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CABLES, CONNECTORS, AND OLD TANDY HARDWARE
------------------------------------------
Some of the most frequently asked questions about the Tandy 1000-series,
some later Tandy PC-compatibles, and old Tandy printers pertain to
connectors, cables, and wiring, as they relate to the IBM PC standard.
This article will attempt to answer some of the questions. For the most
part, I won't be dealing with software questions here. I also won't be
dealing with the Tandy 2000, which though it runs MS-DOS is not really IBM-
compatible.
Basic Electronics
-----------------
Understanding connectors and cables is not difficult, but a little preface
may be in order.
A computer is essentially a DC, or "direct circuit," device, like a
flashlight. If you look at the battery in a flashlight, you will see that
one end is labelled "+" and the other end "-". Electrons flow from the "-"
end of the battery, through the wiring in the flashlight and back to the
"+" end. On the way, they pass through the element in the bulb, where they
encounter "resistance," generating heat and light.
The degree to which a wire is "+" or "-" is called its "potential" and is
measured in volts. 0 volts is neutral, called "ground". The voltages used
in an IBM PC-compatible are +5V, +12V, -12V, and 0V. -12V is only used by
the serial port. +12V is used by the mechanical parts of the PC: the
motor that turns the floppy drive, for example, or the fan. All of the
logic circuits of the PC use +5V and 0V; +5V is a "1" bit and 0V is a "0"
bit. There are wires in a PC that are always +5V, +12V, -12V, or 0V, and
there are wires that alternate between +5V and 0V.
Electricity will always flow from a more "-" (or less "+") location to a
more "+" (or less "-") location. If two wires with different voltages are
connected with no resistance between, the result is a short circuit. A
short circuit can destroy your PC by allowing too much electricity to flow
through it at once.
Printers
--------
Old Tandy printers and some Tandy 1000-series printer ports (the card-edge
type) are wired differently than standard IBM PC printer ports, and it is
important to use the right cable to connect your PC to your printer. Just
because it fits doesn't mean it's the right cable! On an old Tandy 1000,
the wrong cable usually just causes the printer not to work. On a newer
clone (particularly a Packard Bell) with a standard printer port, however,
the wrong cable can destroy the computer.
It's really IBM's fault, not Tandy's. Before the IBM PC ever existed,
there was a standard for connecting computers to printers, and older Tandy
printers (and older Tandy computers) were built to that standard. IBM came
along and decided for no good reason to change some of the signals, while
keeping the same connector. Thus, old Tandy printers supply +5V on pin 18
(one of the cable wires), while IBM PC's expect 0V on that pin. If the
computer is not well-built (and Packard Bell's and some other el-cheapo
clones are not), the result is a short circuit that can permanently damage
the machine. There is also a more common problem with pin 32, the
printer's Init line, which causes an old Tandy printer connected to a newer
clone to think that it is continually being reinitialized - so it does not
work.
For those reasons, it is vitally important to use the right cable. Tandy
sells 8 different printer cables, for old and new printers, old and new
computers, and 6-foot and 12-foot lengths. The following table was posted
in the Usenet newsgroup comp.sys.tandy by:
William K. Walker
North Valley Digital
P.O. Box 1941
Kalispell MT 59903-1941
+1 (406) 257-2306
71066.24@compuserve.com
----------------------------------------------------------------------
PRINTER CABLE SELECTION GRID (PARTIAL)
--------------------------------------
COMPUTER TYPE "OLD": Any Tandy 1000 series except 1000RL,
PRINTER "OLD" "NEW" 1000RLX, 1000RSX.
------- ----- ----- "NEW": All other Tandy IBM compatibles,
DMP 100 A C including laptops.
