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There are 90 ceramic and 18 tantalum
capacitors on the board with only 0.1 volt of
noise on the plus five line.
The memory design has Schmitt trigger
devices on all lines: the address lines have
series termination. The result is a quiet
memory. 200 nS memories are required with
the 4MHz clock.
The character generator ROM can be a
custom programmed 2716 or the TRS-80
character generator. The new Radic Shack
ROM with lowercase descenders also works.
For the language ROMs the two or three
chip Radio Shack set or LNW six chip set
will do. For the six chip set the ROMs are
2716s. The configuration is selected by a
DIP switch of jumpers.
The processor speed is slowed to 1.77
MHz when the ROMs are accessed by in-
serting wait states in the Z80. When run-
ning the Basic interpreter, the speed in-
crease over the TRS-80 is around 40 per-
cent (depending on the application). When
running machine language ROM indepen-
dent software the LNW is more than twice
the speed of the TRS-80.
The LNW-80 allows use of a Radio Shack
keyboard or the LNW keyboard. There are
two connectors on the circuit board. U30
and U34 must be removed to use the Radio
Shack keyboard.
Graphics
The foremost feature of the LNW is the on-
board inclusion of high density black
and white and color graphics consisting of
six additional 4116s with support circuitry.
It is controlled by an OUT command to
port 254.
The graphics pixels use the same ad-
dress space as the Basic ROMs, activated
by an OUT to port 254 with bit 3 set. The
command deactivates Basic making a ma-
chine language driver necessary for
graphics. This is fairly well documented in
the manual. High and low resolution
graphics and characters can be displayed
at the same time.
The screen is divided into an inner and ex-
tended region. The inner region is 384 by
192 pixels. The extended region is another
96 pixels horizontally. Addressing for the
two regions differs slightly.
For color graph ics either a color monitor
or a tv with on-board RFI modulator is used.
Outputs for an RGB monitor can be wired in.
There are two color modes. The first gives
you 128 by 192 pixels in eight colors. The
high resolution mode gives 384 by 192 pix-
els. The bandwidth for this mode requires
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an RGB monitor. The color graphics re-
quires a custom programmed 82S23 ROM,
available from LNW. The ROM contents are
detailed in the manual.
Drawbacks
LNW lists a 24 by 80 character video
screen in its advertising. This is a little de-
ceptive. The LNW-80 has the capability of a
24 by 80 screen by utilizing the high density
graphics, but to get it you have to write a
driver program.
LNW's six chip ROM set costs $120. Con-
sidering that 2716s go for about $6 each . I
decided to program my own. But which ad-
dress blocks go in which ROM socket? I
called LNW and was told the information
was proprietary! Some schematic study
and experimenting revealed the truth as can
be seen in Table 1.
The Expansion Board
The expansion board is of the same high
quality as the printed circuit board,
except the gold plating costs extra. (The plating is
worth it.) It has the same good memory de-
sign and Schmitt trigger drivers. There are
43 ceramic and 12 tantalum bypassing
capacitors.
The board also has a few drawbacks.
The printer port works okay if the printer
is only a few feet from the port. You never
want to hook a long line to the output of a
D-type flip-flop. My printer is 20 feet away
and 74LS175 will not drive it. I had to add
buffering.
They cut corners on the RS-232 port. Wire
jumpers select the default RS-232 parameters.
Why not use a DIP switch? The baud rate is
also a wire jumper, and is not software-select-
able. You can change soldered jumpers about
three times before the pads fall off or the
plating pulls out. LNW should have added
the one extra chip to make baud rates se-
lectable. Or they could have laid out the DIP
switch on the circut board.
The RS-232 connector pads are spaced for
a printed circuit mount right angle 25-pin con-
IC Number ROM Name Address Boundaries
(LNW Manual) Decimal Hex
U75 A1 2-4K 0800-0FFF
U76 B1 6-8K 1800-1FFF
U77 C1 10-12K 2800-2FFF
U78 A 0-2K 0000-07FF
U79 B 4-6K 1000-17FF
U80 C 8-10K 2000-27FF
Table 1.
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nector. Do not solder one or, the board. When
the board is in place inside the LNW cabinet
there is no room to hook up to it with a plug.
You also cannot use a straight connector.
Besides. the connector pads are 16 inches
from the nearest opening in the case. Use
25-wire extension cord.
The system expansion connector for out-
side devices is at right angles to the back of
the case. You have to take the cover off to
use it.
The Keyboard
Here is where the good designers at LNW
went out to lunch en masse. The manual
says the keyboard "is designed specifically
for the LNW-80." The one I got was just a
plain unencoded keyboard. A plastic bag
with engraved keytops was included with
instructions to replace certain keytops with
the new ones. One of the new keytops was
engraved sideways!
The key switches are the same wire
finger switches that gave Radio Shack grief
for so long. With this system, if you do not
have a keyboard debounce fix, you will get
keybounce in spades at 4MHz.
The circuit board is one-sided, with a
large number of wire jumpers on top of the
board. The keyboard has to sit on them and
is difficult to seat fear soldering.
Buy your keyboard someplace else.
Radio Shack Model I keyboards are cheaper
and superior.
The LNW case is an attractive low profile
cabinet made of heavy gauge steel with
room inside for both circuit boards, key-
board, power transformer, a muffin fan and
RFI filter. Unfortunately it is overpriced. Build
your own or buy a cabinet from Cramer.
Conclusion
The LNW-80 is, for the most part, a well
designed, fast, effective computer. I use my
LNW-80 with the Percom Doubler II and
LDOS operating system. I have a time-of-
day clock, plotter controller, music board
and eight bit general purpose port attached
to my expansion bus. I designed and install-
ed a piggyback board for selectable baud
rates on the RS-232. All is working well.
Building an LNW-80 is not a project for a
beginner. The manual is informative, but
this is not a Heathkit.•
Mr. Brocaw was born in Oklahoma in 1944.
He holds a BSEE from New Mexico State
University and is now a senior systems an-
alyst for Public Service Co. of New Mexico.
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