The Tandy 1000s, especially the older ones, are rather reliable as non-IBM manufactured PCs go. They tend to continue working year after year. But inevitably they will fail, and something they cannot be easily fixed because a chip went bad. Tandy was good enough to socket major chips in most of its machines (DIP and PLCC) and frequently reused chips from machine to machine. Tandy sometimes would use off the shelf chips, sometimes custom chips and sometimes programmable logic. In this blog article I will identify the major chips each system uses in order to help people find suitable replacements, if possible.
Before we begin, it should be noted that even though a chip may be deemed "off the shelf", that does not necessarily mean that it may be easy to replace. Chips from Intel, AMD, Motorola, NEC and Texas Instruments are relatively common.
Intel 8088 CPUs (and clones) can be exchanged for an NEC V20 for improved performance and Intel 8086s can be similarly exchanged for an NEC V30.
I have not included the TL/2, TL/3 or any of the RLs, RLXs or RSXs due to a lack of technical documentation for these systems.
Tandy 1000 (25-1000)
The original Tandy 1000 had two custom chips:
- Parallel Port Array (U108), 8041087
- Video Array (U76), 8079000
Off the shelf chips:
- Intel 8088 CPU (or equivalent)
- Intel 8237 DMA Controller (on upgrade board)
- Intel 8250 UART (on upgrade board)
- Intel 8253 PIT
- Intel 8255 PPI
- Intel 8259 PIC
- Intel 8284 Clock Generator
- Motorola 6845 CRTC
- NEC µPD765 or Intel 8272 FDC
- SMC FDC9216 Floppy Data Separator
- Texas Instruments SN 76496 PSG
This computer has four 82S153 programmable logic array chips on the mainboard (U46, U53, U80, U103). The base Tandy 1000 only had 128KiB on the mainboard, and originally Tandy sold two memory boards to increase the memory up to 640KiB. There are two 82S153 programmable logic array chips on the 256K Memory Expansion Board (U5 & U9) (25-1004), which adds DMA and can bring the system to 384KiB of RAM. The DMA chip is a standard Intel 8237 DMA Controller. There is one 82S153 programmable logic array chip on the 512K Memory Expansion Board (U5) (25-1009) which can bring the system to 640KiB of RAM. Finally there is one 82S153 programmable logic array chip on the RS-232 (U5) upgrade board (25-1006).
You can use a generic PC/XT memory upgrade board for the DMA-less 256K Memory Expansion Board if you have the DMA equipped board installed. The DMA chip is necessary because it provides the refresh signals for the DMA on the ISA memory cards. Mainboard DRAM refresh is handled by the video controller chip.
Tandy 1000 HD (25-1001)
The Tandy 1000A and 1000 HD consolidated several of the above off the shelf components into custom integrated circuits to reduce chip count and routing complexity compared to the original Tandy 1000A. The mainboards are identical between the two model designations. The custom chips on these boards are:
- Parallel Port Array (U66), 8041087
- Video Array (U50), 8079001 (may be a replacement for 8079000 and vice versa)
- Address Array (U64), 8079010 or 8079013 (replaces Motorola 6845 CRTC)
- Timing Generator Array (U25), 8079011 (Intel 8284 Clock Generator)
- Keyboard I/F Array (U9), 8079012 (replaces 8255 PPI)
There are two 82S153 programmable logic array chips (U5 & U9) on the Memory PLUS Expansion Board (25-1011) included with the Tandy 1000 HD, which adds DMA and up to 512KiB of RAM and was available as an upgrade for the 1000 and 1000A. The DMA chip is a standard Intel 8237 DMA Controller.
Tandy 1000 EX (25-1050)
Tandy 1000 HX (25-1053)
Tandy 1000 SX (25-1051, 25-1052 & 25-1054)
These computers share the same custom chips:
- "Big Blue" Video Controller (U28/EX, U30/SX, U31/HX), 8040684
- DMA Array, 8075711 (U29/SX, on upgrade board for EX & HX (U13), replaces Intel 8237 DMA Controller)
- "Light Blue" Timing Control Generator (U16/EX, U51/SX, U20/HX), 8075306
- Printer Interface (U32/EX, U55/SX, U37/HX), 8075068
- Keyboard Interface (U10/EX,U22/SX, U13/HX), 8075069
Off the shelf chips:
- Intel 8088 CPU
- Intel 8087 NPU (on SX only, optional)
- Intel 8250 UART (on upgrade boards)
- Intel 8253 PIT
- Intel 8259 PIC
- NEC µPD765 or Intel 8272 FDC
- SMC FDC9216 Floppy Data Separator
- Texas Instruments SN 76496 or NCR 8496 PSG
The EX and HX do not have a socket for a math co-processor and require DMA to be added by a special Memory PLUS Expansion Adapter (25-1062). The sound IC may be either a TI SN 76496 PSG or an NCR 8496 on the SX or EX but will always be an NCR 8496 on an HX.
