READ_VAR blocks freeze

[alekseib]

READ_VAR blocks periodically freeze and communication with Modbus devices is lost. Timeout delays of blocks are set (word 3). There are no errors in communication visible (word 2). The only way that helps is to remove the GEST variable of READ_VAR blocks and assign it back. We have M580 Hot Standby system. What could be the problem?

[MatthewM]

How fast is it being triggered?  What is the timeout?  Is it actually giving no error code?  I suggest you double check and test your trap code for word 2 at the completion of the transaction (FE of word 1 bit 0.  If the next read is already triggered, you might be timing out but never seeing the status word come up.

What you describe with removing the GEST variable seems a bit weird, so I would start with what I said above.

Please share a snip of your code.

[MatthewM]

This is roughly how I would typically setup a READ_VAR, if it is any use to you:

[KJBER]

You can also try using the ModbusPortM / ModbusClient blocks from Schneiders General Purpose Library.   However, remember that it is mandatory that those DFBs do not run in the standby controller.

[RoozeeR · Accepted solution]

Check if the variables (GEST) are excluded from Synchronization with the STBY controller (see HotStandby installation manual). If they are synchronized, a switchover might lock the READ_VAR.

[alekseib · Accepted solution]

The READ_VAR block is triggered every 1 second if Activity bit is 0. Timeout is 20 (2sec).

[alekseib]

I removed GEST and RECP variables from Synchtonization. It's working now, see what happens next.

[suharman_saputr]

hi, i have similar problem but using M580 single controller. the READ_VAR block is freeze, and the only way to solve it is to create new tag for GEST. seems that the first tag that created for GEST is freezing (can not refresh). is there any way to un-freeze this tag without build changes/creating new tag? any help please

[MatthewM]

Have you set a timeout?  What is the state of the Activity bit?

[suharman_saputr]

timeout set 100 (10s) and activity bit is 1

[plcode]

@suharman_saputr did you find a solution to your issue? would you care to post it? thanks.