Tandy 1000A
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| The 1000A main logic board (PCB 8709604, revision B), from the component layout in Tandy's 1985 service supplement | |
| Specifications | |
|---|---|
| Manufacturer | Tandy Corporation, sold by Radio Shack |
| Type | Desktop personal computer (IBM PC compatible) |
| CPU | Intel 8088, 4.77 MHz |
| Memory | 128 KB, expandable with a memory card |
| Storage | One 5.25-inch 360 KB floppy drive |
| Display | PCjr-compatible video |
| Sound | Texas Instruments SN76496 |
| FPU / coprocessor | Socket for an Intel 8087 |
| Power | Internal 54 W switching supply (Tandy 8790070) |
| OS / Firmware | MS-DOS 2.11 |
| Predecessor | Tandy 1000 |
| Model no. | 25-1000A |
The Tandy 1000A (catalogue number 25-1000A) is the revised version of the original Tandy 1000. Tandy's service supplement for it says the new main logic board "was primarily implemented as a cost reduction of the original design". A two-layer board with 73 ICs replaced the original four-layer board, and three new custom chips took over the work of many standard logic parts.[1] The changes a user would notice were a socket for the 8087 numeric coprocessor, a sound volume control for service staff to set, and a software bit that turns the internal speaker off.[1]
History
[edit | edit source]Wikipedia describes the 1000A as a later revision of the original model that fixed bugs, scans expansion cards for bootable ROMs and adds a socket for an Intel 8087.[2] TVDog's model list gives it the same 4.77 MHz 8088 and 128 KB as the original, the catalogue number 25-1000A, and a production date of "1986?".[3] Tandy's service supplement carries a 1985 copyright.[1] In February 1986 the president of the Denver Metro MS-DOS Users Group started a bug report on the "new 1000A" and asked other owners to add to it; its entries run to September 1986.[4]
Wikipedia calls the Tandy 1000 HD essentially a Tandy 1000A with a hard disk fitted at the factory.[2]
Specifications
[edit | edit source]| Item | Detail |
|---|---|
| Board | Main Logic T1000 PC board, revision B, Tandy part 8709604B |
| CPU | Intel 8088 at U28, 4.77 MHz |
| Coprocessor | Socket U29 for an Intel 8087, sold by Tandy as option 25-1012 |
| RAM | 128 KB: sixteen 64K x 1 DRAMs, 150 ns |
| ROM | U51, listed as "128K ROM", Tandy part 8040328 |
| Clocks | 28.63636 MHz oscillator at Y2; 8 MHz crystal at Y1 |
| Video | Custom video array at U50 |
| Sound | SN76496 tone generator (U70), LM386 audio amplifier (U7) |
| Floppy controller | 8272 (U43) with FDC9216 data separator (U57) |
| Timer and interrupts | 8253-5 timer (U8), 8259A interrupt controller (U17) |
| Drive | One 5.25-inch floppy drive, Tandy part 8790127 |
| Power supply | 54 W, Tandy part 8790070 |
| International version | Adds a 0.25 A fuse (F1) and changes Y2 to a 28.500014 MHz oscillator |
Sources: the supplement's parts lists, and the TVDog model list for the clock speed.[5][3]
Changes from the original 1000
[edit | edit source]Custom circuits
[edit | edit source]The supplement includes a copy of the original 1000's schematic, number 8000226, marked to show the logic that each new custom chip replaces, followed by descriptions of the new chips.[1]
| Chip | Position | Tandy part no. | Replaces |
|---|---|---|---|
| Custom Address Array | U64, 68-pin PLCC in a socket | 8079013 in the parts list; 8079010 in the text | Video address generation (sheet 3), address decoding (sheets 5 and 6) and most of sheet 7 |
| Custom Timing Generator Array | U25, 40-pin, socketed | 8079011 | Memory timing, the 8284 clock generator and the system control signal logic (sheets 1 and 2) |
| Custom Keyboard I/F Array | U9, 40-pin, socketed | 8079012 | The 8255 and the keyboard control logic (sheet 8) |
The timing generator runs the 8088 in maximum mode, so its status signals and timing differ from the original board, and it contains a new arbiter that converts the bus HOLD and HLDA signals to the 8088's request/grant line. The address array has an extra address line and an I/O port at 3DD for 256K x 1 RAM chips, but the supplement says the board does not use the feature and that the port should not be changed. The keyboard array adds the speaker control: a 1 in bit 4 of I/O location 0061 turns the internal speaker off.[6]
Parts shared with the original
[edit | edit source]The supplement's assembly list gives the same Tandy part numbers as the original 1000 for the main chassis, the drive support, the option board panel, both reset buttons and the speaker, and it names the same 54 W power supply, 8790070. The case top and the fan have different part numbers.[5][7] The supplement has no sections on disassembly, the power supply or troubleshooting, so those come from the original manual; see Tandy 1000A General Maintenance.
