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SEC TION 4
RS485 SERIAL COMMUNICATIONS
This three-wire RS485-compatible serial communications option is the means by which
communication may occur between the instrument and a master device (e.g. a
computer or terminal).
4.1 PHYSICAL REQUIREMENTS
There are two communications protocols available with this option:
(a) AS CII
(b) MOD BUS
4.1.1 Char ac ter Trans mis sion
Data format is fixed to be seven data bits and one stop bit. The Baud rate may be
selected to be 1200, 2400, 4800 (default) or 9600 Baud. For ASCII protocol, the parity
is even. For MODBUS protocol, the parity is selectable to be even, odd or none.
4.1.2 Line Turn- Round
AS CII Pro to col: The com mu ni ca tions link is op er ated as a multi- drop half
du plex sys tem. When a de vice is trans mit ting, it drives the trans mis sion lines
to the ap pro pri ate lev els; when it is not trans mit ting, its out puts are set to a
high im ped ance in or der that an other de vice can trans mit. It is im por tant that
a trans mit ter re leases the trans mis sion lines be fore an other de vice starts
trans mis sion. This im poses the fol low ing re straints on the mas ter de vice:
(a) The transmitter must release the transmission lines within 6ms of
the end of the last character of a message being transmitted. Note that
delays due to buffers such as those used in universal asynchronous
receivers/transmitters (UARTs) within the master device must be taken
into account.
(b) The transmitter must not start transmission until 6ms has elapsed
since the reception of the last character of a message.
All in stru ments in this range hav ing an RS485 com mu ni ca tions fa cil ity ad -
here to this stan dard; thus, pro vided that the mas ter de vice con forms simi -
larly to the stan dard, there should be no line con ten tion prob lems.
MOD BUS Pro to col: The line turn- round tim ings ad here to the in dus try stan -
dard.
4.2 ASCII PROTOCOL
This protocol assumes half duplex communications. All communication is initiated by
the master device. The master sends a command or query to the addressed slave and
the slave replies with an acknowledgement of the command or the reply to the query.
All messages, in either direction, comprise:
OM067-4 November, 2000 4-1
59039
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-DIN Digi tal In di ca tor Sec tion 4
Product Man ual - Vol ume I RS485 Se rial Com mu ni ca tions