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Chapter 4
Functional Details
Signal connections
All digital inputs and outputs on the PCI-QUAD-AC5 connector are TTL and are fully compatible with
the inputs and outputs on the industry-standard PB-24 rack (Measurement Computing Corporation's
model SSR-PB24).
82C55 emulation
The PCI-QUAD-AC5 board emulates the 82C55 chip. The 82C55 emulation initializes all ports as inputs
on power-up and reset. A TTL input is a high impedance input. If you connect another TTL input device
to the output, it could be turned on or off every time the board is reset.
To establish a consistent TTL level at power-up, use resistors tied to either +5V (pull-up) or ground
(pull-down). There are open locations for pull-up and pull-down resistor SIPs on the board.
When an 82C55 emulation is powered on or reset, all pins are set to high-impedance input. Based on
standard TTL functionality, these inputs will typically float high, and may have enough drive current to
turn on external devices.
Consequently, 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 2.2 K ohm resistor.
Pull-up and pull-down resistors
TTL inputs typically float high, but not reliably. The direction they float is dependent on the
characteristics of the circuit and is unpredictable. If devices such as solid state relays are driven by digital
I/O pins, they can be switched on whenever the computer is powered on or is reset. To prevent unwanted
switching at power-on or reset, force all digital I/O pins to a known state by pulling all pins either high or
low through a 2.2 K ohm resistor tied to either 5V or GND.
The pull-up resistor pulls the input to a high state (+5V) when the board is in input mode, as it would be
on power-up or reset. A 2.2 K ohm resistor draws only 2 mA. A grounded 2.2 K ohm pull-down resistor
pulls the I/O line low when the board is in input mode.
The SIP is made up of eight 2.2 K ohm resistors. One side of each resistor is connected to a single
common point and brought out to a pin. The common line is marked with a dot or line at one end of the
SIP. The remaining resistor ends are brought out to the other eight pins (refer to ). Figure 4-3
Figure 4-3. Eight-Resistor SIP Schematic
2.2 KOhm SIP
Dot
(LO or HI)
I/O Lines
4-1