Applies To:
  • CitectSCADA 5.xx

How do I best set up the Citect Modnet Driver to work with the NR&D modbus-ethernet bridge MEB TCP module ? The MEB TCP is a gateway for various plc protocols (modbus, SquareD, etc) and TCP/IP. The module is housed in a SquareD SY/MAX rack. When we put the system under any load we can get an array of errors from "channel off line", "unit not responding" to "General error 200b", etc.

Also another issue is when any PLC is physically disconnected on the modbus side then Citect does not put the device "off-line".


1. Citect can overrun the buffers in the NR&D MEB TCP unit by making too many requests.

The MEB has 4 physical ports; one ethernet, one modbus+ and two SY/MAX ports. The Modbus Routing table (accessible to any PC that communicates to it) could have up to 8 separate connections simultaneously running throught the MB+ port.

In a larger system, It is recommended that in Citect we do not have a separate "port" for each I/O device defined (for Modbus+ devices connected via the bridge) but configure all Modnet devices through the one port (where possible) and set the driver parameters as follows;


Depending on the complexity of the system you may have to set "maxpending =1" and increase the "timeout=" parameter also.

2. When any Modbus+ target device is dissconnected or powered off, the MEB TCP would respond to Citect with an undocumented error;

Thu Jul 27 12:01:13 2000 18:53:17.723 Error: General error
READ 0008 p1 d1 400001 2
Generic 000008 Driver 00008221 (0x0000201d)

The MEB TCP now has a firmware revision from 01AUG2000 or later that changes the modbus extended response from 0x1d (29), to 0x0b, this is a standard Modbus response; Gateway target device failed to respond.

The Citect Modnet driver V2.04.01 or later will cope with either error condition as follows;

A new parameter is added to the Citect driver to use for a MEB TCP that has firmware earlier than 01AUG2000;

display: #COM
H/W error: plc server i/o device general error
driver error: unit not responding

display #COM
H/W error: plc server i/o device off line, cannot talk
Driver error: Unit off line.

For the new MEB TCP firmware, do not use the above parameter, Citect will put the unit off line.

Note: To set up the MEB TCP module, check the "Applications" section of the MEB TCP installation manual, there is an example of an ethernet bridge for Modbus Plus. Once you set up the Modbus routing, the "Destination Index" is the number that you would use in the Citect I/O Devices form address field to address the target device.