soundararajan

Micrologix 1400 Modbus RTU in Channel 0

5 posts in this topic

Hi,

Am new to the MSG instruction. Am having a problem with Modbus RTU communication in Micrologix 1400 PLC along with the Selec Unix-1 RPM meter. In this, PLC act as Modbus RTU Master, and RPM meter act as Modbus RTU Slave. I tried to connect with Modscan (Selec RPM meter) it's working fine. But, coming to PLC it's not working. Here are attached snapshots of the PLC configuration. Getting a time-out message. Using an 8-pin round connector for communication.

Message Timed Out.PNG

PLC configuration.PNG

Share this post


Link to post
Share on other sites

I'm no modbus expert, but I would ask two questions to check into.

1.  Did Modscan connect at 19200,N,8,2 ? If it connected at some other Speed. parity, Length and Stop configuration there is probably your problem.

2.  Did you try inserting a null modem between meter and PLC.  Modscan and PC might do this automatically.

Share this post


Link to post
Share on other sites

The most common Modbus RTU command is Read Holding Registers (Command 0x03), not Read Input Registers (Command 0x04), so check to be certain that your meter supports Input Registers.    If you are trying to read a Holding Register instead, you need to select Command 0x03, not Command 0x04.

Also, the most common serial framing settings are "19200, 8/N/1", meaning 19200 bits per second, 8 data bits, no Parity bit, and one Stop bit.  

8/N/1 uses a 10-bit frame (1 Start, 8 Data, 0 Parity, 1 Stop), while 8/N/2 uses an 11-bit frame that is less common.

And of course check your wiring.    It's uncommon for any two vendors to label their RS-485 conductors the same, so you often have to try flipping them.     

RS-232 is more commonly labeled correctly, but of course you have to make sure that the devices are a DTE/DCE pair, or if they're both DTE, there's a "null modem" connection crossing Tx and Rx.

If you can post a screenshot of the settings and describe what works correctly in ModScan, that will help.

 

Edited by Ken Roach

Share this post


Link to post
Share on other sites

Is the slave device connected RS485 or RS232?  Your screen shot shows "No Handshaking" which correlates with RS232 pins on CH0, but to use RS485, you will need to choose "No Handshaking (485 Network)".

 

And you'll need a 1763-NC01 or equivalent to access the proper pins on that port.

ml1400ch0rs485.png

Edited by OkiePC

Share this post


Link to post
Share on other sites

Today only tested, due to the non-availability of the line.

Am selected "No Handshaking (RS485 Network)" it's working fine. 

With same 19200, 8, N, 2.

Thanks, OkiePC.

1 person likes this

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now