Instructions

UM-0085-B09 DT80 Range User Manual Page 336
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DO4 – Driving a Relay Using 5D-8D
The active-drive digital outputs 5D-8D (4D for DT81/ 82E) cannot directly drive loads such as relays.
However, an external transistor can be used to increase the current sink capacity so that a relay (or LED) can be
controlled by these outputs.
Figure 160: Wiring for driving an external relay using an external transistor
Action
Use the command
energise relay
1DS0=1
de-energise relay
1DSO=0
Note: the sense is opposite to that shown in DO1 – Driving a Relay (P335) – the DT80 output needs to be driven high in order to
turn on the NPN transistor shown.
DO5 – Latching Relay Output
The DT80’s latching relay output can be used to directly switch loads of up to 1A @ 30V, as shown below.
Figure 161: Wiring for controlling a load using relay output
Action
Use the command
turn load on
1RELAY=1
turn load off
1RELAY=0
Other Considerations
 Reading Digital Outputs
If you read the value of a digital output channel, e.g. by entering 1DSO, then the value returned is the state to which the
output was last set. This will not necessarily be the same as the state returned by
1DS, which reflects the actual state on
the 1D terminal.
If an open-drain output (1D-4D for DT80) is set high (e.g.
1DSO=1), the terminal is not driven by the DT80 and is free to
be pulled low by an external device. If this occurs then
1DSO will still return 1, but 1DS will return 0, the actual state of
the input.
For the DT80's active-drive outputs (5D-8D for DT80), things are slightly more complicated due to the fact that the output
has three states: driving high, driving low, or disabled (tri-stated). The rule is that the output driver is switched on when
the digital output channel is set (e.g.
5DSO=0 or 5DSO=1) and it then stays on. If the digital input channel is
subsequently read (e.g.
5DS), then the driver will be switched off (and it will stay off) to allow the terminal to be read as
an input.
The active-drive outputs include a weak (200k) pull-down resistor. When the output driver is switched off, and in the
absence of any external device driving the terminal, it will therefore normally read low (i.e. 5DS will return 0). Note,
however, that the first time it is read after the output driver is switched off it may still read high (if the output had
previously been set high), due to capacitive effects.