I am trying to figure out how exactly to communicate via this Schneider bridge, and getting nowhere. I pulled the manual off the web, which told me I could get into the configuration menu via the RS232 serial port connected to my PC, but I have been unsuccessful with various cables and configurations. I can get to the configuration just fine via ethernet.

Also, I am trying to just verify that queries sent via the ethernet port are getting to the serial port, so I have been trying to look at the serial port communication by connecting to the com port on a laptop. So far I've seen nothing come across, and I don't know if the problem is in the cabling, or in the configuration, or in the serial port itself. From what I can tell, the serial port is port 502.

Anyway, if anybody has any info that might help me out, it would be much appreciated - e.g. if you've successfully entered the config menu via the bridge's serial port, any other configuration or cabling tips, etc. I've never worked with one of these before, and have little to go on.

Thanks,

Mike

 

The first thing that would help out greatly is more information about the way you are trying to use the bridge. I have used several. If you want to send a Modbus message from ethernet to the serial device, you use the IP address of the bridge. As an example, I had a Quantum PLC (192.168.0.100) sending requests to a Compact PLC (which is attached to the bridge) simply by putting the IP address of the bridge in the instruction on the Quantum.

This was done with the bridge set up for Modbus slaves. In fact, the only setting I changed out of the box was the IP address of the bridge. If the Compact was a Modbus master, you would have to set the bridge up for a connected master and then the device address you send to from the Compact would be a Modbus address which the bridge would translate to an IP address.

I thought the manual was pretty descriptive and the device was very easy to work with.

 

We have used this bridge many times without problems.

First of all: what kind of device do you have attached at the serial port? A Modbus master or a slave (the answer of this question may impose you to change the configuration of the bridge)?

Another point: 502 port there is nothing related with the serial port. This is the Modbus TCP port assigned for the TCP/IP messages. You do not have to use this number, this is transparent to you.

You need the IP address of the bridge and the Modbus addresses of the Modbus slaves attached at the serial port (if you have only one, you just need the IP address of the bridge).

 

Thanks for replying. I think the problem may be at a slightly different level than your responses, however. Basically, I cannot seem to get any sort of communication to occur at the serial port.

There are two LEDs that indicate traffic at the serial port, and the only time I ever see these light up are when I reset the bridge - it looks like maybe a port scan is going on. Any other time, I see nothing. I do see traffic at the ethernet port.

Also, according to the documentation I should be able to connect to the serial port via hyperterminal to enter the configuration program after a reset of the bridge (while holding down the "x" key on my keyboard). The only thing I ever see on my hyperterminal screen is an asterisk that comes up when the bridge is reset - i.e. I can't get into the configuration of the bridge via the serial port. This almost leads me to believe that there's something wrong with the serial port (or in the configuration of the serial port, but according to the documentation, it should be forced into a default configuration when I reset the bridge and hold down the X key from hyperterminal).

So basically all I'm doing at this point is trying to verify that if I send something to the ethernet port, it gets to the serial port, and right now it doesn't look like that is happening. Ultimately, we have a SCADA host that's sending a poll through the LAN to the bridge, and I want to see something happening on the serial port - at this point, I don't care if the messages are formatted correctly, I just want to see that something gets through.

If there's something extremely basic that I may have missed, please let me know. Thanks again for replying, and for any other info anyone can provide.

Mike

 

Being a veteran of the CEV, you probably have the RJ-45 serial cable wrong. It is pretty easy at 9600,8,N,1 from HyperTerminal to get into the config.

I usually recommend people trying "new cables" first wire up a simple pig-tail at the screw terminals that has a 9-pin male connector. You only need 3 wires - TX screw to pin #3, RX screw to pin #2, and GND screw to pin #5. To be safe in your 9-pin, short RTS/CTS (pins 7 & 8) and DTR/DSR (pins 4 & 6), but the CEV doesn't require these. Also be careful when you look at the screw terminal printing on the side of the CEV - I've know a very few people to get a backwards view of this.

Once you do this, a standard NULL-MODEM cable from your CEV to the PC works fine (I normally used one of those $35 Rockwell cables I got with a SLC5/05 - bit of an irony really).

The beauty of this pig-tail to 9-pin male, is now for any device you have, just use the SAME cable that works to your PC notebook. Once you are certain things are talking, that is the time to start making a custom RJ-45 cable. If you try the RJ45 cable first, you can never be sure if things don't work because your port settings are wrong or if your cable is wrong.

best regards
- Lynn

 

Mike, as long as you connect to the CEV with Modbus/TCP, you WILL get data out of the serial port. Period - I know because I was involved with writing the CEV's firmware! The CEV doesn't sit and hold the request until some magic event causes it to go out the serial port. It dumps any Modbus/TCP request out as MB/RTU or MB/ASCII as soon as received As stated in my last email, if you're seeing nothing at your hyperterminal session, you likely have a cable problem.

Also make sure you configure the correct electrical interface (RS-232 or RS-485) at BOTH the software configuration level AND the red switch externally. Neither can do the job alone (a HW issue).

Regards
- Lynn

 

Lynn, i am using this brdige as Modbus Ethernet to Modbus Converter. PC is connected to bridge through HUB. RS485 port is connected to 2 PLCs which are in multidrope fashion. I am able to connect to one of the PLC through concept but not with other. If i switch off the first PLC then the non responding PLC starts communicating.
What could be the reason ??

Avinash Mali
Larsen & Toubro Limited

 

One possible answer is that some devices turn off-their comm port when another slave talks to reduce interrupt loads. So it is possible if you poll PLC #1 & it responds, then the CEV sends an immediate 2nd poll to the 2nd PLC, that the 2nd PLC will still have it's comm port off & miss the first byte or 2. So when you poll both devices, the "2nd" always sees a CRC error but if you poll only the 2nd alone it is Ok. I have seen this many times.

If this is the case, a slight work-around on many bridge products is to enable the RTS/CTS radio modem support and use a "50 msec delay after rise of RTS". On a CEV this may limit you to RS-485 4-wire mode (FW set to RS-232 but HW red-switch set to RS-485). For RS-485 2-wire you'd need an external RS-232/485 converter. This delay prevents this second request from hitting the 2nd slave too fast, but will delay every request by a fixed 50 or X msec.

Anyway, it would be worth your time to try. If this isn't the problem, you'll know. If it is the problem, then you can deal with trying to eliminate the external converter and reducing this fixed delay ;^) But this is a known issue in Modbus bridge products, so check it out.

Best regards

Lynn August Linse, [email protected]
IA Firmware Specialist, Digi Int'l (www.digi.com)