Amateur Radio - Free and Open Source Software

,
Number 8 0n your Feedback c:erd
My SX-64 Runs Digicom!
Low-cost packet solution foryour portable C-64.
by Ted Drude KA9ELV
B
y now, most packet radio enthusiasts are
probably aware of the excellent public
domain software TNC-2 emulator called
D igicom> 64. which runs on Commodore
C-64 computers. It was originally
written by West German hams Stefan Eckart
D L2MDL and Florian Rad le he r r
DLSMBT. (See Barry Kutner W 2U P's
art icle on Digicom in the August 1988 issue
of73.)
Digicom is a great way ofgetting on packet
radio, assuming you have a C-64 computer
or a compatible sys tem (C-64C, C-128,
or C- 128D). Yo u can use several types
of inexpe nsive single chip modems with
Digicom. Most can be built from scratch
or from a kit for under $50. Version 2.0 of
the program has many features not even
found in hardware TNC-2s. including multiconnects.
auto message store and forward.
large text bu ffers. and many BBS-like functions.
While C-64 users have been having a ball
with Digjcom , Commodore SX-64 owners
have been left out in the cold. That is. until
now! If you couldn't fig ure out how to gel
Digicom running on your SX-64 portable,
you can gel the complete story here. including
how to modify Digicom modems to work
wit h the SX-64, and how to ma ke the proper
internal connections.
What Is An SX-64. Any way?
The SX-64, a portable version ofthe C-64,
operates on AC power. It consists of a C·64
compatible CPU, a 1541 disk drive, a five
inch color monitor, and a built-in audio am p
and speaker. All components are housed in a
15" x 16" x 5" metal case with a sturdy
earrying handle. The detachable keyboard
folds up over the screen and disk drive for
easy transportatio n.
The design o f the SX-64, with its ample
intern al power supply and its huge castaluminum
heat sink, makes it more durable
than the C-64. And, unlike most C-64s, you
can leave the SX-64 on for days, usually
without the worry o f overheating or other
problems.
Commodore originally sold the SX-64 for
$995. When price wars drove the home computer
market soft in 19 83- 19 84 , these
machines were discontinued, and ma ny we re
sold through surplus and liquidation chan-
24 73 Amateur Radio • October, 1989
Photo A. This SX-64 (left) is running Digicom>
64 Version 2.10 wuh an internally
mountedpacket modem. External video monitor
(right) displays incoming packets. with
Digicom in monitor mode.
Figure 1. Commodore SX-64 (top view).
1 8 n==
U==
Figure 2.
side).
c::::J
= = 1 "'10
B CI
C
SX-64 CPU Boa rd (component
I
....
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Figu re 3. SX-64l10 Board (component side).
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• I-'
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Figure 4. SX-64 CPU Board power connector
(detail).
nels. However, SX·64 repair parts are still
available, and many SX-64s in good 10 excellent
condition are currently available at
hamfests and computer flea markets fo r
$300-$500.
Why Won't Digicom Run on the SX-64?
The SX-64 is virt ua lly 100% software
compatible with the C·64, bUI the ha rdware is
a different story . Digicom is written 10 add
ress a modem circuit connected to the
C-64's cassette port. That's fine for C-64Cs,
C-128s, or even C-128Ds, all of which have
the nearly obsolete cassette port installed on
them. But Commodore, in its infi nite wisdom,
decided to pu t all C-64 110 po ns except
for the " redundant" cassette po n on the
SX-64's back panel. Almost all the interna l
circuitry needed for the cassette port, however,
it is still present inside the SX-64; it just
isn't brought out to an external pon.
Get the Latest Release
Digicom version 1.42 was originally writlen
to overcome the missing cassette port
problem since it addressed a modem connected
10 the user port. which is present on the
SX-64. However , th is version of Digicom is
much less powerful than versions 2.0 and
later, as it lacks multi-connects, connectlogging
, store and forward , and so on. The actual
release copies of ve rsion 1.42 are buggy
and poorly documented,
I suggest you use the latest version (version
2. 10 as ofthis writing). Besides, many SX-64
owners already use Ihe user pon s for other
thi ngs, such as telephone modems, RS-232
pons, or, as I am doing , for horne-brew Centronics
printer interfaces. The secret is in
knowing how to make the right connections
between a Digicom modem and the SX-64's
internal chips.
Installing the Modem
Beg in by removing the ribbed plastic trim
pa nels on both sides ofthe computer. They're
held in place by four small screws. IWO on
each side, in the middle of the back heal sink
panel. After removing the screws, slide the
panels out toward the rear of the computer.
Next, remove the six countersunk machine
screws, three on each side, that are holding
the top cover in place. After that, remove the
IWOlarge screws at the top comers ofthe back