Honeywell DCP 50 Product Manual page 42

D. Preset Single Register (Write 1 Word)
The message sent to the controller consists of 8 bytes: the address and function
number as usual, the address of the parameter to be written, and the two-byte value
to which the parameter will be set, and finally the CRC16.
The normal response is to echo the message in its entirety.
E. Loopback Diagnostic Test
The controller is sent an 8 byte message consisting of the usual preamble, a two-byte
diagnostic code, and two bytes of data, followed by the CRC16.
Full MODBUS support in this area is not appropriate - consequently, the only
Diagnostic Code supported is code 00. In response to the message, the controller
must echo the message received exactly.
F. Preset Multiple Registers (Write n Words) 16
This message consists of eleven bytes. Only one parameter may be written at a time,
even though this function number is supported. The preamble is followed by the
address of the parameter to be written, and then a two-byte word count (always set
to 1) and a single-byte byte count (always set to 2). Finally, the value to be written is
followed by the CRC16.
The controller normally responds with a eight-byte reply, as follows:
5-4
B
06
08
B