5.0 Serial Communications
The Serial Communications Port is a standard RS485 communications link. Up to 32 standard
RS485 loads may be present in a single loop on this link.
5.1 Data Format/Baud Rate
Data is format as a 10-bit data word i.e. 1 Start bit
7 Data bits
1 Parity bit (even parity)
and 1 Stop bit
Supported Baud Rates are 1200, 2400, 4800, 9600 bits/second
The half-duplex 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.
5.2 Protocol
The protocol operates on a single master basis. The master device initiates all communication.
The hygrometer acts as a slave device.
5.3 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 two message formats; they permit instrument identification, Parameter Read
operations and Parameter Write operations.
5.3.1 Message Form 1
The master device sends a Form 1 message to ascertain whether an instrument occupies a
specific communications address. If there is an instrument at that address, a reply (with a
positive acknowledgement) is received. If there is no instrument at that address or there is a
communications link failure, no reply is received. The message from the master device must
be of the form:
Laa??*
where aa is the address (a two digit hexadecimal number). The reply from the addressed
instrument is of the form:
Laa?A*
Where aa is the same address as in the received message.
dewTEC DS2000 User’s Manual Page 18
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