The Microscan RTU driver is the MODBUS master.
Devices MicroScan is polling are MODBUS slaves.

Modbus Memory Areas

0XXXX     Coils (relays)

Read: Function Code 01
Write: Function Code 05

Multiple Write: Function Code 15 Not supported

1XXXX     Inputs (di inputs - read only)

Read: Function Code 02
Write: Not supported

3XXXX    Input Registers (analogue inputs - read only)

Read: Function Code 04
Write: Not supported

4XXXX    Holding Registers (read/write registers)

Read: Function Code 03
Write: Function Code 06
Multiple Write: Function Code 16 Not supported

General notes:

1. The protocol specifies a starting address for each command - this is zero referenced, but in scada this is always one referenced i.e holding register 40000 does not exist, only 40001. Thus remember when an address is put out in the protocol, its value is one less that the scada address.

2. Block reads are used for reads of sets of modbus registers.
Block writes are not possible.

3. No other modbus commands are supported.

4. The CRC value should always be checked on a command to the remote device.
Microscan driver will ignore messages with incorrect CRC values.

5. The microscan interface does not issue any modbus broadcast commands.

6. Commonly data in both 3XXXX and 4XXXX is the same with both function codes 03 and 04 reading the same data, and code 06 writing to that data.

7. Although separate codes for Coils and DI inputs are provided, this does not mean they have to be used - these memory locations can be setup instead to sit in the 3XXXX and 4XXXX files.

8. The microscan caches reads, so for most efficient use group similarly accessed items together.

9. Integer word data (16 bit) in files 3XXXX and 4XXXX is supported.
The microscan cannot read IEEE values or ascii based strings from these memory areas.

10. Minimum messages needed, for file 4XXXX, function code 03,06.

11. If possible, allow your TX reply time to be adjustable to allow time for slow modbus devices to turn their transmitters off.

12. Use a 2100-IS for modbus commands, and set for modbus operation with S2-7 ON.
Use of 2100-NS not recommended.

13. Modbus message activity can be viewed in MicroScan Driver, View menu, Comms Messages.