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Old July 14th, 2010, 07:45 PM   #1

mihiri is offline
Join Date: May 2010
Location: Newcastle
Posts: 22
Thumbs up Modbus Master not polling the Modbus Slave through an EtherNet/IP Connection

Hello All,

There is this always one last issue before getting everything to work and I am having a similar scenario.

So I got this module from GridConnect (Xport-EIP-MB), this is a EtherNet/IP to Modbus converter. We have designed a Circuit to implement this adaptor and our Modbus Slave device is a Modbus RTU. Xport module has a config file where you can set the following parameters and I have assigned them correctly to match the Slave ;

Modbus Type : RTU
Modbus Slave Address : 1
Modbus Baud Rate : 9600
Modbus Data Bits : 8
Modbus Stop Bits : 1
Modbus Parity :Even

I have confirmed that when mapping the Modbus to EtherNet/IP mapping through explicit messaging, the Slave responds with a EtherNet/IP success response . This confirms that the Modbus master in the Xport queries the slave and the slave responds.

This is all good, now finally I was to read the modbus slave register values through EtherNet/IP (What this module supposed to do). I sets up a EtherNet/IP connection originator (ControlLogix5561 PLC) and adds the Xport module as a generic EtherNet/IP module. The PLC establishes a connection to the Xport module and I can't see any register data on the Input control Tags.

I verified that the Modbus slave has vaild data by polling the slave with a serial Modbus master, so there is no issues in the slave register end.

With the active connection (ControlLogix5561) on to the Xport we observed the link (Data Out pin of the Xport) to the Modbus slave through a Oscilloscope to see any Modbus slave polling and we are not seeing the Xport polling the Modbus slave or sending any queries. According to the Xport manual once a EtherNet/IP connection is established the modbus master within the Xport will poll the slave every 100mS, but we are not seeing any activity.

So this can be two issues, there is a compatibility issue between ControlLogix5561 and the Xport (Could be firmware, etc) even though there is an active connection to Xport in the IO branch, but the Xport does not see a valid EtherNet/IP connection. The second issue could be there is a firmware error within the Xport for not polling the slave (Xport is 3.3V TTL and we have pulled this to 5V TTL to the Modbus Slave, because Modbus slave end had a 5V RS485 serial connection for Modbus Serial )

it is much appreciated if anyone who has encountered a similar issues, put a helping hand out

Thank you very much
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Old July 14th, 2010, 08:24 PM   #2
United States

danw is offline
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Join Date: Oct 2004
Location: midwest, USA
Posts: 2,833
Two things I can think of:

1) If TTL is directly compatible with RS-485, why does Xport list an RS485 version of this device? Pure marketing?

Yours: GC-XPORT-EIP-MB Modbus to EtherNet/IP RJ45
alternative: GC-NET485-EIP-MB RS485 Modbus to EtherNet/IP

Is there some reason you don't thave the RS-485 version if your Modbus slave is RS-485?

2) There's some flow control lines, CTS, DCD & RTS. What happens on these lines (voltage wise) when your direct messaging function that sees the slave works?
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Old July 15th, 2010, 01:02 AM   #3

mihiri is offline
Join Date: May 2010
Location: Newcastle
Posts: 22

1) The Xport-EIP-MB module we have converts a serial Modbus RTU/ASCII (RS485 OR RS232) server into a EtherNet/IP server. This is what we are trying to do is to integrate the Xport module to the RS485 interface. (Please find the attached data sheet for the module)

2)The Xport firmware does not use use the configurable pins (CTS, DCD & RTS), So in our hardware these pins are always pulled high. thus not used.

I have tried many ways to fix this and seems like the Xport is not polling the slave with a EtherNet/IP connection on through the PLC
Attached Files
File Type: pdf XPORT-EIP_Data Sheet.pdf (173.5 KB, 43 views)
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Old July 15th, 2010, 03:41 AM   #4

mihiri is offline
Join Date: May 2010
Location: Newcastle
Posts: 22
Problem Solved

Found the problem, If the Xport config file is unlocked the modbus master stops scanning, just needs to lock the config file. It all works fine
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