Applies To:
  • CitectSCADA 5.xx

How do I communicate to an MTL8000 system via Modbus? 

In an MTL8000 system, you can use analog and digital modules to connect (intrinsic safe) I/O. Via a BIM (Bus Interface Module), you can connect a Modbus (RS485) link. I used a RS485/RS232 converter to connect to the COM port of my computer.

The Modbus mapping is a follows (using four modules with an MTL8000 system):

Digital Input:

Slot 1     Coils 1-8
Slot 2     Coils 9-16
Slot 3     Coils 17-24
Slot 4     Coils 25-32

Digital Output:

Slot 1     Coils 513-520
Slot 2     Coils 521-528
Slot 3     Coils 529-536
Slot 4     Coils 537-544

Analog Input:

Slot 1     Registers 1-8
Slot 2     Registers 9-16
Slot 1     Registers 17-24
Slot 2     Registers 25-32

Analog Output:

Slot 1     Registers 1-8
Slot 2     Registers 9-16
Slot 3     Registers 17-24
Slot 4     Registers 25-32

If you have more modules in, for example, slots 5-8, those modules can be addressed in a similar way by incrementing the above addresses. In my case, the first module was a DI, the second a DO, the third an AI and the last an AO.

So I had the following addresses:

Analog Inputs 1-8 : 30017 - 30024 (in Citect use UINT type)
Analog Outputs 1-8 : 40025 - 40032 (in Citect use UINT type)
Digital Inputs 1-8 : 10513 - 10520
Digital Outputs 1-8 : 00009 - 00016

In Citect, a standard Modbus connection was configured via the Express Wizard, with the following settings:

Boards form:

Type: Comx

Ports form:

Baud Rate: 9600
Data Bits: 8
Stop Bits: 1
Parity: EVEN

I/O Devices form:

Address: Modbus address as configured in the BIM module of the MTL8000 system.
Protocol: Modbus.

The parameter [Modbus]InitType had to be set to 1.