Comm Fail

[MLBerg]

I am new to the plc word and looking for an easy way to check for communication failure between two plc's communicating via radio. I am using Direct Logic's DL205 with MDS 9810 radio.

 

[Pierre]

Unless you already have Quality (Fail/OK) BITs in the PLC functions you have to program a "heartbeat".

PLC "A" Turns a Bit to 1 in PLC "B" which resets it to 0.

When this BIT stays unchanged for a certain amount of time, then you have a com. failure.

Do the same thing from B to A.

 

[Tom Jenkins]

I use a technique similar to Pierre's but instead of cycling a bit I read the real time clock seconds register in the slave PLC. If the time is unchanged for three cycles, I set the error bit.

 

[zmanvortex]

Heartbeat Works

Pierre is correct with the heartbeat theory. I have just completed a project with DL-205 w/ 250 in which one machine upon power up will go out and try to find up to 8 stations. Not every application of this machine will use all 8 stations but we wanted one program to cover all possibilities. Once the master finds all stations that are "alive" it fills in a table to do the communications with. If I get a comm error sp 117 right after trying to communicate with one of the stations I go into e-stop mode until I can get that station back on line.

I also do the heartbeat trick in my program for some signalling. Here is an example psuedocode.

Slave sets bit C0

Master reads bit C0 from slave
if bit C0 on then bit C1 on (not set)

Master sends bit C1 to slave

Slave Reads bit C1
if Bit C1 on Reset bit C0


This works well in my program.
Hope this helps.

 

[MLBerg]

Thanks for the info. Using the clocks minutes and seconds in the slave seems to work well.