Zenith Z-100 Series, Technical Manual

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Page 2.85
CIRCUIT DESCRIPTION
Serial Port B
Serial port B consists of U242 and its surrounding circuitry.
This port is a DTE port and can be used to connect to devices
such as a MODEM or to another computer.
To select this IC, the CPU addresses the following ports:
OECH Rec e i ver holding register (read).
Transmitter holding register (write).
OEDH Sta t u s registers (read).
SYN1/SYN2/DLE registers (write).
OEEH Mod e r egisters (read/write).
OEFH Comm a nd registers (read/write).
The differences between this port and serial port A are minor.
To chip-select this IC, the CPU asserts EPCIBCS instead of
EPCIACS, the EPCI interrupts the Computer through EPCIB-
INT instead of EPCIAINT, and pins 9 and 25 of this port are
clock inputs instead of outputs. This last feature is taken care
of when the CPU initially programs the EPCI. The frequency
can be 1, 16, or 64 times the serial baud rate.
In the asynchronous mode, pins 9 and 25 act as outputs.
Under these conditions, D103 and D104 isolate these pins
from U247A and U247B.