Friday, May 18, 2012

The Other Early Tandy 1000s

Some people may have noticed the prior post in which I extoll the virtues of the Tandy 1000SX.  I recently posted on the later Tandy 1000 models, which increasingly stray from the standards established by the PCjr. and original 1000.  But let me examine the issues with the other systems.

Tandy 1000TX - This system is equal to or better than the SX in every way but two.  It is better in that it has an 80286 CPU, is upgradeable to 768K RAM (to allow full 640K conventional), has a volume control dial for the internal speaker, a front-case mounted headphone jack which reroutes the audio from the internal speaker (no multiplexer nonsense), has a serial port, comes with a 720K drive and boots up a lot faster.  However, it provides power through the drive cable, which needs to be accounted for by cutting holes in the pins feeding power to any floppy drive not supporting that arrangement.  This is only slightly annoying.  Even so, the power supply has one 4-pin Molex connector for the 5.25" floppy drive, which can be split with a common Y-adapter.  

The other issue is that the system is by default faster than the SX, even with a 286 Express accelerator.  While it can cut the speed of the CPU by half, even a 286 at half speed is substantially faster than an SX without an accelerator.

I would estimate that, based on benchmarks of both systems, Tandy 1000TX running at 4MHz would be somewhat faster than a Tandy 1000SX with a V20 upgrade running at 7.16MHz. However, this is still about 2x faster than an IBM PC with an 8088 running at 4.77MHz. Already you have three noticeable speed increases over the original PC and the software written when that was the only thing around. So if the lower speeds are important to you, then the TX may not be a good choice.  Despite running its CPU at 8MHz, the TX and TL machines are approximately 10% slower than an IBM AT @ 8MHz, but have faster video than IBM EGA.

The 286 is in a PLCC socket, so most 386 and 486 upgrade devices work in it, if you can find one.  However, it still has 8-bit slots, so you will need to find VGA, IDE, HD Floppy and Network cards that will work in an 8-bit slot.

Tandy 1000EX - This was released at the same time as the SX, and is essentially a stripped down of the SX.  It even uses the same BIOS as the SX, shares the 7.16/4.77MHz speed switch.  Unfortunately, expansion is really difficult in this machine as it has one bay that is supposed to contain a 5.25" floppy drive.  No reset switch either, no way to use a different keyboard, and due to internal RF shielding it is extremely difficult to get inside to the motherboard.  I also read that installing a RTC chip is also difficult due to lack of clearance that the internal shielding imposed.  No coprocessor can be installed without an extremely rare adapter.  The video is difficult to upgrade at best but it is possible despite what Tandy may have told customers.

The worst issue with these machines are the PLUS slots and finding expansion boards.  A PLUS slot is simply a 62-pin ISA with berg-strip pins and connectors mounted at a right angle to a regular card edge ISA connector.  However, the brackets securing the cards to the chassis are a custom design.  Finally, to upgrade the RAM and add DMA you need a special PLUS board similar to the boards for the original 1000s. The most common upgrades to find would be serial cards and modems, and while specialty companies made other upgrade devices, they are extremely difficult to find.  There is a port for an external 5.25" or 3.5" drive, but the pinout is a Tandy proprietary design.  You can switch the boot drive using the F4 key at bootup.  If you have a 3.5" drive, you will need DOS 3.2 and drivparm in your config.sys to obtain the full benefit from it.

The lack of on-board DMA, which users of other computers took for granted, is extremely irritating.  Without DMA, most hard drive options are out of reach.  Standard MFM/RLL controllers use DMA and memory mapped interface cards like the Trantor T-128 SCSI and the Silicon Valley Computer ADP-50L IDE boards require it to enable their improved performance.  You may be able to use a port I/O SCSI interface like the Trantor T-130B, but that will not work reliably unless you have a V20, V30 or 286 CPU.

The other issue is the floppy drive.  Without DMA, the CPU must handle all the transfers to and from the floppy drive.  This requires tight timing and disabling interrupts, so serial (mouse and modem) and keyboard input may be lost during floppy drive access.  DMA makes using old machines much easier.