DMP 105 A C
DMP 106 A C Tandy cable part numbers:
DMP 107 A C 6 foot 12 foot
DMP 110 A C A 26-0225 26-0222
DMP 120 A C B 26-0289 26-1259
DMP 130 A C C 26-0227 26-0223
DMP 130A A C D 26-0288 26-1258
DMP 132 A C
DMP 133 A C
DMP 134 B D
DMP 135 B D
DMP 136 B D
DMP 200 B D
DMP 202 B D
DMP 2102 A C
DMP 2103 B D
DMP 2110 A C
DMP 2120 A C
DMP 2130 B D
DMP 240 B D
DMP 300 A C
DMP 302 B D
DMP 430 A C
DMP 440 A C
DMP 442 A C
DWP 230 A C
DWP 520 A C
LP 950 B D
LP 990 B D
LP 1000 A C
IBM type B D
printers
EPSON type B D
Panasonic type B D
----------------------------------------------------------------------
Cable type "D" is the standard cable that you can get from your local
computer store. If you need cable type "A", "B", or "C", you need to get
the special Tandy cable. If your Tandy printer is not listed, you need to
contact Tandy MS-DOS System Support at 1-817-878-6875 (or by email at
support@tandy.com) to find out which cable you need. If your non-Tandy
printer is not listed, use cable "B" or "D", depending on what type of
computer you have.
It is possible to make your own printer cable, but it is certainly not
worth it, considering the modest cost of the Tandy cables. You can also
install a standard parallel port in an expansion slot, if you have a system
with a card-edge printer port.
Floppy Drives
-------------
All Tandy PC-compatibles use standard 5-1/4" floppy drives. The system may
not support high-density floppy drives, however. You cannot interchange
low-density (360k) and high-density (1.2M) 5-1/4" drives, so if your system
only supports low-density drives, you need to replace an existing drive
with another low-density drive.
3-1/2" drives are another story. Most Tandy's use nonstandard 3-1/2"
floppy drives that are designed to draw power through the data cable.
Normally, an IBM PC-clone connects to the floppy drive via two cables, a
data cable and a power cable. Tandy's usually use only one, a data cable,
using wires that an IBM PC does not normally use to supply the power.
Unlike 5-1/4" drives, you can use a high-density 3-1/2" (1.44M) drive in a
system that only supports low-density drives; it will function as a low-
density (720k) drive, however.
To find out whether your Tandy needs a special 3-1/2" drive, look at the
way your current drive(s) are connected. If you already have a 3-1/2"
drive in your system that you would like to replace, or you want to install
another one, see whether there is only one cable connected to the drive or
two. If there is only one, you will need a special drive. If you have a
5-1/4" drive in your system, another way is to look closely at the data
cable (that is the flat one; the power cable is four differently-colored
wires). If you see holes punched in the data cable leading from the
motherboard to the 5-1/4" drive, you need to use a special 3-1/2" drive.
If your system uses special Tandy floppy drives, and you attempt to install
a standard drive in it, it may destroy the drive and your computer. (You
will not normally be able to install a special Tandy drive in a normal
clone since there is no power connector on a Tandy drive.) The problem is
that the Tandy floppy data cable carries +12V on 3 lines, and +5V on 5
lines, that are 0V on a normal clone. A standard floppy drive will connect
all these wires, creating an even worse short-circuit than with the wrong
printer cable.
Tandy-style 3-1/2" floppy drives were made by Sony and Teac. The Teac
drives are still available. Tandy may still be selling the drives. At any
rate, the following companies sell them as well:
PC Enterprises
2400 Belmar Blvd Bldg B-16
Belmar, NJ 07719
(800) 922-7257
Pacific Computer Exchange
1031 S.E. Mill St., Suite # B
Portland, OR 97214
(503) 236-2949
DCS Industries
5265 Hebbardsville Road
Athens, OH 45701
(800) 537-3539
If you have a system that requires special 3-1/2" drives, you need to use a
special floppy cable as well. This is true even if you do not have any
actual 3-1/2" drives in your system. The Tandy floppy cable has holes
punched in the end that leads from the motherboard to the 5-1/4" drive.
