Measurement Computing PCM-D24/CTR3 rev.3 - Manual (Page 17)

NOTE ON 82C55 ** PLEASE READ **
The 82C55 when powered up or reset, defaults all data lines to the
input mode. This presents a high impedence. With some TTL input
chips, the 82C55 high impedence input can cause the input to float
high and turn the device on. If you are using the PCM-D24/CTR3
to control a digital device and it is critical that it remain off (inputs
at ground) on power up or reset, please attach 2.2K Ohm pull down
resistors to each output line.
Remember, if you have the 82C55 interfaced to an input chip, when the computer is
powered up you have an input connected to an input. This is an undefined connection.
BASE + 0 82C55 Port A
The direction of this port is set by writing a control word to BASE + 3. The port is
byte wide only and must be set as 8 in or 8 out. Data is written to and read from this
port in bytes.
If programmed for output, a write to this register will update the outputs. A 0 sets the
output to TTL Low. A 1 sets the output to TTL High. NOTE: TTL High is not nec-
essarily 5V. The current high/low bit settings may be read back from the port by read-
ing it.
If programmed for input, a read will capture the current state of all 8 input lines (8
bits) of the port. Writing to an input port has no function.
A0 A1 A2 A3 A4 A5 A6 A7
0 1 2 3 4 5 6 7
BASE + 1 82C55 Port B
The direction of this port is set by writing a control word to BASE + 3. The port is
byte wide only and must be set as 8 in or 8 out. Data is written to and read from this
port in bytes.
If programmed for output, a write to this register will update the outputs. A 0 sets the
output to TTL Low. A 1 sets the output to TTL High. NOTE: TTL High is not nec-
essarily 5V. The current high/low bit settings may be read back from the port by read-
ing it.
If programmed for input, a read will capture the current state of all 8 input lines (8
bits) of the port. Writing to an input port has no function.
B0 B1 B2 B3 B4 B5 B6 B7
0 1 2 3 4 5 6 7
14