MicroLogix 1400 + Modbus RTU using 1763-NC01 cable

[Epy]

I am using a 1763-NC01 Ch0 to RS485 cable along with a ML1400 processor. I'm trying to use it as a Modbus master to communicate with a cut meter slave. I've already contacted tech support for the meter company, but was wondering if anyone knew of any sort of gotchas for this configuration. I'm of course using the MSG command to try and pull the Modbus registers, but haven't made any progress. On the RSLogix side, it says "timed out on the local processor", and on the meter side the screen just shows a byte error of 2 with each attempt. I've tried virtually every combination possible within the channel 0 configuration.

Thanks in advance.

 

[Mickey]

One of the first things to do with RS485 is to swap terminals A&B or +&- or transmit/receive
how ever they are labeled. Different manufactures have different standards.

 

[Ken Roach]

The principal gotcha is the enabling of the RS-485 portion of Channel 0. The default is for the controller to use the RS-232 pins, so you have to select the RS-485 pins by configuring the Handshaking value to "No Handshaking (485 Network)".

This might fall under the "I tried every setting" but let's make sure it forms one of the pillars of your serial port configuration.

The secondary gotcha is universal to RS-485: no two vendors seem to label things the same. Always test by switching Data A and Data B.

The MicroLogix implements RS-485 with a dedicated data common pin "COM", while many other devices rely on the chassis ground as a data common reference.

As there is no such thing as "two wire" RS-485, when you're mixing a 1763-NC01 with a ground-referenced device, connect the COM pin on the -NC01 to ground.

The MicroLogix implementation was done to provide compatibility on Data Highway 485 with SLC controllers, which have used the dedicated data common pin for decades.

 

[Epy]

We tried that briefly yesterday but since you suggested it I will try it again and try all the different configurations. Thanks

Edit: This was in regards to Mickey's message

 

[Ken Roach]

I realize this might be overkill for a single MicroLogix controller and Modbus link, but for the benefit of future readers of this thread I want to mention that the absolute best value in serial troubleshooting tools are the EZ-Tap and Versa-Tap from Stratus Engineering.

http://stratusengineering.com/

Having the EZ-Tap and the Versa-Tap in your toolbox is like being the guy at an Amish barn-raising with a nail gun.

 

[Epy]

The principal gotcha is the enabling of the RS-485 portion of Channel 0. The default is for the controller to use the RS-232 pins, so you have to select the RS-485 pins by configuring the Handshaking value to "No Handshaking (485 Network)".

This might fall under the "I tried every setting" but let's make sure it forms one of the pillars of your serial port configuration.

The secondary gotcha is universal to RS-485: no two vendors seem to label things the same. Always test by switching Data A and Data B.

The MicroLogix implements RS-485 with a dedicated data common pin "COM", while many other devices rely on the chassis ground as a data common reference.

As there is no such thing as "two wire" RS-485, when you're mixing a 1763-NC01 with a ground-referenced device, connect the COM pin on the -NC01 to ground.

The MicroLogix implementation was done to provide compatibility on Data Highway 485 with SLC controllers, which have used the dedicated data common pin for decades.

I noticed that the only handshaking setting that made anything happen is the no handshaking (485 network) option so I've been sticking to that.

Success! I flipped A/B and re-tried with 485 network for handshaking and the default factory settings on the unit. Yesterday the unit (and the CPU) had been configured for 19200 baud, even parity, and that didn't seem to work with the wires flipped, or maybe I didn't select 485 network.

In regards to what you are referring to about ground/common pins, right now I have A & B hooked up appropriately and I have the CGND pin on the cable term block connected to earth ground. Is this correct? It works currently so I'm hesitant to change it.

Thanks guys.

 

[Mickslan]

The principal gotcha is the enabling of the RS-485 portion of Channel 0. The default is for the controller to use the RS-232 pins, so you have to select the RS-485 pins by configuring the Handshaking value to "No Handshaking (485 Network)".

Thanks Ken! This had me stumped. I kept looking at the config of Channel 0 and thinking "yep, no handshaking", I hadn't even realised there was another 'no handshaking' option...