Measurement Computing CIO-DO24DD - Manual (Page 10)

4.2 CABLING TO THE CONNECTOR
The CIO-DO24DD connector is accessible through the PC/AT expansion bracket.
The connector is a standard 50-pin header connector. A cable with mating connectors
(C50FF-##) can be purchased from Measurement Computing Corp. The second
output connector for the CIO-DO48DD is on the end of the card opposite the
expansion bracket.
4.3 CONNECTOR PIN ASSIGNMENTS
The connector is arranged so that there is generally a ground return for each control
line, and each 8 lines of a port are together on the cable.
The signal P ull Up# is the inp ut to the pull-up resistor SIP associated with a p ort.
Applying a voltage to that pin will pull up all lines of a port to that voltage through the
user-installed SIP resistor when the circuit is open.
The signal Port n Clamp is the clam ping voltage input for that port. For example, if
the circuit you are switching at port A is a 24 volt circuit but is subject to 100V
spikes, connect 24V to the Port A Clamp input and any signal over 24V will be
clamped to 24V.
4.4 THE OUTPUT CIRCUIT
The heart of the CIO-DO24DD is a Darlington pair of transistors. It is composed of a
ULN2803 and a few passive components. It works much the same as a solid state
relay. A simplified schematic of the circuit is shown in figure 4-2.
The load device is wired to the Output Pin so that your device is energized ON when a
path to ground is completed. If desired, a clamping voltage can b e supplied to the
Port N Clamp input.
The circuit is switched by the PC setting a register bit. When a 1 is written to a bit in
the register for a particular port, the Darlington transistors associated with that bit are
turned on and your circuit is completed via a path from the output pin to PC Ground.
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