These holes cut the wires in the cable that carry power. If a normal
floppy cable is used with a 5-1/4" drive in a Tandy system, it may destroy
the drive and the computer. For this reason, you should only replace your
floppy cable with another floppy cable designed for your system and
supplied by Tandy. To order a replacement floppy cable, call Tandy
National Parts at 1-817-870-5600. You will need to give them the catalog
number of your machine, ex., 25-1601; you will find the number printed on
the back or bottom of the computer.
If you ever replace or remove your floppy cable, take note of its
orientation. When you install the new cable or reinstall the old one, the
red or blue stripe must be on the same side of the connector as when you
removed it. If you attach the cable upside-down, the drive light will come
on (and stay on) as soon as you turn the computer on, and the drive will
not work.
Another thing you need to be aware of when installing floppy drives is
that, unlike other clones, Tandy floppy cables are not usually "twisted."
On the IBM PC, the floppy data cable is cut lengthwise between the first
and second floppy drives, and the center part of the flat cable is turned
upside down. The effect of this is to switch some wires around so that the
second drive, which is set by a jumper or switch (called the "drive select
jumper" or switch) to be drive A: will instead be seen by the system as
drive B:. Hence, on an IBM PC, you never need to mess with the drive
select jumper. On a Tandy, the cable does not have the twist, so if you
install two drives you need to be sure to set the jumper or switch on one
of them so that it will be drive B:. If you don't have the switches set
right, then when you try to use either drive, both drives' lights will
light, and neither drive will work. The switch will usually be labelled "0
1 2 3" or "1 2 3 4". Set one drive to "0" and the other to "1". A jumper
is a movable plug that connects two metal prongs, and it works the same way
as a switch - just move the plug as needed.
If you are up against it and need to install some standard hardware in a
Tandy (such as a floppy tape drive in my case), or if you feel confident of
your abilities and want to make a Tandy drive work in a normal clone, it
can be done with a little care. I don't recommend it unless you absolutely
have to, but here is what you need to do.
The flat data cable going to the floppy drives has 34 "pins," or wires.
The pins are numbered from 1 to 34, and 1 is marked in red or blue on the
cable. On the Tandy cable, pins 3, 5, 7, 9, and 11 carry +5V; on a normal
clone, they are ground (0V). On the Tandy, pins 29, 31, and 33 carry +12V;
on a normal clone, they are ground also. Using a utility knife, you can
very carefully cut the cable lengthwise between the wires to get at the
individual wires and pull them apart from the rest of the cable. Note that
the wires in the cable are very thin, and it is easy to cut through them if
you are not careful.
To get a standard 3-1/2" drive to work in a Tandy that uses special drives,
extract the wires carrying power as described above and cut them, doing
this between the two floppy connectors. Connect the standard drive to the
second connector, set its drive select switch/jumper as needed, and connect
a power cable to the drive. If you also have a Tandy drive, connect it to
the first connector so that it can still draw its power through the data
cable.
Getting a Tandy 3-1/2" drive to work in a standard clone is a little less
straightforward. It involves cutting the end off of a power cable and
soldering pins 3, 5, 7, 9, and 11 from the data cable (below the twist) to
the red (+5V) wire, and soldering pins 29, 31, and 33 from the data cable
to the yellow (+12V) wire. Always remember that a Tandy drive *must* get
power through the data cable, and a non-Tandy drive *must not* get power
through the cable. If you are competent to do this kind of work, you know
what I mean - if not, don't try it. In my case, I had to graft two data
cables together to connect my floppy tape controller in the middle (that's
what the tape controller needed), bypassing the power around the tape
controller to get it to the floppy drive. Again, if you understand what
I'm talking about, go ahead; if not, don't run the risk of destroying your
system the first time you turn it on.
The companies listed above also sell secondary controllers for Tandy that
support standard high-density drives. If the setup program for your
machine does not permit disabling the built-in floppy controller, you will
need to get a secondary controller that is specially made for Tandy.
