Applies To:
  • CitectSCADA 5.20

Modbus is a commonly used defacto industry standard.  It was primarily designed to operate over serial links but is commonly being requested for Ethernet (TCP/IP) links. This KB article answers some frequently asked questions. 

Q. Does Citect support Modbus over TCP/IP.

A. Yes. There are actually two drivers that are relevant in this regard, the MODBUS and MODNET drivers. The MODNET driver is based on the "Open Modbus / TCP Specification". This specification can be found on Modicon's web site as openmbus.doc. The MODBUS driver is the standard Modbus serial protocol, simply transported over TCP/IP.

Q. Why would I use the standard MODBUS driver over TCPIP?

A. There are several possible reasons including:

a) You have a serial port server that acts as a remote serial port. To access these servers you either setup a COM port redirector (not applicable to this conversation) using windows drivers or send modbus packets over Ethernet using TCP/IP.

b) You have a device that, say, supported the serial modbus driver and have upgraded it without respect to the "Open Modbus / TCP Specification".

For each of these reasons, you need to define TCPIP as your driver in the Citect's board form rather than COMX and also define the TCP/IP address and port in the Citect's port form. See also KB article Q2633 & Q1982.

Q. Why would I use the MODNET driver?

A. The MODNET driver has been designed specifically for use over TCPIP and is based on the "Open Modbus / TCP specification". It is documented in Citect V5.21, or in Citect V5.20 after installing Citect V5.20 Driver Pack B. See also KB article Q2605.