ML1400 Modbus RTU Not Sending Data

I have an Allen-Bradley MicroLogix 1400, and have been working with it for a few months now and understand its basics pretty well. Up to now I've been using Modbus TCP (EtherNet) to communicate to a SCADA system through a Field Processor.

However, now I have to use Modbus RTU (RS-232) and am encountering difficulties. It's not sending data. In one of the attached images, the embedded display is shown and the box corresponding to channel 2 (RS-232, for Modbus RTU) is not checked (sorry for the poor quality picture).

As far as I know, the only settings I have to configure are those I've shown in the attached images. The node, baud rate, parity, stop bits, and data bits all match those set in the FP and SCADA system, and all of it looks good to me. The serial cable being used is also a reliable one, I've used it with remote I/O devices with no hassle just yesterday.

Transmitting the data I need with Modbus TCP is fine, but RTU is just not transmitting data for me. If anyone has any advice it would be greatly appreciated.

EDIT One: More Information
The SCADA system is the Master, and as far as I can tell it is requesting data from the Slave (the ML1400) appropriately. Although I've just noticed that in the "Channel Errors.png" image attached, it isn't showing any messages received. That seems like a problem.

EDIT Two: Messages Received
I left the PLC online to do some minor edits to the program, and apparently it received some messages when I wasn't looking (please see attached image "Channel Errors with Msg Rec.png"). I made no changes to the communications, so I'm not sure what happened there, but it happened, which is good proof that at least there's connectivity.

I don't know if the red outlined setting



is the 'silent' end interval that comes at then of both master and slave RTU messages.

Graphic from the Modbus serial standard:


a typical RTU message sequence:

My first troubleshooting step would be to put a Modbus RTU Slave emulator on my computer so I could see the request frames coming from the SCADA Master.

Are you sure that the SCADA system is only reading Coils and Input registers ? It is far more common in my experience to read Holding Registers, and some devices perform a "ping" to read Holding Register 1 to verify that a device is online.

The Channel 2 RS-232 port is a DTE type, so make sure that your modems and/or cables are configured to plug into a DTE device.

Dan, I think if you leave the Inter-Character delay value at zero, the MicroLogix will use the default 3.5 character-width intermessage timeout. I would put a value of 4 or 5 there.

Pre-Transmit delay can also be important; some DCS systems wait a while before switching to listening on their Modbus RTU port. It can't hurt to put 5 or 10 ms there.

Well it's an embarrassing find, but it turns out the error was with the FP in use. One of the engineers accidentally left the drivers for their previous testing in the system and told everyone that they'd properly cleared it.

Embarrassing to admit, but might as well close the thread. The program was fine, just human error.