Afternoon people, i hope someone can help.

I'm currently in saudi working in a bev. can plant and i've only recently picked up the 5000 software and have written the line controls for the plant.

Now, they wish to have the DH+ network installed to allow m/c interlocks for safety and line control.

Now, we have the problem, as i just can't seem to get the hang for the messaging between processors, i can write the message blocks and fill in the comms path but for some reason the processors will not communicate with each other.

Is there a link available that gives a step by step guide? as the DH+ manual from the Allen Bradly site doesn't shed much light for me.

Or if anyone could send me a description to my email would be very bery much appreciated!!

many thanks in advance
Michael Wilson

 

Hi,
I am unaware of any regulatory framework which would allow you to use DH + for safety circuits.
Maybe someone on the list with more AB experience can enlighten us.

Marc Sinclair

 

USE MESSAGE READ / WRITE INSTRUCTION.

 

Using Message instructions for interlocks between machines is not safe, message instructions are queued in a buffer and are not deterministic. Better to use a DCM card. Make sure the write and read types match PLC5 typed Write in the sending PLC and PLC5 typed Read in the receiving PLC. In the PLC5's you had to set an address N10:0)in the port configuration for the status word in order for the message instructions to work.

 

Welcome to the deep end. See if you can find out who threw you in.

DH+ is the wrong choice for safety interlocks, and really not a very good choice for peer-to-peer networking with ControlLogix.

Logix uses a tag-based memory, while DH+ messaging is based on PLC/SLC data registers. You generally use the "Map PLC/SLC Data" function in RSLogix 5000 to set equivalency between arrays of INT or REAL data tags and PLC data files.

I'm not sure if you can message Logix tags natively back and forth between Logix controllers... I've never seen somebody use DH+ to network Logix controllers when PLC-5s weren't involved as well.

It would be a lot better to use Ethernet and produced/consumed Tags to perform peer-to-peer interlocks in this system. That's what it was made for!

A-B has an office over in Dubai. Give them a call.

 

G'day Michael,

To do messaging between two CLX processors is not too tricky. As I mentioned in a previous post I find it better to do message writes rather than a read. The hardest thing is that DH+ is octal.

Create an array or UDT identical in both processors, use a MSG with 'CIP Data Table Write'. Enter the tags you are using as the source and destination. Enter the CIP path, note that the browse function does not serve much use, and check 'Cache Connections'. There is an AB technote regarding the cache connections. In the CLX you are writing to, you only need to had the same size and structured array/word/udt and nothing else.

I also use some code to check for active nodes so that I only write to an active node. The code also resends the message when a node becomes active again, and we only send the message when data changes.

The DHRIO module is of help to understand the CIP path used in CLX messaging, but there is not much in actual messaging. If you want I can send a sample program of the logic I use.

I would assume the interlocks are not time crucial, and a couple of seconds in delay will be okay. If you need to be deterministic use scheduled ControlNet, it can be a little tricky to get going but it is reliable.

Regards,

Trevor Ousey
BJH Controls P/L
[email protected]