I am trying to modify a timer, that has a preset that is loaded from a MOV command. The source is an N21 bit.

Here's the kicker, if I am offline, I can change the timer and cycle the machine, and the machine runs the way the engineering staff wants it to. When I put the machine back online, with the rest of the equipment, my timer reverts back to what the old source was. No where, in the programming, is N21 called out, other than as the source for the timer.

 

Does the PLC have a memory module installed?

If it does, you will have to save the change to the module to keep it.

 

Since you have no programming that changes the value of register N21, it seems likely that there is a HMI or other online source that is sending a timer preset value directly to register N21. Although you can manually change the value in register N21 while offline, once you go back online, a new value will be received that overwrites your value and sets the register back to the previous value. If this is the case, you will have to figure out which online device is responsible for sending this timer preset value to the PLC and then enter your new preset value using the online device.

 

Dear Don,

Another possible causes that prevent you from change the N21 register
current value which that can't be cross referenced, other than HMI
mentioned by Dick, are :

- The N21 register are included on destination registers of COP instruction.

- The N21 register are included on destination registers of MSG instruction.

- The N21 register are included on destination registers of BTR instruction.
For above three possible cause, please verify if above type
instructions are exist in your SLC-04 logic program, then check the their destination register and data length.

Also, you can disable suspected instruction by put AFI instruction on beginning if their rung, and then check if you are now able to change value of N21.

- The N21 register are included on destination registers of remote MSG
instruction through DH+.
Check and verify if some other PLCs write to it through DH+. Try to remove DH+ connector from CPU DH+ port and check if you are now able to change value of N21

Hope this help
Aan

 

> Does the PLC have a memory module installed?

> If it does, you will have to save the change to the module to keep it.

Can you change the timer while online? Go in the status file under memory module and look to see if a status bit is set to load from the memory module you can temporarily change it if need be. If you do have a memory you can only save to it while the controller is in program mode, it will ask you to do this if you try to save it by going to comms then under EEPROM.