Serial Ports
------------
The serial ports in all Tandy PC-compatibles are completely, totally, and
100% IBM-compatible. Many Tandy's have slow UART's, however, or, rather,
they contain integrated circuit chips that simulate slow UART's. (The
UART, or "universal asynchronous receiver/transmitter," is the main chip
controlling the serial port.) For that reason, the built-in serial ports
in most Tandy's are quite suitable for a mouse, but not good for use with
high-speed external modems. It is better to install an internal modem in
an expansion slot (and cheaper too). MSD.EXE, distributed with newer
versions of DOS, and available on the Internet, can tell you if you have a
slow UART (8250 or 16450) or a fast one (16550).
Some early 1000-series models have a port on the back marked "light pen."
It is just that, a light pen port, not a serial port, though it looks like
one. There are rumored to be some old programs out there that will use a
CGA light pen, and it is said that there were once light pens made that
would plug into the port. That's what it's for. (A light pen is a type of
pointing device, serving the same purpose as a mouse, but programmed
differently.)
Joysticks
---------
On some Tandy's, the joystick ports use a 6-pin round DIN connector rather
than the 15-pin DB15 (rhomboid) connector used on most IBM clones. On
those Tandy's, each of the two joysticks plugs in to a separate connector,
while on IBM's both joysticks plug in to the same connector. Apart from
the connectors, the joysticks are completely the same, and a Tandy joystick
will work in games as well as an IBM one.
There is no adapter available, however, so the best way to go is just to
get a Tandy joystick. The Tandy Color Computer uses the same joystick with
the round connector, so there are quite a few used ones around, and they
are very cheap ($5-10). Also check with the companies listed above, or
with Tandy.
If your Tandy has the round joystick ports, you will not be able to install
a standard game port in an expansion slot, since the new port will conflict
with the old ones, and the built-in joystick ports can't be disabled.
If you *must* have that WAY k00l flight simulator joystick you saw in your
local computer store, you might find a way to make it work if you do some
soldering. The IBM-standard joystick pinout is like this:
--------------------------
\ 1 2 3 4 5 6 7 8 /
\ 9 10 11 12 13 14 15 /
----------------------
pin assignment
1 +5V
2 stick 1 button 1
3 stick 1 X-axis
4 ground (0V)
5 ground (0V)
6 stick 1 Y-axis
7 stick 1 button 2
8 +5V
9 +5V
10 stick 2 button 1
11 stick 2 X-axis
12 ground (0V)
13 stick 2 Y-axis
14 stick 2 button 2
15 +5V
There are two round joystick ports on the Tandy, and they look like this:
----+----
/ x H x \
| x |
| x x |
\ x /
\_______/
... or something like that (I'm not great at ASCII art). The x's are the
pins. The pinout is:
pin assignment
1 Y-axis
2 X-axis
3 ground (0V)
4 button 1
5 +5V
6 button 2
Unfortunately, I don't know which pin is which; a voltmeter might help.
Always make sure that you don't connect ground to +5V; you run the risk of
destroying the joystick and sound circuitry.
IDE Hard Drives
---------------
Most Tandy's (the newer ones) that have an IDE hard drive connector on the
motherboard use standard AT IDE drives. Normally, we're talking about
regular IDE, maximum 504MB. You can get those drives anywhere.
The Tandy 1000TL/2, TL/3, RL, and RLX have an IDE connector on the
motherboard that looks identical to a normal IDE connector. It isn't. An
AT IDE drive can be plugged in, but it won't work. Instead, these systems
use XT IDE drives. Those drives are quite scarce nowadays, and none was
made larger than 40MB. The Seagate ST351A/X is a 40MB drive that will
work. This drive is no longer being made, but it is still available from
KLV in New Jersey (201) 812-1118. Jumper settings for this and other
Seagate drives are available by faxback from SeaFAX at (408) 438-2620.
Some other drives that will work are the Seagate ST325X, ST325A/X, and
ST351X, and the Western Digital WD93028, WD98028, WD93038-X, WD93044-X, and
WD98044-X. Tandy once sold a full line of XT IDE drives, and your local
Radio Shack may be able to locate one as well. There can be problems with
40MB XT IDE drives in some cases; if that happens to you, contact Tandy MS-
DOS System Support (see above).
