Modbus - Ethernet Bridge Problem

I am trying to bridge modbus registers coming off a gas chromatograph onto the ethernet network; but the bridge is kicking my tail. I can physically connect my serial to 485 microflex converter to the wires and successfully poll the registers using modscan & omnimbt on my laptop but when I set up MOXA switch with same configurations as a TCP/IP Server (19200; 8; N; 1; RTU) I get nothing. I even went as far as getting a Schneider ConneXium ethernet bridge but still neg results. All I get with either one is "Target device did not respond"

Anyone run into anything similar? Also, I noticed drawings for the Schneider show a 120 ohm resistor at device, which we do not have. Would a resistor cause the problems with the bridge but not my serial-485 converter for my laptop?

Appreciate any comments.

its important to note the different types of modbus. modbus tcp and modbus rtu over tcp. Im not familiar with the moxa or connexium, but if your driver is expecting true modbus tcp on port 502(or vice versa), you need to make sure that the moxa is converting modbus rtu over tcp into modbus tcp. Either way if you are using modscan you should have the option to try either method.

I wouldnt rule out the resistor but it is unlikely that is the culprit in my experience.

maybe post some screen shots of the moxa config and also your settings in modscan.

Here are some. Not on site so couldn't get one of the main config screens on the moxa. Port 1 is set as TCP Server; port 4001; command port 966...

Also attached are successful polls from modscan and omnimbt

More

Jason Ozolins said:

Here are some. Not on site so couldn't get one of the main config screens on the moxa. Port 1 is set as TCP Server; port 4001; command port 966...

Also attached are successful polls from modscan and omnimbt

Click to expand...

ok so yeah, just make sure you are doing modbus rtu over tcp/ip using port 4001.

try and download a program called modbus poll master simulator
http://www.modbustools.com/download.asp

set it up the same way you have been setting up modscan but tell it to be modbus rtu over tcpip and put in your ip and port. I looked at the moxa and the other device and it doesnt look like either is capable of actually converting to modbus tcp. they do what is called tcp sockets(or tcp server or modbus rtu over tcpip), which is essentialy just packing the serial data into a tcpip stream. if you only have the capability of polling with modbus tcp, then you will need to use a digi one iap or lantronix xpress dr instead of what you are trying now.

anyway, try modbus poll and see if that works. if you need some help setting it up ask.

Thanks a lot. I will let you know tomorrow how it turns out.

Still neg results with the Modbus Poll program you listed. No slave ID response. Seems like either bridge just absolutely is not picking it up, although I am sure I have correct pin out/terminations. I can see my TX light flashing on the correct port I have it going to but nothing ever comes back on RX.

Just blows my mind it works fine when I hook up directly to wires with my microflex 485 converter...


Oops, sorry for upside down image... iPhone...

Nevermind, I got it. Damn Ground on pin out was juicing everything. Appreciate your responses!

can you post a wiring diagram? I assume you are using 2 wire 485? some devices are setup for 4 wire, and you use jumpers to make them 2 wire. that may be the case with this moxa. you would jump rxd+ and txd+ togetheer, and that would be the positive(or B) side going to your device, then you would jump rxd- and txd -, and that would be the negative(or A) side going to your device.