Specifications

CHAPTER 3
I/O Features
ADB Port 23
Pin 9 on each serial connector provides +5 V power from the ADB power supply. An
external device should draw no more than 100 mA from that pin. The total current
available for all devices connected to the +5 V supply for the ADB and the serial ports is
500 mA. Excessive current drain will cause a fuse to interrupt the +5 V supply; the fuse
automatically resets when the load returns to normal.
Both serial ports include the GPi (general-purpose input) signal on pin 7. The GPi signal
for each port connects to the corresponding data carrier detect input on the SCC portion
of the O’Hare custom IC, described in Chapter 2. On serial port A (the modem port), the
GPi line can be connected to the receive/transmit clock (RTxCA) signal on the SCC. That
connection supports devices that provide separate transmit and receive data clocks, such
as synchronous modems. For more information about the serial ports, see Guide to the
Macintosh Family Hardware, second edition.
ADB Port 3
The Apple Desktop Bus (ADB) port on the Macintosh Performa 6400 computer is
functionally the same as on other Macintosh computers.
The ADB is a single-master, multiple-slave serial communications bus that uses an
asynchronous protocol and connects keyboards, graphics tablets, mouse devices, and
other devices to the computer. The custom ADB microcontroller drives the bus and reads
status from the selected external device. A 4-pin mini-DIN connector connects the ADB
to the external devices. Table 3-2 lists the ADB connector pin assignments. For more
information about the ADB, see Guide to the Macintosh Family Hardware, second edition.
Note
The total current available for all devices connected to the +5 V pins on
the ADB and the modem port is 500 mA. Each device should use no
more than 100 mA. ◆
Table 3-2 ADB connector pin assignments
Pin number Name Description
1 ADB Bidirectional data bus used for input and output. It is an
open-collector signal pulled up to +5 volts through a 470-ohm
resistor on the main logic board.
2 PSW Power-on signal that generates reset and interrupt key
combinations.
3 +5V +5 volts from the computer.
4 GND Ground from the computer.