Measurement-computing PCM-D24/CTR3 rev.3 Manuel d'utilisateur télécharger pdf (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.

A0A1A2A3A4A5A6A7

01234567

BASE + 1 82C55 Port B