What is the best way to verify messaging between PLC's?

[yam785]

I have a couple of questions about 'Messaging' between PLC's using DH+. I'm currently working a project, which involves communication between PLC5s, SLC5-04s, and 1756-L55. Maybe others have the same questions. I normally only read, and learn on this site. As I want to learn to do things the right way, and I am in a time crunch, I am asking for help with the following questions:

1. What is the best way to set-up timing/sequencing of MSGs in PLCs? Is it important to not have them enabled at the same time?

2. What is the best way to verify communication between PLCs? This project relies on communication, so if heartbeat was lost, machinery needs to shut down.

Any help would be greatly appreciated. I would gladly help others, if I had something worth adding.

Thanks

 

[curlyandshemp]

I have a couple of questions about 'Messaging' between PLC's using DH+. I'm currently working a project, which involves communication between PLC5s, SLC5-04s, and 1756-L55. Maybe others have the same questions. I normally only read, and learn on this site. As I want to learn to do things the right way, and I am in a time crunch, I am asking for help with the following questions:

1. What is the best way to set-up timing/sequencing of MSGs in PLCs? Is it important to not have them enabled at the same time?

2. What is the best way to verify communication between PLCs? This project relies on communication, so if heartbeat was lost, machinery needs to shut down.

Any help would be greatly appreciated. I would gladly help others, if I had something worth adding.

Thanks

I think a heartbeat pulse is the way to go to verify communication. A heartbeat ensures that the other PLC is actually in RUN mode. A message instruction will successfully read a target PLC's registers even in the Program mode, and even a Produced/Consumed tag setup in the Contrologix world will yield the same result.

For DH+ comms, I always set up a timer or bit shift register to stagger the communications

Ian


Ian

 

[rdrast]

A 'quick and easy' heartbeat might be using a timer, and a bit in one of the registers you are transferring. Have that bit echoed back by the receiver, and use a one shot to reset the timer every time it cycles...

Off the top of my tired head:

Rung:
xio Echoback_Bit ote Send_Bit
Rung:
xic Echoback_Bit ONS OneShot_Bit RES Comms_Loss_Timer
Rung:
TON Comms_Loss_Timer
Rung:
XIC Comms_Loss_Timer.DN OTL Comms_Fault_Latch

 

[Tom Jenkins]

WE use a method similar to rdast's. We copy the system clock seconds to a register and include it in the data being transferred. We copy it to a holding regiser in the slave device. Each time the value in the register changes we repeat the copy. If it is the same we start a timer, and if the timer times out we set a comm fail coil.

 

[yam785]

Thanks for the help. I just wanted to make sure I was going about it the right way. One other question I have, are the control block bits the same in the PLC5s as they are in the SLC500s? bit15=enable, bit 13=done, bit12=error, etc. I would like to go with the existing set-up in the bitmap included, but I need to figure out a way to time the cycle, so it is not reading so often. Time is not critical, and there is a ton of processors on this data highway, and it is very slow already. I don't want to slow it down much more, with the messaging I need to add.

Thanks again,
Bob

 

[yam785]

Trying to figure out how to get bitmap to show up in post. Still no luck.