2.7.6 Serial Communications Output
The cable used should be suitable for data transfer at the selected rate (1200, 2400, 4800 or
9600) over the required distance. Transmitters/receivers conform to the recommendations in
the EIA Standard RS485.
The “A” terminal on the hygrometer (Terminal 17) should be connected to the “A” terminal
on the master device; the “B” terminal on the hygrometer (Terminal 16) should be connected
to the “B” terminal on the master device and the “Common” terminal on the hygrometer
(Terminal 18) should be connected to the “Common” terminal on the master device.
Where several hygrometers are connected to one master port, the master port transceiver in
the active state should be capable of driving a load of 120kΩ per indicator. The masterport
transceiver in the passive state must have pull-up/pull-down resistors of sufficiently low
impedance to ensure that it remains in the quiescent state whilst supplying up to ±100µA each
to the hygrometer transceivers in the high impedance state.
2.7.7 External Autocal Control
An external autocal potentiometer can be added to control the setting remotely.
Connect a 20kΩ multi-turn potentiometer (20 turn or better) using a twisted pair or screened
cable between pins one and three. Wire the external potentiometer to increase resistance when
turned clockwise (Increasing the resistance will increase the display value).
Note, to use the external control, the internal control on the rear terminal needs to be turned
fully clockwise (see section 4).
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dewTEC DS2000 User’s Manual Page 8
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