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Released in 1987 the Tandy 1000 SX was an evolution of the original Tandy 1000 which itself was a clone of the IBM PCjr. The Tandy 1000 line featured several advantages over standard IBM compatibles of the time such as built-in “Tandy video” which offered CGA video modes along with 160×200 and 320×200 16 color modes. The Tandy 1000 also offered enhanced Tandy 3- voice sound. Both Tandy video and audio were very widely supported in games.

The 1000 SX is an enhanced version of the original Tandy 1000 offering many improvements that more or less correct every issue that the original Tandy 1000 had such as making upgrading and expanding the PC easier and adding a faster CPU “turbo mode” as well as a DMA controller to speed up certain processes. Many retro PC enthusiasts consider the Tandy 1000 SX one of if not the best bang for your buck IBM compatibles of the 1980s.

The 1000 SX case when compared to a IBM PC is noticeably smaller which saves desk space. It’s also mostly made of plastic which also makes it a significantly lighter PC. The SX came stock with dual 360k 5 1/4 floppy drives in its dual 5 1/4 bays though the built-in floppy controller also supports 720k drives. We will detail the process of upgrading to one of these larger capacity drives later in the article.

At the bottom of the front of the case there are two screws on the far left and right. Removing these is all that is necessary for removing the cover. On the bottom left of the front we have a large red reset button. Next to this we have a keyboard port. The Tandy 1000 SX uses a proprietary style keyboard port like most of the early Tandy 1000 line. This port is not compatible with standard IBM type PC/XT/AT keyboards so a Tandy keyboard is required though there are modern adaptors that allow the use of standard PS/2 keyboards.

To the right of the keyboard port are dual round 6-pin joystick ports. These joystick ports are also proprietary and require joysticks compatible with the Tandy 1000 and Tandy color computer line.

Turning the Tandy 1000 SX around and we can see the various rear connectors. The power switch like the one on the IBM PC/XT/AT is on the rear left side though the switch itself is not visible in this image. Starting from left to right we have a standard 3 prong connector for the power cord. Next is a card edge style connector located directly under the model label. This is actually an edge-style parallel port and is intended to be used with specific Tandy printers. Adaptors were made to connect non-Tandy parallel port devices to the edge connector. Next is the seldom if ever used DE-9 lightpen connector followed by the built-in video and audio jacks.

First up is a standard DE-9 connector for connecting to TTL RGB monitors. Tandy made several Tandy specific monitors of differing quality like the CM-5 that I’m using and the higher quality CM-11 but any standard CGA type monitor will work. The video output of the Tandy 1000 SX is based on the video capabilities of the failed PCjr but since the PCjr failed this video standard became known as TGA or Tandy Graphics Adaptor. The Tandy 1000 SX can output standard CGA as well as the TGA modes which allow 16 colors on screen at one time. This more or less resembles the look of EGA in low resolutions though they are not the same.

Tandy playing EOB in CGA mode
Tandy playing EOB in TGA 16 color mode

CGA compatibility on the Tandy 1000 line is almost 100% and is CGA register compatible though due to slight differences in text characters used very few games (ex. ICON : Quest for the Ring and The Seven Spirits of RA) may show some minor graphical issues. As for the extended TGA modes available the Tandy 1000 also supports 160×200 with 16 colors, 320×200 with 16 colors and 640×200 with 4 colors out of the palette of 16 colors. A very significant number of games do support Tandy 16 color graphics. Some of these games allow the user to choose the color mode between CGA and TGA but some games will automatically detect if a Tandy is present and force 16 color TGA mode. Some games such as ArcticFox only support 16 color mode via the Tandy graphics adapter.

Next up we have the RCA style A/V jacks for video and audio. At first glance it looks like stereo audio jacks but the red jack is for color composite out for connecting usually to a TV but also to some monitors. Some games may also show differences in colors when displayed in color composite mode on a Tandy 1000 as opposed to an IBM compatible CGA card.