These computers have one 82S153 programmable logic array chip on the mainboard (U3 on EX, U16 on HX, U18 on SX). The RS-232 Serial Board for the EX and HX (25-1014) also has an 82S153 (U5). As the EX and HX have keyboard built-in they have an 8048 microcontroller on their mainboards with internal ROM (U6/EX, U9/HX). These 8048s should be identical to the ones in the detachable Tandy 1000 keyboards.
Tandy 1000 TX (25-1600)
This computer has the following custom chips:
- "Big Blue" Video Controller (U36), 8040684
- "Night Blue" Timing Control Generator (U52), 8040810
- Printer Interface (U69), 8075068
- Keyboard Interface (U17), 8075069
- Floppy Disk Support Logic (U64), 8041401/8041404
- DRAM/DMA Control (U53), 8040142
Off the shelf chips:
- Intel 80286 CPU
- Intel 80287 NPU (optional)
- Intel 8237 DMA Controller
- Intel 8250 UART
- Intel 8254 PIT
- Intel 8259 PIC
- NEC µPD765 or Intel 8272 FDC
- NCR 8496 PSG
- CHIPS 82A205 286 Signal Buffer
This computer has one PLS173 programmable logic array chip on the mainboard, 8075173 (U19).
Tandy 1000 TL (25-1601)
This computer has the following custom chips:
- Tandy Video II (U19), 8079020
- "Night Blue" Timing Control Generator (U17), 8040810
- Parallel, Serial, Sound & Joystick (PSSJ) (U11), 8079021/8079036
- Keyboard, Floppy, Interrupt, & Timer (KFIT) (U13), 8079019
- Floppy Disk Support Logic (U15), 8041401/8041404
- DRAM/DMA Control (U22), 8040142
Off the shelf chips:
- Intel 80286 CPU
- Intel 80287 NPU (optional)
- Intel 8237 DMA Controller
- NEC µPD765 or Intel 8272 FDC
- CHIPS 82A205 286 Signal Buffer
- Dallas DS1215 RTC
This computer has two PLS173 programmable logic array chips on the mainboard, 8077173 (U42) and 8076173 (U44). The Tandy Video II chip is a 100-pin QFP chip soldered to the mainboard.
Tandy 1000 SL (25-1401)
Tandy 1000 SL/2 (25-1402)
These computers have the following custom chips:
- Tandy Video II (U26), 8079020
- "Buffer Blue" Timing and DMA Control (U41), 8079024/8079035
- Parallel, Serial, Sound & Joystick (PSSJ) (U40), 8079021/8079036
- Keyboard, Floppy, Interrupt, & Timer (KFIT) (U30), 8079019
- Floppy Disk Support Logic (U18), 8041401/8041404
Off the shelf chips:
- Intel 8086 CPU
- Intel 8087 NPU (optional)
- NEC µPD765 or Intel 8272 FDC
- Dallas DS1216E RTC (optional)
These computers have one Programmable Array Logic 16R4 chip (U136). The Tandy Video II and Buffer Blue chips are 100-pin QFP chips soldered to the mainboard.
All Tandy 1000s mentioned before, except for the TL, can be upgraded with the Tandy Smartwatch, which is a Dallas DS1216E RTC module. The SL and SL2 have an empty socket at U13 for the Smatwatch, the earlier systems must install it underneath a ROM chip. See Page 33 of Tandy 1000 Tech Notes and & Jumper Manual Volume 1 for the correct location for your system.
Bonus: IBM PCjr,
The IBM PCjr. has one custom chip, the video gate array (1503730/L1A0091) at ZM36. It also has two PROMs acting as logic, a TBP28L22 at ZM52 and a TBP24S10 at ZM56 (The IBM PC/XT also has a TBP24S10 PROM acting as logic at U44.) The PCjr uses a Texas Instruments SN 76489 sound chip but otherwise will have the same off the shelf chips on the mainboard or on an internal expansion card as the Tandy 1000 model 25-1000 has, DMA excepted. Nothing, except for the CPU, is socketed on the mainboard. Even the CPUs were soldered in later runs of the mainboard.
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