Measurement Computing CIO-INT-32 - Manual (Page 15)

5.0 ELECTRONICS AND INTERFACING
This short, simple introduction to the electronics most often needed by digital I/O
board users covers a few key concepts. They are:
Pull up/down resistors
Voltage dividers.
Low pass filters for digital inputs.
IMPORTANT NOTE
WHEN THE Z8536 IS POWERED ON OR RESET, ALL PINS
ARE SET TO HIGH IMPEDANCE INPUT.
The implications of this is that if you have output devices such as solid state relays,
they may be switched on whenever the computer is powered on or reset. To prevent
unwanted switching and to drive all outputs to a known state after power on or reset,
pull all pins either high or low through a 10K resistor.
To install pull up/down resistor packs, see the following section.
5.1 PULL UP & PULL DOWN RESISTORS
This discussion deals with pull-up/pull-down resistors and Z8536 digital I/O chips on
CIO-INT32 boards.
Whenever the Z8536 is powered on or reset, the control register is set to a known
state.
When used as an output device to control other TTL input devices, the Z8536 applies
a voltage level of 0V for low and 2.5V to 5V for high. It is the output voltage level of
the Z8536 that the device being controlled responds to.
The concept of voltage level of an Z8536 in input mode is meaningless. Do not
connect a volt meter to the floating input of an Z8536. It will show you nothing of
meaning. In input mode the Z8536 is in 'high Z' or high impedance. If your Z8536
was connected to another input chip (the device you were controlling), the inputs of
that chip are left floating whenever the Z8536 is in input mode.
If the inputs of the device you are controlling are left to float, they may float up or
down. Which way they float is dependent on the characteristics of the circuit and the
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