Applies To:
  • CitectSCADA 5.x - 7.0


Citect Modnet (modbus TCPIP) driver communicating to modbus serial devices via QUCM gateway. Occasionally Citect recieves Modbus exception 0x01 "Illegal function code" (0x2001 in Citect syslog.dat)

Analysis of packet captures shows that citect is sending a correct request. The same request had been accepted by the device on other occasions.


The Ethernet switch used to connect the Citect system to the QUCM module had been configured to force the port which connected to the QUCM to 10Mbps full duplex.

Niobrara technical support has advised that the QUCM Ethernet port is 10Mbps half duplex only.

This intermittently caused the QUCM to miss receipt of a packet which caused the QUCM to send the Modbus Exception "Illegal Function"
packet to Citect

Fixing the switch port to half duplex resolved the issue. Auto negotiate may also have been appropriate.