Micrologix 1500 Modbus

Can anyone give me an easy explanation on how to use modbus/rtu with 1761-net-aic module, for micrologix 1500. If it isn't very easy then maybe give it to me straight. Just trying to take in registers from a Siemens model 2100 tank sensor to the PLC and use it for process variable. Maybe point me in the right direction on some good literature. Thank in advance

Since you mention using the Net-AIC we assume you are using it to convert from the RS232 output of the Micrologix to RS485 from its port 3.

1. Using the Net-AIC manual hook the 1500 to it using the proper cable. You can use power through the PLC cable or supply 24VDC seperately. Set the power switch accordingly.

2. Wire from port 3 to your device. I have no idea what your 'tank sensor' connections look like.

3. In the PLC program, while offline, set the PLC port to Modbus RTU Master. Set the other parameters (baud rate etc.) as needed by the end device.

4. In the program create a MSG to read a known value from the device. Look in the device documentation for the proper Modbus address. (The address you end up using may possibly have to be 1 higher.) Each Message will have to use an MG:xx file

5. Select the channel, the Modbus command (for more on Modbus itself go to www.modbus.org), the data table within the PLC where the information will end up and the size in elements (probably 1 for the time being)

6. Enter the Modbus address of the device and its node address.

7. The commend is edge triggered. Don't try to re-trigger it until it DN or ER bits turn on.

8. A good troubleshooting method if all else faile is to swap the two RS485 data lines.

Thanks Bernie, first time doing a complete project. Normally someone else does this but I have been wanting to program for a while. This is very reassuring.

Obviously what I wrote is just the start. Play around with a single command, changing it to a write to see how to send information. Use the multiple Modbus commands to get down reading or writing multiple areas with a single message. Finally learn how to link multiple messages (basically the DN or the ER from a previous message starts the next - when the last one completes clear all flags and start over).

I would also get myself a USB to RS-485 cable and a copy of Modscan (Converter: http://www.usconverters.com/index.php?main_page=product_info&cPath=75&products_id=280 Modscan32: http://www.win-tech.com/html/demos.htm)

It is very helpful to be able to watch what is going on at the comm level when troubleshooting.

Read enough of the Modbus documentation to get the concepts down, this way when things don't start up like they should you will be better prepared to figure out why.

Thanks I'll probably pester a few people before its all said and done, haha. I can already think of a million things I want to ask everyone.

Just realize that Modbus in not a super high speed connection. Think information which is ok if it's updated every second or so (assuming you have a bunch of devices you are interfacing with.).

But otherwise, once you get the hang of it, its really easy. As long as the Modbus locations for information are given and there is a valid RS485 connection then you are good for up to 32 devices. As this distance increases you may want to lower the baud rate. Be sure to use the proper termination resistors on the physical ends of the network. The NET-AIC has a termination available if you want to use it.

The only problem we've hit is when end devices have incompatible permitted settings. (One device wanted only 2 stop bits another only 1 stop bit. One wanted 7 data bits while the others wanted 8 data bits.)

bernie_carlton said:

The only problem we've hit is when end devices have incompatible permitted settings. (One device wanted only 2 stop bits another only 1 stop bit. One wanted 7 data bits while the others wanted 8 data bits.)

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Working for Schneider that was norm. If you ever want to get indepth training on Modbus, that's the place to go.

Connecting our stuff to our stuff wasn't entirely horrible (not entirely, just often horrible), but connecting our stuff to other vendors (who often included just enough of the Modbus standard to get their stuff working with their stuff) could be a pain.

As far as the Modbus standard goes, about the only thing "standard" about it is how you spell "Modbus".