Message Formats - Honeywell DCP 50 Product Manual

5.3

MESSAGE FORMATS

The first character of every message is an instrument address. The valid range of
such an address is 0 to 255. The second character is always the Function Number.
The contents of the remainder of the message depends upon the function number.
In most cases the instrument is required to reply by echoing the address and function
number, together with an echo of all or part of the message received (in the case of
a request to write a value or carry out a command) or the information requested (in
the case of a read parameter operation). Broadcast Messages (to which the
controller responds by taking some action without sending back a reply) are
supported at instrument address zero. Commands which can be broadcast are
marked with the symbol .
B
Data is transmitted as eight-bit binary bytes with 1 start bit, 1 stop bit and optional
parity checking (None, Even or Odd). A message is terminated solely by a delay of
more than three character lengths at the given Baud Rate, and any character
received after such a delay is treated as a potential address at the start of a new
message.
The following individual message formats apply. Since only the RTU form of the
protocol is being supported, each message is followed by a two-byte CRC16.
Details of how the checksum must be calculated are given at the end of this section.
A. Read Coil Status (Read n Bits) 01/02
The message sent to the controller will consist of 8 bytes, as follows:
The normal reply will echo the first two characters of the message received, and will
then contain a single-byte data byte count, which will not include itself or the CRC.
For this message, there will be one byte of data per eight bits-worth of information
requested, with the LSbit of the first data byte transmitted depicting the state of the
lowest-numbered bit required.
5-2