Skip to Content

Register Login

Industry Online Support

Technical Forum

Navigation

Modbus RTU 485 CB1241 problem

Created by: AutoGen_9633495 at: 6/19/2024 5:59 AM (2 Replies)

Rating (0)

Thanks 1

3 Entries

6/19/2024 5:59 AM	Rate (0)
AutoGen_9633495 Joined: 6/19/2024 Last visit: 11/7/2024 Posts: 1 Rating: (0)	Hello, I'm trying to setup a communication between a couple of gas purity sensors and S7-1200 (CPU 1211 DC/DC/Rly). The problem is that I can't get Modbus communication going. Slave device settings (from the datasheet) are: 19200 baud Even parity 1 stop bit RS 485 (Half-duplex) Read Input Register (04) ----Register 30001----Length:2----sensor unit 1 live reading I've tried reading registers of the device using Modbus Poll and was successful. After that I've started setting up the communication with MB_COMM_LOAD and MB_MASTER (located in MODBUS LIBRARY folder) . The MB_COMM_LOAD block initializes OK (DONE bit is TRUE and without error), however MB_MASTER seems to be stuck in BUSY mode (the block never gets DONE - I've checked with counter) with error 16#80C8 (Slave timeout. Check the baud rate, parity and the connectors on the slave.) I've tried changing the baud rate and parity, but no success. The second option I've been trying is Modbus_Comm_Load and Modbus_Master (from MODBUS (RTU) library). When using this option the DONE bit in Modbus_Comm_Load stays false. I've tried putting Modbus_Comm_Load block in the startup OB but no success. This program is located in Modbus RTU library folder. Any help would be highly appreciated. Regards, D Attachment programs.zip (424 Downloads)

Suggestion To thank Quote Answer

6/25/2024 8:45 AM	Rate (0)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 10/21/2024 Posts: 1547 Rating: (537)	Unfortunately, you have fallen into the common Modbus trap. Slave devices will just ignore requests that they do not understand, and the slave will not report what the issue is to the master. There are many reasons for the slave to fail to respond. The request might not have got to the slave device, the request may have been corrupted due to incorrect wiring or serial port settings, or the request could be addressed to wrong slave device. Check and double check that the serial settings match the slave device settings (baud rate, parity, start bits, stop bits, data bits). Check that the wiring is correct - this is especially prudent with RS485 interfaces as the nomenclature of the connections and there polarity is often confused. You could try swapping the +/- (A/B / hi/lo) wires over. Check that slave address in the master matched the slave address of the slave device. If all else fails, you would have to get a serial line snooper to capture the data on the wire. This can be done by using a RS485 convertor for a PC and then using serial port monitor software to capture the data (there are many free applications on the Internet to do this). Good luck.
	Programming today is the race between software engineers building bigger and better idiot proof programs, and the universe producing bigger and better idiots. So far, the universe is winning...
Suggestion To thank Quote Answer
This contribution was helpful to 1 thankful Users danw

6/26/2024 7:38 PM	Rate (0)
danw Gold Expert Joined: 3/28/2010 Last visit: 11/19/2024 Posts: 1081 Rating: (214)	>I've tried changing the baud rate and parity, but no success. One would only change the baud rate and parity to make the settings on both ends (master and slave) identical. They must be identical or the master's messages will be ignored by the slave. Trying swapping the driver lines one end to see if the polarity is backwards (compared to the 485 adapter you used with Modbus poll. The attached doc might be of help for an S7-1200. Modbus RTU seems to start on page 29 (pdf). Attachment Modbus TCP and RTU S71200_system_manual_en-US_en-US.pdf (476 Downloads)

Suggestion To thank Quote Answer

|

Share this page:

Share this page on...

Related topics by keywords:

ModbusRTU

Follow us on