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TRS-80 Model 100

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TRS-80 Model 100
Radio Shack TRS-80 Model 100 with case (Tekniska museet)
TRS-80 Model 100 with its Radio Shack carrying case (photo: Anders Folkeson, Tekniska museet, CC BY 4.0)
Specifications
ManufacturerKyocera, for Tandy Corporation (sold through Radio Shack)
TypePortable computer
ReleasedApril 1983
CPUOki MSM80C85A (80C85), 2.4576 MHz
Memory8 KB static RAM (26-3801) or 24 KB (26-3802), expandable to 32 KB in 8 KB packs
StorageCassette interface at 1,500 bit/s
Display240 x 64 dot LCD, 40 x 8 characters
SoundBuzzer
ROM32 KB masked ROM (LH535618); socket for a 32 KB option ROM
PowerFour AA alkaline cells or an external adaptor (5.5 mm plug, centre negative)
Dimensions11-4/5 x 8-4/9 x 2 in
Weight3 lb 13.5 oz
OS / FirmwareBASIC, TEXT, ADDRSS, SCHEDL and TELCOM in ROM
Model no.26-3801, 26-3802

The TRS-80 Model 100 is a battery-powered portable computer that Tandy sold through Radio Shack from April 1983. Kyocera made it; in Japan it was sold as the Kyotronic 85, and the Olivetti M10 and NEC PC-8201 were built on the same Kyocera design.[1] It has a 71-key keyboard, an eight-line LCD, a built-in 300 bit/s modem and its programs (BASIC, TEXT, ADDRSS, SCHEDL and TELCOM) in ROM. Files live in battery-backed RAM, kept alive by a rechargeable cell on the main board when the AA cells are out.[2]

Radio Shack sold two versions. The 26-3801 has 8 KB of RAM, one RAM pack in position M9; the 26-3802 has 24 KB, with packs in M9, M8 and M7.[3]

Specifications

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TRS-80 Model 100 specifications
Item Detail
CPU MSM80C85ARS (M19), code and pin compatible with the 8085; 4.9152 MHz crystal X2, 2.4576 MHz at the CLK pin
ROM 32 KB standard (LH535618, M12); a socket under a cover in the base takes a 32 KB option ROM
RAM 8 KB standard, to 32 KB in 8 KB packs; each pack is four 2 KB static RAMs on a ceramic carrier
Display 240 x 64 dot LCD, 1/32 duty; ten HD44102 segment drivers and two HD44103 back-scan drivers on the LCD board
Keyboard 71 keys in a 9 x 8 matrix
Clock µPD1990AC calendar clock (M18) with a 32.768 kHz crystal
Interfaces RS-232C (DB-25, to 19,200 bit/s); 300 bit/s modem (8-pin); cassette (8-pin, 1,500 bit/s); Centronics printer (26-pin); bar code reader (9-pin); 40-pin system bus socket
Power Four AA alkaline cells, five days at four hours a day; adaptor socket 5.5 mm, centre negative
Memory backup Rechargeable battery on the main board, trickle-charged; about 40 days with 8 KB, about 10 days with 32 KB

Sources: the specifications and connector list,[2] the clock figures in the troubleshooting guide,[4] the LCD board parts list,[3] and the memory description.[5]

Hardware

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Boards

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The case holds three boards: the main board in the bottom half (marked PLX110CHIX in the service manual's board drawing), and the LCD board and keyboard in the top half, joined to the main board by cables.[6][7] All the electrolytic capacitors are on the main board; the LCD board uses ceramic chip capacitors.[3]

Power supply

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The logic runs on +5 V (VDD) and −5 V (VEE) from a DC/DC converter on the main board: transformer OT2 driven by transistors T21 and T22, rectified by D13 and D15 and smoothed by C84 and C85. A third rail, VB, is held up by the backup battery when the computer is switched off, at 2.0 to 4.0 V, and keeps the RAM and the calendar clock running.[4][8] A low-battery LED lights when the supply from the AA cells or adaptor falls to 4.1 V ±0.1 V, and the converter is shut down at 3.7 V ±0.1 V.[9]

Memory backup battery

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The memory backup cell is a nickel-cadmium battery, part P-36, listed as "Battery Ni-Cad 3-51FT", Radio Shack part ACS-0100.[3] In 1990 Tony Anderson described it on CompuServe's Model 100 forum as a stack of three 1.2 V button cells in shrink-wrap, lying on its side behind the RS-232 port.[10] Jeff Birt's 2018 parts sheet gives the original as a Yuasa 3-51-FT-A, 3.6 V, 50 mAh.[11] The MEMORY POWER switch on the base disconnects it; with the switch off the computer will not run and the RAM is not backed up.[2]

Known faults

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  • The original Ni-Cad memory battery and the main board electrolytic capacitors leak with age. Owners on the Model 100 mailing list report electrolyte under C82, C85 and the four 10 µF capacitors C49, C50, C54 and C55, and recommend replacing the original battery before it leaks.[12][13] See the TRS-80 Model 100 Capacitor Replacement Guide.
  • On several machines reported on the Model 100 mailing list, a faint or blank screen on a computer that still beeps came from leakage at C82 and C85: a low −5 V rail, or a damaged trace under C82 that runs to the contrast control.[14][15] See the TRS-80 Model 100 Troubleshooting Guide.
  • A machine that sat unused with a flat backup battery can appear dead until the battery is charged and the RAM cleared with a cold start.[16]

