Applied Technology and Microbee adverts. - The MESSUI Place

A program incorporated in the Basic Interpreter
determines which pixels of the
character cell need to be illuminated and
where the character must be placed on
the screen to create the display required.
The same program keeps track of how
many of the 128 programmable
characters have been used and which of
these can be over-written as more of the
screen is filled.
As long as displays are confined to vertical
and horizontal lines this method
works well. Diagonal lines exhaust
memory space more quickly, and circular
patterns or complex, full screen
graphics are not possible.
Internal details
Opening up the MicroBee reveals two
circuit boards, the upper containing a
mixture of RAM and ROM and the lower
carrying the Z80 microprocessor, video
display circuitry, cassette interface, tone
generator, keyboard and interface
connectors.
A standard Microbee has 16K of programmable
memory, with room for onboard
expansion to 32K. The other half
of the memory board has space for up to
28K of ROM. MicroWorld Basic occupies
16K, with the remaining space available
for utilities such as an editor/assembler,
network communications program or a
word processing package. Our review
machine contained the word processor
(of which more later).
A feature of the Microbee is its use of
CMOS memory which draws very little
current. A 4.5V camera battery provides
sufficient power to allow the memory to
be maintained on standby while the
mains supply is off. The program in
memory is retained, whether the user
wants it or not.
An RS-232C interface is provided for
connection of a printer or other serial
device. Two other openings at the rear
of the case provide access to the circuit
board, which is pre-drilled to allow addition
of an expansion connector for the
Z80 bus and parallel interface circuitry. A
parallel port can be added by installing a
25-pin D-type connector and a single integrated
circuit.
One of the reasons why the Microbee
is so compact is that it has an external
power supply in the form of a 12VAC
plugpack. This feeds rectifiers, filters and
three 5V regulators in the Microbee. As
such, this arrangement could be improved
upon. For a start, the plugpack
voltage of 12VAC means that a relatively
high DC voltage is fed to the regulators,
lading to higher than necessary power
dissipation in the case.
High resolution (512 x 256) graphics displays can be over-printed with text as shown
here. The incomplete design results when all programmable characters have been
used.
And it would seem that when the
Microbee is fully optioned up, the power
supply is not quite up to the task. While
some owners have gone to the trouble
of building a more rugged external supply
it would seem that these problems
could be overcome by providing a
plugpack with a lower output voltage
and a higher current rating, ie, with the
same overall power rating.
Power is supplied to the Microbee
from the 12VAC plugpack with a 5-pin
DIN connector. The same connector is
also used for the cassette interface connections
and direct video output. There
is no RF modulator, so attempting to
connect the computer to a standard
television set is doomed to failure.
The manual accompanying the
machine makes no mention of the need
MicroBee specifications
for a direct entry video monitor. In the
same fashion it is not until page 124 of
the 144 page booklet that instructions
are given for connecting a cassette
recorder — and even then the manual
refers to a red plug which is actually
grey. Considering that a demonstration
tape intended for classroom use was included
. with the computer, this is a
serious failing.
All in all, the manuals supplied with the
Microbee suffer from the common failing
of those written by someone too
close to the subject. Features not commonly
used are treated in loving detail
while vital information for the beginner
is either glossed over or missing entirely.
At the time of this review an extensive
re-writing of the manuals was underway,
so these comments may not be ap-
Processor: Z80
RAM: 16K, expandable on-board to 32K, with battery back-up
ROM: 16K, expandable on-board to 28K
Interfaces: RS232C serial port, optional parallel port and S-100 expansion.
Keyboard: 60 keys including spacebar, typewriter style
Display: 64 x 16 upper and lower case, underline and inverse video modes
Graphics: High resolution, 512 x 256 using programmable characters Low
resolution 128 x 48
128 programmable characters, 8 x 16 pixel matrix
Sound: 25 tones with duration in increments of 143 second.
Expansion: 64K memory version, S-100 interface, disk drives
Editor/Assembler, word processor and network communications software is
available in ROM
Documentation: Incomplete at time of writing.
ELECTRONICS Australia, February, 1983 95