West Control Solutions N8080 (for 2xxx) - Manual (Page 12)

3 SERIAL COMMUNICATIONS
The Serial Communications option is a standard RS485 communications link. Up to
32 standard RS485 loads may be presented to a single loop on this link. Each
Indicator presents ¼ standard, therefore up to 128 may be connected to a single
loop (ignoring the load presented by the master device). However, addresses are
restricted to the range 1 to 99.
3.1 DATA FORMAT/BAUD RATE
Data format is fixed at one start bit, seven data bits, 1 parity bit (even parity) and
1 stop bit i.e. a 10-bit data word. Baud rates supported are 1200, 2400, 4800 and
9600. The half-duplez line turn-round time is fixed at 6ms regardless of Baud rate.
The maximum inter-character delay is 120ms. The No Reply timeout is 2 seconds.
Data is expressed as a five-digit signed hexadecimal number in which the
following characters are permitted:
0 1 2 3 4 5 6 7 8 9 A B C D E F
Note that all the non-numeric characters are upper case. The detection of any
characters other than these will be regarded as a syntax error. Where a value
carries a decimal point, the point position is implicit and the responsibility for
interpreting it lies with the user.
3.2 PROTOCOL
The protocol operates on a single master basis only. All communication is initiated
by the master device.
The communications addresses available are in the range 1 - 99. Address 0 is
used for broadcast Parameter Write operation messages. When a message is
broadcast, the receiving instruments will attempt to implement the instruction but
will not reply.
3.3 MESSAGE FORMAT
Each message starts with a Start of Message character (L) and finishes with an End
of Message character (*). A reply from the addressed instrument will contain either
a positive acknowledgement or a negative acknowledgement. A positive
acknowledgement has the character A immediately preceding the End of
Message character; a negative acknowledgement has the character N
immediately preceding the End of Message character.
There are three message formats; they permit instrument identification, Parameter
Read operations and Parameter Write operations.
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