Technical information

Read This First. OPTO DIO Reference
Which type of chip is fitted on the opto card is determined
by the part number used to order the card. The cards are
supplied with the SOURCE drivers UDN2984 fitted as standard,
unless the user specified SINK at the time of ordering. The card
can be easily changed between sink and source specification by
exchanging certain chips and setting a pair of jumpers.
The difference between these two types of driver, their
output current capabilities, their power on reset condition and
optional inverting configuration is explained below.
Sink And Source Explained._________________________
The output ports of the card are optically isolated from the
rest of the PC so that wiring defects or accidental connections to
high voltage or high current devices do not damage the PC. In
addition the outputs are via high voltage, high current driver
chips. The lines of these driver chips are connected by the user
to a load. As the users programs sets the output port lines high
and low the output driver chips attempt to pull the loads to a
high and low voltage.
When current flows out of the driver output terminal and
into the load, the device is said to "source" current. This is a
source driver. Conversely current flows from a load into a "sink"
driver.
Source drivers can pull outputs both up and down.
When the PC is powered off whilst the external power is ON
the port lines are ON.
Sink drivers can only pull an output down to ground,
they cannot pull an output up. When the PC is powered off
whilst the external power is ON the port lines are ON.
With sink drivers the outputs only go high when the external
circuit pulls them high. eg via a resistor attached to a positive
voltage source.
Sink drivers can provide both source and sink functions. Sink
drivers cannot provide source driver functions. However, they
can cope with significantly higher current loads.
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