Omega CN1166 manual Read Coil Status Read n Bits 01/02, CRC16

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:

		Addr. of
Addr.	Func.	1st Bit		No. of bits		CRC16
	1/2	HI	LO	HI	LO	HI	LO

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.

Addr. Func. Count 1 - 8 9 - 16 17 - 24

1/2

Last	CRC16
	HI	LO

4-2 Volume II O077-4