Motherboard Theory Of Operation - NorthStar HORIZON Manual

MOTHERBOARD THEORY OF OPERATION
MOTHI~RBOARD
HARDWARE
1. Power Regulators: Three-terminal regulators, types 7805,
7812K, 78L12, and 79L12 supply regulated +5V, +12V, +12V,
and -12V, respectively to the motherboard electronics and
the floppy disk drive electronics. Tantalum and disk
ceramic bypass capacitors are used to suppress oscillations
in the regulator outputs and to filter noise on the power
lines.
2. 5-100 Bus Interface: Octal Bus Drivers/Receivers, at
locations 6A and 7A, are used to buffer and drive the 5-100
bus Data Output and Data Input buses on the 5-100 bus to and
from the motherboard internal bidirectional data bus (U0-
U7) .
S-100 bus address lines A3-A7 are matched with the code
specified on the port select header (5B) to select the
motherboard during Z80 IN and OUT instructions. Address
lines A0-A2 are decoded (9D) to select one of the eight
motherboard ports. See the Motherboard I/O Port Addressing
section for details.
Bus Control signals SINP, SOUT, SWO/, and PDBIN are used to
direct data flow to and from the internal bus, peripheral
interfaces, and the S-100 backplane lines.
The power-on-clear (POC) signal is used to reset all logic
on the motherboard to a known state when the computer is
reset or when power is turned on.
3. Serial Interfaces: The HORIZON motherboard includes two
serial I/O interfaces. One comes standard with the HORIZON
and the Second Serial Interface is provided as an option.
The are both identical in capabilities.
Eu~L
serial interface is based on the 8251 integrated
circuit Universal Synchronous/Asynchronous
Receiver/Transmitter (U5ART). The primary function of the
USART is to perform serial/parallel conversion of data
between the computer and the I/O interface. In addition the
U5ART can perform a wide variety of control function under
program control from the processor. In particular, the
USART can perform standard asynchronous and synchronous
communication, generating and detecting start bits, stop
bits, parity bits, sync bytes, and interface control
signals. For details of using the U5ART, consult an 8251
data sheet.
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North Star HORIZON Computer