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dCS 904 User Manual Manual for Software Version 1.5x and 1.36
dCS Ltd June 2000
Manual part no: DOC135904 iss 2B2
Page 53
135904ma2b2.pdf file available from website
Contact
dCS
on + 44 1799 531 999 email to: more@dcsltd.co.uk
(inside the UK replace + 44 with 0) web site: www.dcsltd.co.uk
RS-232 Remote Control Interface
Overall Description -
dCS 9xx units can be controlled using a simple serial protocol, via the RS-232
ports, using the control format described below. All commands available from
the front panel (and a few others, dCS use only) of a unit can be remotely
controlled using this approach. Each unit must have a unique ID (in the range 0
to 99) which must be set up by hand using the menu system on the front panel.
The units remember their ID when powered down, so this setting up only has to
be done once.
Physical Interface -
Units are all connected in a RS-232 daisy chain, up to a maximum of 11 units,
with a serial cable (DB9 pin female straight cable) connected between the Serial
Out and Serial In ports of the units. The same type of cable is used to connect
the Serial In port of the first unit on the chain to the COM port of a PC.
By default all units are configured to operate at 1200 baud. Standard RS-232
signal levels are used. Bytes are transmitted with 1 start bit, 8 message bits, 1
stop bit and no parity.
Units can be switched to 1200, 2400, 4800 or 9600 baud. An RS-232 break will
reset all units on the daisy chain to 1200 baud. A special command and ID is
used to configure the units to other baud rates (see “Special Commands and
Protocols” below). The following rates are recommended:
3 or less units 4800 baud
4 to 7 units 2400 baud
8 to 11 units 1200 baud
Operation of the daisy chain at higher than the recommended rates may result
in incorrect behaviour of the system – either because the units misinterpret
commands, or, more likely, because the controlling computer misinterprets their
replies. Units will revert back to 1200 baud if they are switched off and on - they
do not remember what they were last set to.
9600 baud is currently not fully tested over all temperatures. It can be used for
single units operating in a benign environment.
Timing Accuracy and Warnings -
The units use clock dividers derived from their crystals to produce the RS-232
signals. The frequency of operation is measured to be better than 2% for all
baud rates with both crystals. Some of the commands, however, switch clock
frequencies, and these may be controlled by phase locked loops with long time
constants. While this is happening, correct RS-232 timing cannot be
guaranteed, and the units should not be addressed – a period of 30 secs should
be allowed after switching clock frequencies for timings to stabilise.
Units acknowledge and repeat back their actions on receipt of a command. The
acknowledge should be waited for and checked before proceeding to the next
command – see Acknowledge Message below