SLC500 MSG blocks

[jed]

Does anyone know why the MSG blocks stop working and sometimes show "waiting for que space" in the status and how to start the block working again without restarting the processor?

 

[bernie_carlton]

Does your code wait for the previous message to either be in the done (DN) or error (ER) state before beginning the next?

 

[jed]

Several of the PLCs have only one MSG block which use a flip flop timer of about 4 seconds to control the block, but the others which have more than one block only use a rolling register so block 1 is done when the register equals 1 and so on.

 

[Ken Roach]

This sort of thing is most common on systems where the communications link between controllers is unreliable and the logic and configuration has not taken into account the effect of disconnected or failed messages.

What network(s) are you using; Ethernet, DH+, DH485, DF1 ?

Disclaimer: I'm very good with SLC and Logix communications but this is "as I understand it". I don't work on the development team and have no special understanding of the firmware.

The SLC-5/0x controllers have a 4-message "buffer" and a 10-message "queue".

In the buffer, the messages are processed by the communications subsystem and actually sent out onto the network and wait for replies. For MSGs that have a Write data payload, the data is taken from the source registers and held with the MSG in the buffer.

In the queue, the messages are not yet processed by the communication subsystem and the data from the Write source files has not been copied. These messages are waiting their turn.

In theory you could execute fourteen MSG instructions in one scan and they'd all run to completion.

There are some variations on this, considering the different stacks and tokens and arbitration and handshaking of the various networks available for the SLC family.

 

[Norml]

I have a program with 6 MSG instructions that use this method.
Everything works fine without having to cascade them but I'm not sure what the buffers/queues are for the Micrologix 1100.

 

[jed]

SLC504 MSG blocks

Hi, thanks for the replies.
My network is 7 SLC504s with DH+ over fibre optic modules. For years the network was always dropping various nodes on and off but after talking to Rockwell they said to connect 150ohm resistors to all the processors and all the fibre optic wired connections as they are all classed as an end which solved the problem. Just recently 3 more SLC504s have been added by another company which is causing the network to drop out again after a couple of days of being OK. This causes the MSG blocks in some of the nodes to stop operating showing "waiting for queue space" and the only way to start them is to cycle the power. I think there is also now a problem with one of the fibre optic modules as when the 3 new nodes are connected the activity LEDs become half as bright and all the existing nodes drop off.