Documentation

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  • Radio Shack TRS-80 Model 100 Service Manual: Radio Shack's 144-page service manual for catalogue numbers 26-3801 and 26-3802, with theory of operation, troubleshooting flowcharts, parts lists, schematics and board drawings.
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References

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  1. ↑ Wikipedia, "TRS-80 Model 100" (retrieved 2026-10-21). Introduction in April 1983; Kyocera manufacture and the Kyotronic 85; the Olivetti M10 and NEC PC-8201 on the same platform.
  2. ↑ 2.0 2.1 2.2 Radio Shack, Service Manual: TRS-80 Model 100 Portable Computer, catalogue numbers 26-3801/3802, Section I, pp. 1-3 to 1-7: the five built-in programs, specifications, connectors and switches, and the MEMORY POWER switch for the Ni-Cad backup battery. Hosted as File:Radio Shack TRS-80 Model 100 Service Manual (26-3801, 26-3802).pdf.
  3. ↑ 3.0 3.1 3.2 3.3 Service Manual: TRS-80 Model 100, Section VI, parts lists, pp. 6-3 to 6-12: main board capacitors (pp. 6-3 and 6-4), integrated circuits with the note on which RAM packs the 26-3801 and 26-3802 carry, the miscellaneous parts including the Ni-Cad memory battery P-36 (p. 6-11), and the LCD board parts, all chip capacitors, with drivers LM1-LM10 (HD44102) and LM11-LM12 (HD44103) (pp. 6-11 and 6-12).
  4. ↑ 4.0 4.1 Service Manual: TRS-80 Model 100, Section V, Troubleshooting, pp. 5-1 to 5-12: the troubleshooting flowchart, the checks for each of the 13 functions (power and DC/DC converter p. 5-4, LCD p. 5-5, keys and buzzer p. 5-6, clock and reset p. 5-7, memory protection and printer p. 5-8, cassette and bar code reader p. 5-9, RS-232C p. 5-10, modem p. 5-11) and the check list and test program (p. 5-12).
  5. ↑ Service Manual: TRS-80 Model 100, Section IV, Theory of Operation, pp. 4-2 and 4-3: the 80C85 CPU, the RAM packs (four 2 KB RAMs each, standard pack M9, expansion in M6, M7 and M8) and the 32 KB ROM.
  6. ↑ Service Manual: TRS-80 Model 100, Section II, Disassembly/Reassembly, pp. 2-1 and 2-2: four case screws, the case halves held by snaps, the LCD and keyboard connectors, the buzzer connector on the LCD board, five keyboard screws, four LCD board screws and seven main board screws (M3 x 8).
  7. ↑ Service Manual: TRS-80 Model 100, Fig. 7-3, "Main P.C.B. - Component Side", p. 7-3. Rendered as File:TRS-80 Model 100 main board component side (Service Manual Fig. 7-3).png.
  8. ↑ Service Manual: TRS-80 Model 100, Section IV, "Clock Control Circuit", p. 4-15: VB is applied to the µPD1990AC (M18), so the clock runs with the power switch off.
  9. ↑ Service Manual: TRS-80 Model 100, Section IV, "Power Supply and Auto Power Off Circuit" and "Reset Circuit", pp. 4-32 to 4-34: the DC/DC converter (OT2, T21, T22, D13, D15, C84, C85), VDD feedback through T13, D4, R121 and C92, VEE stabilised by R97 and D14, the 4.1 V and 3.7 V low-power thresholds, the ten-minute automatic power-off, and the reset delay set by R103 and C78.
  10. ↑ Tony Anderson, reply in the thread "Model 100 Memory battery", CompuServe Model 100 SIG, February 1990, captured as NICAD.THD (Library 7), Living M100SIG archive (retrieved 2026-10-21). Community source. The cell is three 1.2 V button cells in shrink-wrap, behind the RS-232 port in the Model 100; Radio Shack part ACS-0100; float-charged from the AA cells or adaptor.
  11. ↑ Jeff Birt, TRS-80 Model 100 Parts List Updated, capacitor map and order list, 27 May 2018 (retrieved 2026-10-21). Community source. Positions of the 17 main board electrolytics on the service manual's board drawing, with values, sizes and Nichicon equivalents; the original memory battery as a Yuasa 3-51-FT-A, 3.6 V, 50 mAh.
  12. ↑ Jeffrey Birt, "New member - question on 'half' alive Model 100", M100 mailing list, 29 April 2018 (retrieved 2026-10-21). Community source. A bad Ni-Cad battery removed; C82, C85, C49, C50, C54 and C55 replaced because all had started to leak.
  13. ↑ Brian White, "Re: Model 100 inspection/repair services", M100 mailing list, 9 February 2019 (retrieved 2026-10-21). Community source. Check inside for corrosion and replace the original Ni-Cad before it leaks, or clean up after it if it already has.
  14. ↑ Eric LK, "Need help fixing VEE on M100", M100 mailing list, 21 March 2021 (retrieved 2026-10-21). Community source. Very weak LCD contrast cured by replacing C82 and C85, which restored −5 V on VEE; VEE back down to −3 V six months later.
  15. ↑ George Hines, "Non working display on my m100", M100 mailing list, 9 July 2019 (retrieved 2026-10-21). Community source. C82 and C85 had leaked; a trace under C82 running to the contrast control was damaged, and patching it restored the screen.
  16. ↑ "Recovering an Unresponsive Laptop", Bitchin100 DocGarden wiki, last edited 25 September 2013 (retrieved 2026-10-21). Community source. A flat Ni-Cad, RAM in a random state and the contrast knob mis-set; overnight charge, memory power off for about 15 minutes, then a cold start.


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