Nole
Member
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.
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.