DH485 network issues

Hello All,

I am currently troubleshooting a communications issue regarding a DH485 network. The network contains PLC1, PLC2, and PLC3, all AB SLC-5/03s. PLC2 reads from PLC 1 and 3, and then outputs the signals read from PLC1&3 as well as some of its own signals through a Prosoft MCM3150 Modbus Card to an RTU. The PLC2-RTU comms are working fine, but PLC2 cannot read from PLC 1 or 3 on the DH485 network. I have no idea how long this has been an issue, so I cannot narrow down a smoking gun as to what could have caused it. The network drawing is attached, except the SCADA has been disconnected (knife terminals opened), because the SCADA PC has completely crashed (again, no one seems to know when or why).

Here is the info I have gathered so far:
- The Inter PLC Comms Logic in PLC2 is written in such a way that the MSG Block for PLC 1 Read executes. Once MSG DN or ERR changes to 1, the MSG Block for PLC 3 Read executes. Once MSG DN or ERR changes to 1 for that MSG, a short timer executes, while also unlatching the 'Ignore if Timed Out' Bit on both MSG Blocks. The cycle then starts all over again once the timer is done. (Refer to attached JPG for logic.)
- The logic should cycle continuously, but is locked up. I have tried to manually clear the MSGs various times in order to try and get the cycle going again, but they always just error out, and the cycle stops.
- However, if PLC 2 is unplugged from the 1747 AIC Link Coupler, the logic begins to cycle again, as it should if it were operating normally. (this makes me think there could be a hardware/wiring issue.)
- The message blocks for comms with PLC 1 and PLC 3 both have the same error, stating "(Hex 7) Target Node not responding.".
- The DH485 lights on both PLC 1 & 3 blink slowly.
- All wires on the network were end to end tested, so wire integrity is healthy. This does not mean they are wired correctly, though, and it almost seems as if the DH485 network wires are 'new' relative to the rest of the wiring in the panels.

The more I think about it the more I am starting to think it could be a network wiring issue. When the SCADA PC was disconnected they simply opened the knife terminal blocks. If this is the case, would a non terminated end to the DH485 network cause issues? Or is it a code issue (I can't imagine that this code NEVER worked and was never verified after it was programmed)? I was told the Comms haven't worked in years, but no one knows when specifically, or if there was a potential event that made the comms fail.

ANY troubleshooting help or tips would be appreciated, as I don't have many ideas left.

If there are any questions, please don't hesitate to ask, and I'll answer them the best I can.

Get your cable terminations done correctly. Your print is rather hard to view but you need either the SCADA reconnected or put in the jumpers on the 1747 at PLC1. Check the AB DH485 documentation.
I have an issue with the 'knife terminal blocks'. Just what are they? I always tried to keep all comm cables intact to proper connectors to keep interference low.
You can check the DH485 for comm errors and overruns in RSLinx.

Why would you open knife terminals on a DH485 network?

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1770-um022_-en-p.pdf

In the link coupler for each port on the network, there is an-inbuilt "termination" terminal if this is end of line.....

Thanks for the info fellas.

I have not have the opportunity to try and properly terminate the network yet, but once I do I will be sure to let you know how everything turned out.

FYI, the Belden 9842 cable is only run from the 1747 AIC to the neighbouring Marshalling cabinet, at which point it is terminated at 'knife terminals' and the runs between each PLC Marshalling cabinet (by far the longest parts of the run) is done over basic cable with no shielding.

Quite honestly the site is a mess, but I'll get their telemetry comms working at least.

Thanks again,