Practical Issues with the Tandy 1000 SX and TX

I have recently become the owner of a Tandy 1000 TX, and I'd like to share my experiences with using the machine.  Since the system is very similar to a Tandy 1000 SX, much of what I will say will also apply to that system.  Overall, the system is much more straightforward than the Tandy 1000 TL.

Memory Upgrade  (SX and TX differences)

The TX comes with 640KB but can be upgraded to 768KB.  The extra 128KB is dedicated solely to the video chip and allows you to have the full 640KB conventional free for MS-DOS.  There are four DIP-18 sockets on the motherboard, and you must insert a 64Kx4 DRAM chip, 120ns or better into each of them.  Once all four are inserted, you must remove the jumper at E9-E10.

Not all games and software work with this upgrade.  I know of several games that either do not show graphics or are unplayable with the upgrade. Unlike the TL, you do not need to remove the chips.  Just replace the jumper at E9-10 and the upgrade will be disabled.

The SX comes with 384KB and can be upgraded to 640KB.  Eight DIP-16 sockets are on the motherboard and you must insert a 256Kx1 DRAM chip, 150ns or better into each of them.  One all eight are inserted, remove the jumper at E1-E2.  There are no compatibility issues with this upgrade.

If you add an EGA or VGA card, the on-board video and its RAM stealing is disabled and you will also be able to use the full 640KB of conventional memory.  Most games will not recognize an EGA or VGA card in the Tandy 1000s without running a simple program called VGAFIX on startup.

Case Opening (SX and TX differences)

The SX and TX's case is held together by two screws at the front.  The case shell is made of plastic, so be careful of stepping on it or letting it fall when you are working inside it.  Unlike the TL and SL and later machines, the keyboard and joysticks attach to the front of the machine, so you will need to remove them to get at the insides.  When you have removed the front screws, just pull the plastic portion of the case forward.

Unlike the SX, which had a metal bar over the expansion slots to allow a monitor to sit on the case, the TX has a metal shield completely covering the expansion bays.  Two screws attach the shield to the chassis, and once removed, you have to pull the shield up and then to the left.  You may need a small screwdriver to help pry it up.  The shield is bendable, so take care with it.

The SX has a metal bar to support the weight of a monitor, and should have two plastic guards to protect someone from slicing their fingers on the metal edge.  Its expansion slots use slotted hex screws.  The back plastic plate should be removed when installing expansion cards, but one can get at the screws with a small slotted screwdriver.

Tandy mostly used slotted or phillips head hex nut screws inside the SX and TX, and I highly recommend using a 1/4" hex nut driver to remove them.  You won't strip a screw using a hex nut driver.  The drive cage is fixed to the chassis with approximately five screws.  You will need to remove any expansion cards to get at the screws.  The SX has a metal plate parallel to the the expansion slots that should be removed to get at some of the screws holding the drive cage in place.

The TX and TL use the same small Phillips head screws for the expansion cards.  Jameco sells compatible screws, look for Jameco Item # 2185871.  Unlike the SX, you probably will not need to remove the back plastic plate to unscrew the screws.  They also work in the SX.

The drive cage supports only two 5.25" drives.  Some drives do not have the appropriate mounting holes.

CPU Upgrade (TX and SX differences)

The TX, like the TL, has a socketed 80286-8 in an 68-pin PLCC socket.  Unfortunately, the location of the 286 is close to the front edge of the bezel, and a longer accelerator will not fit.  Improve Technologies Make-it-486! will fit, but it has no math coprocessor.

The 80287 math coprocessor should be rated for 8MHz, but a 6MHz will probably be fine.  Games that run on an 8MHz 286 generally do not use one.  No jumpers or dipswitches need be set.

The SX has a socketed 8088-2 in a 40-pin DIN socket.  It can be upgraded with an NEC V-20, μPD70108-8 for a modest speed boost and compatibility with many programs that use 286 instructions.

The SX will support a special accelerator board called the 286 Express Card.  This card takes up an expansion slot, requires the removal of the 8088 and inserting a ribbon cable with a DIP-pin connector into the CPU socket.  The 8088 goes onto the daughtercard with the ribbon cable.  This upgrade allows and defaults to the use of the 8088 and acts like it is not there unless activated via its DOS driver.  It has 8KB of cache, which can be disabled for compatibility.

The 8087 math coprocessor should be rated for 8MHz operation, but 5MHz will probably be fine (its runs at 7.16MHz/4.77MHz).  Remove the jumper at E3-E4 if you install one.

Floppy Drives (SX and TX differences)

The ideal 5.25" 360KB double density floppy drive for the Tandy 1000 SX or TX is the Teac FD-55BR. This is a beige/off-white version of the FD-55BV, which is also a good choice.  Teac made a 1.2MB 5.25" high density drive called the FD-55GFR, and it looks identical to the lower density drive.  Make sure you get a double density drive, high density drives do not work with the SX or TX's built-in floppy controller.  The SX and TX have very short floppy cables and the card edge placement on the Teac drives (and they are good drives) work beautifully with the stock cable.  The molex power connector is also close to the left edge of the drive, and the one connector from the power supply is extremely short.

The proper drive to use with a TX  is a Sony MP-F63-01D 720KB double density 720KB floppy drive.  This drive is a 3.5" drive enclosed in a 5.25" bay adapter.  The bay adapter provides the front faceplate for the drive.  The drive uses a large "lip" as the ejector button.  It is identical to the drive in the HX, but that has a different faceplate and designation.   Tandy provided a 3.5" upgrade for the SX that uses the same drive but has an adapter board that converts the power-in-drive connector on the drive to a card edge and molex connector.