The audio out is very useful if you don’t want to output sound through the internal speaker and instead wish to use an external speaker. The audio out though is disabled by default and some games may fail to initialize. there is a program called  tdyspkr.com which should allow you to select the A/V audio jack as active on boot up for those programs which fail to see it.

Since we are talking about sound It’s worth mentioning the Tandy 1000 SX “Tandy sound” which is a bit more advanced than the standard PC speaker and uses 3 voice channels and 1 sound channel is a clone of the PCjr sound. Tandy sound or Tandy 3-voice sound as it’s also known is generated by either a SN76496 or NCR8496 Chip. Some games such as Thexder only support PC speaker or Tandy speaker sound with Tandy sound being noticeably superior and less grating on the ears. If you opt to not use external speakers Tandy 1000 SX also sports a nice sounding large cone speaker at the front of the case

Removing the case to get inside is relatively easy and only requires removing two screws located on the front face of the case.

Image was taken before math co-processor and memory upgrade

The only expansion card for my Tandy 1000 SX is an I/O card for a serial and parallel port and this is the only card that was installed in this machine when I received it. The metal bar going across the motherboard from the side to the dual 5 1/4 bays is actually a support beam that is easily removed. Its purpose is to help support a monitor if one is placed on top of the case as was a common practice of the time.

Removing the dual 360k 5 1/4 floppy drives gives us a better view of the motherboard.

Like the original IBM PC the 1000 SX only has 5 8-bit ISA slots. Unlike the IBM PC though, the Tandy 1000 SX has the floppy drive controller as well as the video built-in. The motherboard of the 1000 SX also has a built-in DMA chip to speed up certain processes, something the earlier Tandy 1000, 1000A and 1000 HD lacked.

1) CPU – The 1000 SX uses an 8088 like previous models although the SX runs default at 7.16MHz with the ability to downclock to 4.77MHz for compatibility with older software. The SX has no turbo button so to slow the CPU back down to 4.77MHz you need to use the DOS commands “MODE SLOW” and “MODE FAST” in order to switch speeds. The default speed is 7.16MHz though at boot up you can also press the F4 key on the keyboard to boot into 4.77MHz slow mode. The MODE command which sets the CPU speed is only available in the OEM version of Tandy DOS 3.x.

2) Math Co-Processor – The SX does have a socket next to the CPU for adding a math co-pro. I elected to add an 8MHz capable 8087-2. Adding a math co-pro is hardly necessary and only a handful of programs and games from the time can take advantage of it.

Next to the CPU is a small switch box. These switches can be used to disable several features mostly for the purpose of adding discrete cards. All switches are set to “on” by default. switch 1 disables the built-in video. Switch 2 controls the IRQ the built-in video uses. Switch 3 disables/enables the internal floppy controller and lastly switch 4 will enable/disable the parallel port.

3) RAM – By default the 1000 SX came with 384k of memory but is easily upgradable to a full 640k of memory. My 1000 SX only came with the original 384k of memory but I upgraded to the full 640k via eight 256k x 1 150ns (or faster) DRAM chips. After adding the additional memory jumpers E1 and E2 also need to be removed for the system to detect the change.

640k should be enough memory for any game that will run well on the SX and there is even evidence that expanding past 640k on a 1000 series can cause issues and incompatibilities with games.

4) System ROM BIOS / Smartwatch Battery – This socket is used for both the system BIOS chip on the SX as well as the battery which holds time for the system clock.

System ROM BIOS

you’ll notice that on my machine the BIOS chip is in a socket that is a little high. that’s because my SX has a separate battery module in between the motherboard socket and the BIOS ROM. I don’t believe all SX’s have this module but mine does. its purpose is to hold the date which needs to be entered every time Tandy DOS 3.x loads up on boot. To access the battery you have to first remove the ROM chip which is socketed over it.

You can still purchase a modern replacement for this battery HERE which allows the use of a coin-style battery.

5) Speaker Volume control – Unfortunately there is no way to control the volume of the speaker from outside of the case but Tandy did include a volume adjustment on the motherboard. If turned down all the way the speaker can be completely muted.