The machine does have two small benefits over the SX.  First, it has an external volume dial that can control the headphone or internal speaker.  The SX has a potentiometer inside to adjust the volume, but it isn't very practical to open the case.  Second, due to the design you will always get sound output from the jack, unlike the RCA audio output of the 1000 or 1000SX.  On the latter machines you may have to set bits in the audio multiplexer.  No 8087 coprocessor socket.

Tamdy 1000HX - This update to the EX shares most of its benefits and drawbacks, but adds more in both directions.  It was released alongside the TX.  Unlike the EX, it has two internal 3.5" bays, although if you want to use them for anything other than Tandy drives you may need some ingenuity in getting other things to fit in them.  It can support a third external floppy drive, which can be a 5.25" or a 3.5" drive in addition to the two internal drives.

It has better clearance for installing a clock chip and less RF shielding to contend with.  However, it uses power-in-drive cabling for the floppy drives and there are no molex connectors available for powering something else.  Although it only comes with DOS 2.11, Tandy modified that DOS to provide full support for 3.5" floppy drives.  In theory I suppose that could confuse 3.5" DD floppy programs that say "requires DOS 3.2".  It also has DOS-in-ROM, so you don't need a DOS floppy in the drive for simply running applications or have to wait on slow floppy disk accesses to your DOS disk.  The machine boots instantly to a menu allowing you to select to boot to a program in drive A:, start from the C: (ROM) drive, or enter Deskmate.

Tandy 1000/A/HD - This is the original model, and it sort of a trial run for the later machines.  They have three slots and can expand memory through the use of ISA boards.  The original Tandy 1000, (non-A version), has no socket for a numeric 8087 coprocessor.  You can burn and insert EPROMs to upgrade to the 01.01.01 BIOS of the 1000A.

The built in graphics cannot be disabled and and using an EGA or VGA card is tricky at best.  I do not think that the early 1000s supported the use of a monochrome or Hercules card because there would be no BIOS support for it.

There are only 3 ISA slots, and at least one is required to expand the system beyond 128K and add DMA. Tandy originally released two RAM expansion boards, the first with the DMA chip and up to 256K (25-1004) and the second without.the DMA chip and up to 256K (25-1009).  Any RAM card with settable memory addresses like an AST Six Pak Plus (short versions) will work in place of the second card.

For the 1000HD, Tandy included a new board (25-1011) with the DMA chip and allowing you to add up to 512K.  It also has a PLUS header allowing you to use a PLUS card, like the serial card, without having to take up another precious slot.  The 1000HD also comes with a 10MB MFM hard drive (Tandon TM-252) taking up one of the two 5.25" bays and the interface card (WD1002S-WX2).

Like the SX, the Tandy 1000/A/HD can use the 286 Express Accelerator.  The accelerator consists of an ISA card and a daughterboard with a ribbon cable to the motherboard's CPU socket.  The Tandy 1000A and 1000HD share the same motherboard, and can use the came daughtercard that the SX can use.  The original 1000 requires a different daughteboard, and the regular card is hard enough to find as it is.    An NEC V20 will give a modest speed boost.

The Tandy 1000/A/HD system does not assign bootup functions to the F1-F4 keys as the later Tandys do.  Since there is only one speed, there is no function for the fast/slow speed, and swap drives is not available (since only one type of drive was supported during these systems' life).  Neither is the mono mode, and TV mode (40 columns with 200 lines) requires pressing F12 at bootup.  However, the 1000A or the 1000 with upgraded BIOS will allow you to use F1 to enter the mono mode.

There is no volume control and the same issues with the sound multiplexer and the RCA audio out on the SX will be encountered in these machines.  Unlike the SX, there is no switch to use IRQ2 instead of IRQ5 for the video (so you can use an IRQ2 hard drive controller).  The Power Supply is rated for 54W compared with the 65W of the SX and TX.  None of these machines have a Real-Time Clock (RTC), which must be added via a plug-in module.  The Dallas DS-1215 is the usual choice.  

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