Seeking help with AB PLC5 to RS-485 Modbus through a Digi1 IAP

[krakenfan69]

I have a PLC5/80E that I want to poll a Modbus device into using a Digi1. I have setup the message as a multihop going to the IP adress of the Digi and then to a compactlogix backplane to the node address 5 (the end device is set to 5 as well).


Using MDbus I can poll through a 485 converter into my PC.



I can't for the life of me get the setup in the DIgi correct.


In the Digi I set it up with a Network Master DF1 Ehternet to Modbus RTU using Port 1 and the screw terminals. I connected 2/4 & 3/9 on the terminals. As far as I can tell looking at the lights the Digi isn't even passing the message to the serial port.



Any chance someone can shed some light on this for me?


Thanks,


****** Fan #69

 

[Ken Roach]

Does the MSG instruction in the PLC-5/80E complete successfully, indicating that it connected to the Digi One IAP and received a response ?

>a multihop going to the IP adress of the Digi and then to a compactlogix backplane to the node address 5

I think that means that when you address this kind of message to a Digi One IAP, it uses the Slot number of the CIP Path as the Modbus Slave ID of the Modbus RTU poll it executes on the serial port side. So in the Multi-Hop (=CIP Path) configuration section of the MSG instruction in the PLC-5, it would look as though you were sending a message to a ControlLogix in Slot 5 of its backplane.

>In the Digi I set it up with a Network Master DF1 Ethernet to Modbus RTU using Port 1

I think the Digi nomenclature is a little confusing.

On the serial side, they would call this a "Serial Connected Slave", so that's straightforward.

What they call "Network Master DF1 Ethernet" really means that the Digi One IAP behaves as though it's an A-B controller using PCCC protocol. PCCC and DF1 are not strictly synonyms, but I understand why they use them.

I don't know for sure if that's the right selection when you're using "Multi-Hop"; it might be enabling the old A-B proprietary CSPv4 A-B Ethernet transport (TCP port 2222) instead of the modern Rockwell EtherNet/IP transport (TCP Port 44818).

You can figure out which ports are open on the Digi One IAP pretty easily: use Windows PowerShell and the "tnc" command from a PC on the same Ethernet network to check to see if TCP Port 2222 and TCP Port 44848 are open on the Digi One IAP.

>2/4 & 3/9 on the terminals.

The screw terminal port on the Digi One IAP is described on page 23 of the user manual for RS-485 mode:

Pin 1 RTS+
Pin 2 RTS-
Pin 3 TX+
Pin 4 TX-
Pin 5 GND
Pin 6 RX+
Pin 7 RX-
Pin 8 CTS+
Pin 9 CTS-

RS-485 can be tricky. You generally need at least THREE conductors: Data A, Data B, and Data Common. The Digi One IAP gives you a differential +/- pair for each signal so you can use "4 wire" RS485 or "2 wire" RS485.

If you have a "2 wire" device, in general I would connect Pins 3 and 4 to "Data A" and Pins 6 and 7 to "Data B", then Ping 5 to Data Common or Data Ground. Switch Data A and B to test: it's always a coin-flip about the labels for RS-485 data channels.

 

[danw]

For 4-wire to 2-wire conversion, the (+}’s are jumpered together and the (-)’s are jumpered together on the 4- wire side.

Jumpering should be
3 and 6, Tx+ and Rx+
4 and 7, Tx- and Rx-

I’d start with (-}’s going to A and (+)’s going to B.