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Chapter 4
Functional Details
Signal connections
All digital inputs and outputs on the
82C55 emulation
The
To establish a consistent TTL level at
When an 82C55 emulation is powered on or reset, all pins are set to
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
The
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
2.2 KOhm SIP
Dot
(LO or HI)
I/O Lines