The TX and HX are the first systems that incorporate Tandy's power-in-drive cable, so you need to use your own cable with holes punctured through pins 3, 5, 7, 9, 11, 29, 31, 33 if powering a 3.5" drive that uses a mini-molex power connector.  You also need to untwist the twist in the floppy cable.  Tandy drives get selected by dipswitches and the F3 key in the SX and TX.

The TX cable has one pin connector and one card edge connector.  The SX has two card edge connectors. The SX does not use power in drive cables and has two molex power connectors.

Hard Drives (SX and TX)

The SX and TX requires a hard drive interface.  An MFM controller like the WD1002A-WX1 is small enough to fit in the Tandy and can select IRQ2, but you may need to obliterate a trace that hardwires IRQ5.  However, the SX and TX can disable the video's use of IRQ5.  The hard cards sold during or near the TX's time were were the 20MB 25-1029(A-B) and 25-1032(A-E) and the 40MB 25-4059(A-B).  They use MFM or XTA-IDE drives.  They can save a slot, but are bulky, noisy and can prevent long cards from being installed in nearby slots.

The best contemporary IDE interface was the ADP-50L, which allows the use of any 16-bit IDE drive up to 504MB.  With a compact flash adapter, you can save a drive bay by using a slot bracket.  Modern solutions include XT-IDE, some of which combine a CF card interface, saving you a slot.  Unfortunately, while you can buy the board and parts cheaply enough, you will have to solder the CF connector and other surface-mounted components yourself.

Serial Port (TX only) / Light Pen Port (SX only)

The TX replaced the useless light pen port on the SX with a serial port.  This is a standard XT-8250 port, so do not expect speeds about 9,600 baud.  It defaults to COM1 and can be set to COM2 by removing the jumper at E3-4 or disabled by removing the jumper at E1-2.

The SX uses a DE-9 Male no-screw connector for a light pen, which would suggest that Atari 8-bit or Commodore 64 light pens would work.  The pinouts for those light pens do not correspond to the pinout for the Tandy 1000.  Tandy did offer light pens for the 1000 series, but they must be beyond hard to find today.

Jumpers (SX and TX)

JP1 sets the system to use an MDA or Hercules compatible card.  I am not sure whether it totally disables the onboard video, or just initializes the monochrome monitor for text like on an IBM PC.  If its the latter, programs can still use the built-in video when they require it.  It is NOT the same as pressing F1 on startup, that just sets the Tandy video to use composite-monochrome friendly colors.

JP2-5 disables the use of IRQ5, 6 & 7 for the system board devices, freeing them up for the expansions slots.  The system board uses IRQ5 for the video interrupt, which was rarely used with Tandy graphics.  IRQ6 is used for the floppy controller.  Even if you disable the built-in floppy controller's IRQ6, you will need a secondary floppy controller that can use an alternative floppy controller address.  Ditto for a printer port on an expansion slot for IRQ7.  Leaving IRQ7 enabled is typically harmless.  Sound Blasters don't need it disabled.

Keyboard (SX and TX)

The SX and TX is designed to use a 1000 keyboard.  Some games (Snow Strike) and programs, like Tandy 1000 Deskmate, require it.  The default text mode is an 80x25 column by line mode with 225 lines, pressing F2 at startup will give you a 40x20 column by line mode with 200 lines, suitable for a composite video monitor.  You can also do this by using the Tandy DOS 3.2 or 3.3 command MODE TV If you use the command MODE 200 command will give you 80x25 column by line mode with 200 lines (assuming you are in an 80-column mode).

F3 sets the speed to 4MHz (TX) or 4.77MHz (SX), which can also be done with MODE SLOW.  If you want to play a game like Lode Runner on the TX, you will need the slower speed.  F4 swaps the boot drive, which is quite useful with a 5.25" and a 3.5" drive.

I am not the biggest fan of the 1000's keyboard.  The key action is mushy and the Shift keys are too small.  The Hold key is a particular nuisance, especially as it is right next to the Enter key.  Essentially it is the "freeze" key, if you press it (intentionally or otherwise), your program may pause and it may seem like the computer has frozen up and crash.  Press it again and the program should resume.  Shift + Print acts acts to Print the screen (I believe its Ctrl Print on a IBM PC).  There is no Scroll Lock key, you need to use Alt + Break.

Some programs will only use the dedicated cursor keys for movement, others will use both the cursor keys and the numeric keypad, and others will only use the numeric keypad.  On the IBM PC 83-key Keyboard, there is no dedicated cursor keys, and games do not have this problem.  However, unlike that keyboard, there are status LEDs for Num Lock and Caps Lock.

The ~ and ` and the | and \ are split up on the Tandy 1000 keyboard, making for an adjustment when coming from a PC keyboard.  The \ key is used frequently in DOS, and the | is also used.  (TYPE README.TXT |MORE) More annoyingly, there is no separate *, so typing something like *.* in DOS is a hassle.

Any Northgate Omnikey Keyboard with dipswitches should be able to work with an SX or TX.  They are very expensive, and required a special cable.  I believe the cable just passively mapped the common pins (DATA, CLK, +5v and GND) from the IBM standard to the Tandy standard.  The Northgates can provide full 101 Key capabilities.

Sound Output (SX and TX differences)

The SX and TX both have volume controls for the internal speaker.  The SX's volume control is inside the case and requires opening the case to get at it.  The TX's is in the front of the case and can be accessed with ease.  Both have RCA video and audio jacks, but the TX also has a headphone jack.  Due to the audio routing design of the TX, all audio will always output to the RCA jack.  If something is plugged into the headphone, the internal speaker is disabled.  The volume control will still work for the headphones, but has no effect on the RCA output.

On the TX, there is no need for a program like TDYSPKR to set the internal and external audio options. On the SX, you will need that program to hear 3-voice sound out of the RCA jack for games that do not set the multiplexer (LucasArts' SCUMM games) or to shut off the internal speaker.