Applies To:
  • CitectSCADA 5.00
Summary:
The Modbus Slave protocol driver will be released with Citect version 5.00. This driver allows Citect to emulate a MODBUS compatible PLC. This is useful when you want Citect to communicate to a device which normally communicates to a MODBUS compatible PLC. This could typically be another SCADA software package or Distributed Control System. 

Solution:
The MODBUS driver communicates with MODBUS PLC with the master-slave RTU protocol with CITECT acting as the master. The PLC will only respond when it is addressed by CITECT.

The MBSLAVE driver uses the MODBUS RTU protocol, but with the driver acting as the slave. The MBSLAVE will behave like a MODBUS PLC will the following data tables defined.

00001 to 09999 Output Status
10001 to 19999 Input Status
40001 to 49999 Output Register
30001 to 39999 Input Register

CITECT can read and write to all the above data tables. The external device can read from all the above data tables, but can only write to the output tables.

Modbus functions codes 1,2,3,4,5,6,15 and 16 are supported.

[MBSLAVE] Parameters

AlwaysOnline=1
The MBSLAVE I/O device will comes on line immediately and never goes offline.

WatchTime=40
If AlwaysOnline is 0. The MBSLAVE I/O device will only comes online when the external device writes to any of its output table. When starting up, MBSLAVE will wait up to 75% of the WatchTime for a write from the external device to any of its output table. It will then wait a further 25% of WatchTime for the output tables to be updated fully from the external device before putting the I/O device online.

Once the I/O device is online, the external device must perform at least one write to any of output tables every WatchTime, otherwise the I/O device is put offline.

ReqDelay=50
Delay every reply to Citect by 50 milliseconds.

StringReverse=0
If StringReverse is 1 the bytes in each word of a string is swapped.

FloatMode=0
Controls the byte ordering of IEEE floating point numbers

0 is 1, 0, 3, 2
1 is 3, 2, 1, 0
2 is 0, 1, 2, 3
3 is 2, 3, 0, 1

I/O Devices Form:

Address: n
Where n (decimal) is a number between 1 and 247. This I/O device will only act on and respond to requests addressed to n. It will act on but not respond to requests address to 0 (broadcast address).

Protocol: MBSLAVE

Driver Specific Error (decimal):

40 Data is out of date, no data was received within the WatchTime.

Frequently asked questions:

Q. Can the MBSLAVE driver be used over TCP/IP?
A. No. For use over TCP/IP, you would need to use the MNSLAVE driver. Please contact Citect Support for further information.

 
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