There are 8-bit controller cards for AT IDE drives, including some
specially made for Tandy. Tandy still sells these, as do the suppliers
listed above. There are also some 8-bit AT IDE controllers that will not
work in some Tandy's, especially those with built-in XT IDE controllers,
and early 1000-series models such as the original 1000 and 1000A. If you
have one of those systems, be sure to get an AT IDE controller that is
specially made for Tandy.
1000EX/HX Expansion Slots
-------------------------
The "plus"-style expansion slots in the 1000EX and 1000HX are electrically
the same as an 8-bit IBM-standard expansion slot - with one exception: the
EX and HX, as they came from the factory, do not have DMA. A DMA chip is
included on the expansion cards that increase memory above 256k, available
from Tandy and from the suppliers listed above. Hence, if you have more
than 256k RAM in your EX or HX, the only thing different about your slots
is the connector.
Unfortunately, expansion cards that use the "plus" connector tend to be a
lot more expensive than the same cards that use the standard card-edge
connector. (The companies listed above sell such cards.)
If you're handy in electronics, you can make an adapter to plug in a
standard 8-bit expansion card. I got this third hand; a gentleman named
Jay Wigginton originally posted this on some online service, perhaps
America Online, in 1989. You need the following parts:
(1) Radio Shack Plus Adapter Board, catalog number 25-1016. It was $14.95
in 1989; you might still get it from 1-800-THE-SHACK. This was a standard
8-bit card to which one of the special "plus" cards made for the 1000EX and
HX could be attached - essentially the reverse of the adapter you're
making.
(2) Female "plus" header connector, part number 8519257. This is a part
off of the 1000EX/HX "plus" memory expansion card referred to above. Get
it from Tandy National Parts (see above). (You still need to have the
memory card itself if you want to get DMA.)
(3) 31/62 pin edge connector, part number 8519236. This is a part off of
the 1000SX. It comes from Tandy National Parts also. It is the female
(motherboard) side of a standard 8-bit slot connector.
According to Mr. Wigginton, you first remove the male plus connector (by
desoldering) from (1), "being careful not to damage the board." Next, cut
the top off of (1) down to just above where the "plus" connector was. Cut
the "edge finger" (male edge connector) off of the bottom of (1), "leaving
about 1/8 inch of the gold fingers exposed below the solder mask (the green
covering on the board)" (Wigginton notes that that cut is optional).
Now solder the female edge connector (3) to the holes left when you removed
the "plus" connector from (1). "It will be necessary to bend the pins so
that they fit," Wigginton notes. The connector should be attached on the
side of (1) with printing.
Finally, solder (2) to the edge fingers on (1). "Check all pins for shorts
to other pins. Use a continuity checker. This is very important; it will
not function if any pins are shorted together."
I have Wigginton's original post if anyone wants to see it. Email me at
the address below.
Others
------
People often ask about the pinouts for the video connector on Tandy
systems. All PC-compatible Tandy systems use standard monitors. There are
5 standard types of monitors for IBM PC's: mono TTL, CGA composite, CGA
TTL, EGA, and VGA (including the various SVGA monitors). At least one of
these 5 monitor types will work on a Tandy or on any Tandy video adapter
card. In every case, the pinout is the IBM standard, for one of the 5
standards. The built-in video controller may have nonstandard features,
but that is a software issue. EGA, CGA TTL, and mono TTL monitors use a 9-
pin D-sub connector. VGA monitors use a 15-pin connector. CGA composite
monitors use a round plug.
Some Tandy's have microphone and earphone plugs. Both are standard. If
you connect a radio, tape deck, or keyboard directly to the microphone plug
via a dubbing cable, you will need to move a jumper inside the machine;
consult your owner's manual. The wrong jumper setting can destroy the
sound hardware.
I hope the above is helpful - Jeffrey L. Hayes <tvdog@agate.net>