Modbus - Computer Busses - page 298

18.2.3 Read holding registers (03)
The function reads the binary contents of holding registers (4 x references) in the slave. Hold-
ing registers are identified starting from 40001, which is addressed as register 0000. Register
40002 is addressed as register 0001, and so on.
For example, to read two values, starting at address 0E2 (register 40226), then the follow-
ing is used:

Address Function Start address (hi) Start address (lo) No. (hi) No. (low) LRC
:12 03 00 E1 00 02 05

The response gives 16 bits for every register value. An example response is,

Address Function Number of bytes Data value (40226) Data values (40227) LRC
:12 03 04 BA A2 FF 10 7c

18.2.4 Read input registers (04)
This function reads the binary contents of input registers (3 x references) in the slave. Input
registers are identified starting from 30001, which is addressed as register 0000. Register
30002 is addressed as register 0001, and so on. The response gives 16 bits for every register
value.

18.2.5 Force single coil (05)
This function forces a single coil (0 x reference) to either an ON or an OFF state. For example,
to force coil at address 101 (65h) to be ON, then the following is used:

Address Fun. Start add. (hi)Start add. (lo)Force data (hi) Force data (low) LRC
:12 05 00 65 FF 00 85

A value of FF00 sets the coil ON, while a value of 0000 sets the coil OFF. The response
should just be the echo of the query.
18.2.6 Preset Single Register (06)
This function presets a value into a single 16-bit holding register (4 x reference). For example
to preset register 40226 (address E1) to 021F then:

Address Function Start address (hi) Start address (lo) No. (hi) No. (low) LRC
:12 06 00 E1 02 1F 05

The response should just be the echo of the query.

18.2.7 Read Exception Status (07)
This function reads the current status of the slave. Normally the settings for the addresses
and the bits within the addresses are normally manufacture defined. For example, for the
Honeywell 2500 series chromatograph the returned status codes are:

Coil Assignment Coil Assignment

1 Shutdown 2 Unknown fail

3 Power fail 4 Unacknowledged alarms

5 Starting 6 Running

7 Warm start 8 Cold start