assign three registers, one for 'Old_Value', one for 'Scan_Count_Delta', and one for 'Total_Counts'. Every scan, subtract the 'Old_Value' from the actual module value, and store it in 'Scan_Count_Delta', then move the actual value into 'Old_Value'.

Now, you have a delta count from scan to scan. You can use a bit to enable counting or not. If counting is enabled, then every scan, you add the 'Scan_Count_Delta' to the 'Total_Counts' register. If counting is disabled, you don't.

To force a reset, you set 'Old_Value' = Module Value, and 'Total_Counts' to zero.

This is very easy and reliable on counters that operate on integers, as the rollover is already handled. It also gives an easy way to hold counts through power-downs/resets without having to monkey around with saving and restoring the Counter Module current value.