Hardware - Model 4/4P Parts

ROM

Tuc's Workbench is now selling a USB programmable ROM replacement. They write

The most flexible ROM replacement for your retro computer. It is highly configurable and low-cost. Get them fabbed at JLC for under $5 each in a quantity of 10 or more.

Based on a Raspberry Pi RP2350 (One ROM Fire 🔥) or STM32F4 (One ROM Ice ❄️).

One ROM is available as 24 pin and 28 pin versions, emulating nearly all systems' 24 and 28 pin ROMs, including any possible chip select configurations, and can serve different images to multiple ROM sockets simultaneously.

Replaces failed ROMs in Tandy, Commodore 64s, VIC-20s, PETs, Ataris, BBC, TIs, disk drives and many other types of systems.

ROM types replaced:

24 pin: 2704, 2708, 2716, 2732, 2316, 2332, 2364

28 pin: 2764, 27128, 27256, 27512, 23128, 23256, 23512

One ROM was formerly known as Software Defined Retro ROM (SDRR).

Key Features:
  • Based on sub-$2 microcontrollers. Choose from Raspberry Pi (Fire) or STM (Ice)
  • Fast enough for Tandy, PETs, VIC-20s, C64s, 1541s, IEEE drives, etc. Support faster systems with higher end microcontroller.
  • Same footprint as original ROMs - doesn't overhang the socket like other solutions.
  • USB version - no programmer required, program from your browser.
  • Quick Programming- build and flash in <10 seconds.
  • eflash in situ- no need to remove the ROM from the host when reprogramming.
  • Software configurable chip select lines - no hardware jumpers required.
  • Stores up to 16 rom images of different sizes and chip select configurations. Image selectable via jumpers.
  • Replace multiple roms with one a single One ROM can replace up to 3 original ROMs e.g. all of C64 kernel, BASIC, character set.
  • Dynamic Bank Switching - switch between ROM images on the fly, e.g. different char ROMs.
  • Hackable at runtime - change the ROM images being served and access telemetry from the ROM at runtime using AirFrog or other SWD probes.
  • Images combined automatically - no need to manually build up your own larger PROM image containing multiple retro ROMs.
  • Two layer PBC - component on single-side, limited BOM for low manufacturing cost/complexity
  • Open Source software and hardware

You can find the page at https://shop.tucs-workbench.com/products/one-rom-one-rom-to-rule-them-all.

Model 4/4D/4P Power Supply

Jay Newirth (of newSOFT) is selling a Model 4 power supply using modern power supply components while retaining the use of the existing wiring inside the various Model 4 computers.

This revision works for all Model 4's including the 4P, and has power for both a FreHD and a GoTek.

For more information contact Jay either at JAYNEWIRTH@VERIZON.NET or visit his website at NewSoft Store.

Video/Keyboard

Matt Boytim

Matt Boytim is developing a small board which can be used to replace the keyboard and/or monitor of a TRS-80 Model 3 and 4.

The project, which can be found at this GITHUB will permit using a PS2 keyboard and/or a VGA or HDMI monitor.

Matt has advised this is still very much a work in progress.


Jay Crutti

In addition, Jay Crutti has designed a full Model 4 keyboard replacement. You can get more information at https://www.jaycrutti.com/hardware-projects/tandy-trs-80-replacement-keyboards.

RAM

The Model 4/4D/4P uses 4164 chips. These are still generally available although you need to be careful of the source. Fake chips (or chips that will fail) are very much a risk, so it is important to source them from someone who has run them through a RAM tester. There are people who sell based on how much they have on hand, and you can always try your luck on eBay.

The following list was compiled by Patrick of Texas Tandy Restorations.

4164

ManufacturerModelRAM Type
FUJITSUMB82644164
MOTOROLAMCM6665B4164
MOSTEKMK4564N4164
NECuDP41644164
OKIDATAM3764N4164
OKIDATAMSM37644164
SAMSUNGKM4164B
Note: "A" will not work
4164
TANDY/MOTOROLA80406654164
TOSHIBATMM41644164

4116

ManufacturerModelRAM Type
FUJITSUMB8116E4116
MOTOROLAMCM4116BP4116
MOSTEKMK4116N4116
TANDY/MOTOROLA8041016A4116
TEXAS INSTRUMENTSTMS41164116
NECuDP4164116
AMDAM90164116

Feet

Jay Newirth has discovered that the EVERBILT 1001-687-347 Rubber Bumpers are an excellent replacement for the feet on a Model III/4/4D. Jay recommends using washers at the bottom to avoid the screws digging into the feet.