6) floppy connector – Built-in connector for connecting floppy disk drives to the SX. The built-in controller supports both 360k 5 1/4 disk drives as well as 720k 3 1/2 style disk drives.

7) Tandy sound chip – As mentioned earlier the Tandy 3-voice sound is generated by either an SN76496 or NCR8496 Chip. For the Tandy 1000 SX it can be either chip. My SX uses the NCR8496 which is a clone of the original Texas Instruments SN76496 chip.

It is almost a perfect clone though there are slight audible differences in the noise channel. I suppose since it’s socketed an original SN76496 could be sourced and swapped out with the clone chip. If you are interested more information on the sound chips can be found HERE.

UPGRADES

There are common upgrades that apply to most all PC and XT machines such as an NEC V20 for a small speed boost or XT-IDE adaptor. There isn’t any place to really mount a hard drive and I use a XT-IDE compact flash adaptor myself which works well. I am using a 32MB card with Tandy DOS 3.2 installed but DOS 6.22 with a larger CF card works just as well.

A hardcard or hard drive on an ISA a card would be a more period correct, though slower option.

I don’t recommend adding an EGA or VGA video card to the SX though you certainly can I just find the whole point to using a machine like this is to take advantage of the Tandy sound and video that so many games supported.

A serial card or bus mouse card could also be useful if you are planning on using a mouse with your Tandy.

I did however add a Sound Blaster 1.5 w/ CMS chips. I’ll be using this machining mostly for older titles that support PC speaker or Tandy sound only though I wanted the option for games that do support digital sound and or adlib/CMS sound.

The upgrade I would definitely recommend is replacing one of the 360k 5 1/4 floppy drives with a 720k 3 1/2 drive. I replaced my drive B with a 720k drive as opposed to drive A since the primary format of the time was 360k disks and thus many “booter” type games came in that format which may require being read from drive A. Since there are no 3 1/2 size bays on the SX case you will need a 5 1/4 to 3 1/2 bay adaptor. Keep in mind you can press the F3 key on bootup to swap the A and B drives.

Some things to consider when adding a 720k drive is that the cable the SX uses is very short and also uses edge connectors so it’s very likely you will need some kind of adaptor and or extension to reach for the floppy drive or you’ll need to use a different cable altogether.

Original 1000 SX floppy cable.

Another thing to keep in mind is the Tandy 1000 SX uses floppy cables without a twist in them so you will need a 720k floppy drive with jumpers that allow you to designate what drive it is.

D0 designates the drive as drive A while setting the jumper to D1 sets the drive as the B drive.

If you can’t find an adaptor and or extension which allows you to use the original short floppy cable that came with the Tandy 1000 SX you will either need to find a longer floppy drive cable without the twist or modify a more common “twisted” floppy cable by untwisting the cable as seen below.

Unfortunately, I had no luck getting my modified cable to work though I know others have. I did end up finding an edge connector adaptor that also had an extension cable attached to it which allowed me to use the original cable that came with my SX and everything works fine.

So is the Tandy 1000 SX a great computer for 80’s PC gaming? I’d say so and I would at the very least say it was one of if not the best value for the money at the time. The built-in Tandy sound and video were far superior to PC speaker and CGA common at the time and many, many games supported the standard. There are a number of games that are just best experienced with Tandy color and or sound. You could have bought an EGA card but those would have been quite expensive and sound cards weren’t really supported until the late ’80s. The option to run the CPU at both 7.16MHz and 4.77MHz for compatibility is also a nice bonus. The 1000 SX is also lighter and takes up a little less desktop space than comparable IBM models and as mentioned at the beginning of the article the SX fixes most of the issues that hampered the original Tandy 1000. I like the Tandy 1000 SX so much I’ve actually replaced my primary 8088 based XT PC with it despite that machine still holding a few advantages over the Tandy.

One Comment

  1. It looks like a very cozy machine. I miss the compact desktop experience. I didn’t know about the Tandy 1000 until now–it’s before my time, though the look of this one is very iconic in my opinion.


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