Measurement Computing PC104-AC5 - Manual (Page 12)

3.2.1 Output Ports
Ports configured for output hold the output data written to them. This output byte can
be read back by reading a port configured for output.
IMPORTANT NOTE
THIS BOARD EMULATES THE 82C55. WHENEVER A
82C55 IS POWERED ON OR RESET, ALL PINS ARE SET
TO HIGH IMPEDANCE INPUT. SO DOES OUR
EMULATION.
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 application note.
3.2.2 Input Ports
In 82C55 mode 0 configuration, ports configured for input read the state of the input
lines at the moment the read is executed, transitions are not latched.
3.3 CONTROL REGISTER
Configure A, B & C
Base Address +3
Group B Group A
CL B N/A CU A N/A N/A N/A
0 1 2 3 4 5 6 7
The Ports A, B, C High and C Low may be independently programmed for input or
output. The most commonly used mode for an 82C55 is mode 0, input / output mode.
This is the only mode supported by the PC104-AC5. The codes for programming the
emulated 82C55 in this mode are shown in Table 3-2.
NOTE: D7 is always 1 and D6, D5 & D2 are always 0.
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