Applies To:
  • CitectSCADA 4.xx, 5.xx
Summary:
The X-link unit is a protocol converter which converts between disparate protocols such as DH+ and TiWay. The unit itself is normally a box with two ISA slots; one for each interface (DH+ and TiWay). A serial connector is also provided and thus the unit could be used to convert between, say, DH+ and a serial protocol such as Modbus with a single ISA card.

Some care must be taken when configuring such a device for use with Citect, and it is not simply a matter of treating such a converter as a pseudo PLC. 

Solution:
The mapping between the registers of the two PLC types is performed using an application which runs on a PC and is downloaded to the converter via the CONFIG serial port. A null modem cable between the PC and the X-Link converter is used to transfer this information.

Whilst it is obvious that mappings must be provided to map Citect read and write requests into read and write requests for the foreign PLC, many third party integrators aren't aware that it is necessary to map the register for Citect's on-line check. Thus, the first point to note when using a protocol converter is to determine the register used by Citect for its on-line check and map this to the corresponding register in the foreign PLC.

In addition, the configuration software which comes with the X-link sometimes won't allow this particular register to be mapped (as Citect will often attempt to read a special status register to determine a unit's on-line status). Two cases where such a situation may occur are with the Modbus protocol and the ABTCP protocol.

The Modbus protocol (and its derivatives such as Modbus+) provide a means of specifying an alternate register type to use for the on-line check. This is in the form of an optional "InitType" INI parameter. See KB article Q1146 for a description of this.

The ABTCP protocol provides a similar option in that an alternate (integer) register may be used by specifying it with the "AltInitReg" option in the Citect.ini file. Normally, the S2:0 resister is used to determine whether a unit is on-line.

Note that in the case of ABTCP, the S2:0 status register can be mapped to one of the internal X-link status registers to make the default on-line test work. 
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