ADLINK PCI-9524 - Manual (Page 54)

38 Signal Connections
PCI-9524 ADLINK Technology Inc.
User’s Manual Copyright 2008
3.3 Isolated Digital Signal Connection
PCI-9524 provides three opto-isolated encoder input channels,
three opto-isolated pulse-command outputs, eight channel opto-
isolated digital inputs as well as eight channel isolated digital out-
puts. Also, a built-in isolated power supply can be used as a resis-
tors pull-up source.
3.3.1 Signal Sources and Terminal Devices
Open-collector and open-drain outputs
Open-collector or open-drain output stages are commonly used in
industrial I/O. Open-collector/open-drain output stages made of
NPN or N-MOS type transistors are for sink-type drivers, while
those made of PNP or P-MOS type are for source-type drivers. A
sink-driver sinks current from the external pull-up resistor when it
is activated, and floats when it is inactivated; conversely, a source-
driver sources current to the external pull-down resistor when it is
activated, and floats when it is inactivated. High-side voltage can
usually go as high as the output transistor can tolerate, and hence
offers a wider, more versatile output voltage selection. The draw-
back is that when the output stage is inactivated, either the signal
fall-time of a source-driver or the rise-time of a sink-driver, is deter-
mined by the RC time-constant formed by the pull-up/pull-down
resistor and the stray capacitance. The asymmetrical rise/fall-time
somehow limits the frequency response of the output stage.
Push-pull outputs
Push-pull output stages are comprised of a complementary tran-
sistor pair, say, a PNP plus a PNP, or a P-MOS plus an N-MOS.
Unlike open-collector output stages, they can sink or source cur-
rent and hence a symmetrical rise/fall-time that is independent of
the external load resistance. Push-pull output stages can generally
toggle at a much faster speed than open-collector output stages.