Honeywell DCP 50 Product Manual page 41

This function will be used largely to report controller status information, and so a bit
set to 1 indicates that the corresponding feature is currently enabled/active, and a
bit reset to 0 indicates the opposite.
If an exact multiple of eight bits is not requested, the data is padded with trailing
zeros to preserve the 8-bit format. After the data has been transmitted, the CRC16
value is sent.
B. Read Holding Registers (Read n Words)
The message sent to the controller to obtain the value of one or more registers is an
eight-byte message as follows:
The reply sent by the controller echoes the first 2 characters received and then
contains a single-byte data byte count, the value of which does not include either
itself or the CRC value to be sent. For this message, the count equals the number of
parameters read times two. Following the byte count, that number of parameter
values are transmitted, MSB first, followed by the CRC16.
C. Force Single Coil (Write 1 Bit)
The message received by the controller is 8 bytes long, consisting of the standard
preamble and the address of the bit to force, followed by a two-byte word whose
MSB contains the desired truth value of the bit expressed as 0xFF (TRUE) or 0x00
(FALSE).
Generally, this function will be used to control such features as Auto/Manual and
Tuning. The normal reply sent by the controller will be a byte-for-byte echo of the
message received.
03/04
B
05
5-3