Expansion and compatibility
[edit | edit source]The Tandy 1000-series FAQ makes these points about the 1000A:[8]
- Like the original 1000, it has no DMA controller on the main board. A memory expansion card supplies both the extra RAM and the DMA chip, and the memory must be expanded before a hard disk can be fitted.
- Its slots use IRQ 5 for vertical sync, so a hard disk controller must use IRQ 2. Unlike the 1000 SX and TX, the 1000A has no switch to free IRQ 5.
- It takes an 8087 in its socket; the original 1000 needed a daughterboard.
- It has no serial port, but a standard 8-bit serial card works.
- It cannot use VGA, because its BIOS does not scan for a video ROM or switch off the built-in video.
- It needs a Tandy-style keyboard; the interface is not the IBM standard.
- The latest BIOS for it, 01.01.00, is the version it shipped with.
- An NEC V20 in place of the 8088 works in about 70% of original 1000s and 1000As; the rest fail to boot or run erratically.
Known faults
[edit | edit source]- An owner reported in September 1986 that early 1000A boards had a 68-ohm resistor pack at RP12 where 33 ohms was intended, which could blur the picture on a CM-4 colour monitor.[4] The parts list gives RP12 as a 33-ohm 8-pin SIP pack.[5]
- GW-BASIC version 1.01 switched off all sound on the 1000A, and the sound stayed off after returning to DOS.[4]
- The power supply is the original 1000's 54 W unit, so its faults and tests are on Tandy 1000 Troubleshooting Guide.[5]
Details and fixes are on Tandy 1000A Troubleshooting Guide.
Documentation
[edit | edit source]- Tandy 1000A Service Manual Supplement (1985): introduction, descriptions and pin-outs of the three custom chips, parts lists, the component layout and schematics. File:Tandy 1000A Service Manual Supplement (1985).pdf
- The original Tandy 1000 Service Manual (25-1000) covers disassembly, the 54 W supply and the keyboard; see Tandy 1000 Documentation.
Related pages
[edit | edit source]- Tandy 1000A General Maintenance
- Tandy 1000A Troubleshooting Guide
- Tandy 1000A Capacitor Replacement Guide
- Tandy 1000
- Tandy 1000 HD
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 Tandy Corporation, Supplement to the Tandy 1000 Service Manual (Cat. No. 25-1000), covering the Tandy 1000A, 1985, hosted as File:Tandy 1000A Service Manual Supplement (1985).pdf. Section 1, "Introduction", p. 1.
- ↑ 2.0 2.1 "Tandy 1000", Wikipedia (retrieved 2026-10-22). Section "Tandy 1000".
- ↑ 3.0 3.1 Jeff Hayes, "models.txt", TVDog's Tandy 1000 archive (retrieved 2026-10-22). Model, processor, standard memory, catalogue number and approximate production date for each 1000-series model.
- ↑ 4.0 4.1 4.2 J. R. Applegate and others, "Tandy 1000 model A Bug Report", February to September 1986, TVDog's Tandy 1000 archive (retrieved 2026-10-22).
- ↑ 5.0 5.1 5.2 5.3 1000A Service Supplement, Section 3, "Parts List": Main Logic Main Assembly, p. 15; Main Logic Sub Assembly, pp. 16-20; Optional Parts, p. 21; Parts Added Or Deleted For International Version, p. 22.
- ↑ 1000A Service Supplement, Section 2, "Custom Circuits": Custom Address Array, pp. 3-6; Custom Timing Generator Array, pp. 7-12; Custom Keyboard I/F Array, pp. 13-14.
- ↑ Radio Shack, Service Manual: Tandy 1000 Computer, catalogue no. 25-1000, 1985, hosted as File:Tandy 1000 Service Manual (25-1000, 1985).pdf. Main Logic Parts List (continued), p. 99.
- ↑ Jeff Hayes, "Tandy 1000-series FAQ", version 2.52, 25 October 2005, TVDog's Tandy 1000 archive (retrieved 2026-10-22). Sections II.A.1, II.B.1, II.D.1, II.E.1, II.F.1, II.F.2, II.G.4, II.I.3 and III.E.2.