Modicon multidropping with fix32 as MMI ??


Thread Starter

Avinash Mali

I am using Modbus Ethernet to Modbus protocol converter of Modicon (174 cev 300 10). My four PLCs are multidropped on RS485 and connected to RS485 port of bridge. Modbus ethernet port is connected to PC having Fix32 as HMI. I am using MBE 7.16 driver which supports Unit_Id field. But at a time i can get data from only one PLC provided other is off. Does anybody this ?? How to get data from all four PLCs simultaniously using this bridge and fix32 as HMI ??
I'll start with the basics. If you can talk to any one PLC alone I presume your wiring and serial port settings are correct. Do your PLCs all have different Modbus addresses? Is your bridge configured for Message Address Routing (see manual 890USE15500 section 1.2.2, Mapping for a Modbus Slave)?

I have a similar setup working with both iFix and Concept. It uses RS232 from the bridge to the first PLC, which then acts as a bridge onto Modbus Plus to the second PLC.

Robert Willis

What type of RS485 Network are you using (2 Wire or 4 Wire)? It also would be helpful if you would include the manufacture of your RS232 to RS485 converter that you are using. Who is the manufactiure of the PLC? You should be able to communicate to all four devices without the need to turnthem off. The fact that you can only get one device to function correctly provided the other is off potentially indicates that this is a RS485 Network issue.

Robert Willis
Square D Company
Have you made sure each PLC has a different slave ID number on the MODBUS network in question?
Meir Saggie

Lynn August Linse

(This is a duplicate response to Avinash Mali's "Re: COMM: Communicating via Schneider 174 CEV 300 10" question [See <> - Moderator], just in case people don't see both threads.)

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.

[I can add in this question's context that I'm pretty sure the HMI will be sending 4 requests at once in a "pipeline" mode even though this kind of violates the spec. MB/TCP OPC/DDE servers have done this from day #1 because it works MOST of the time and has a large improvement in performance. This means the CEV can send out a new request immediately/shortly/very fast after receipt of the last byte of the provious response as there are at least 3 waiting in "queue"]

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 (