ZiLOG Z80 Handbook

combination of the many kinds of external memory available today.
RAM (random access memory) is semiconductor memory that can
be both read and written into. ROM (read
only
memory) is a pro-
duction-type memory that contains a program or data or both which
can be read but not altered. PROM (programmable read only
memory) may be
programmed
in the field with inexpensive equip-
ment, but may not be altered once programmed. EPROM (erasable
programmable read
only
memory) may be programmed for a read
only operation, but may be periodically erased under ultraviolet
light. Many wags have suggested another type, a WOM or write
only
memory, but in most cases the former memory types are com-
monly used.
The Z-80 microprocessor and associated CPU circuitry interface
to I/O
device controllers
along with external memory. I/O device
controllers perform several functions. Firstly, the I/.O device con-
trollers
buffer
data passing between the Z-80 CPU registers or ex-
ternal memory and the I/O device. The buffering matches the high-
speed data-transfer rate of the Z-80 CPU to the relatively low-speed
rate of the I/O device. It is important for the CPU not to have to
wait until the I/O device accepts data, as the wait time may repre-
sent tens of thousands of Z-80 instructions. A Teletype Corporation
ASR-33 Teletype, for example, accepts data at the rate of 10 bytes
per second. While waiting for the Teletype to accept a byte of data,
the Z-80 microprocessor could be executing 1/10 second worth of
instructions or about 30,000 instructions. The Teletype controller
allows the Z-80 to pass a byte in several microseconds and signals
the Z-80 when the Teletype is done processing the data from the
Teletype device controller.
Another function performed by the I/O device controller is for-
matting of the data. A floppy disc transmits data as
a serial bit
stream. The floppy disc controller, among other functions, converts
the serial bit stream into 8-bit
parallel
bytes in proper format for
transmission to the Z-80 CPU over the data bus.
A third function of the I/O device controller is that of
level con-
version.
Data from CPU logic is in TTL (or Transistor-Transistor
Logic) signal levels, which are nominally 0 volts and 5 volts. A Se-
lectric I/O typewriter may require 24 to 48 volts to drive the sole-
noids of the Teletypes and obviously some voltage level conversion
is required.
Other functions of the I/O device controller are timing, synchro-
nization, control-signal
handshaking,
and transmission of device
status. A wide range of I/O devices interface to the Z-80 through
their respective device controllers, ranging from 5 character-per-
second Teletype equipment, audio cassette equipment, analog-to-
digital converters, and 100,000 byte-per-second graphic display
24

ZiLOG Z80 Specifications

General

Manufacturer	ZiLOG
Introduction Year	1976
Clock Speed	2.5 MHz to 20 MHz
Data Width	8-bit
Address Width	16-bit
Instruction Set	Z80
Transistor Count	8, 500
Package	40-pin DIP
Category	Microprocessor
Memory Address Space	64 KB
Voltage Supply	5V