KiwiSinceBirth has released v2.0a of his redesigned Model I motherboard. He reports the following changes:
This project is an upgraded / evolved TRS-80 Model 1 main board replacement. It was designed to provide a reliable, compact, and modern replacement, removing some limitations of the original 1970’s product, while still remaining faithful to the original technology (i.e. no emulation)
v2.0a provides significant improvements in video, not only does it use dual port RAM for snow free video, but it changes the video sync generation circuit bringing back the picture adjustment controls not present in V1. Also supported is software switchable fonts, for alternate languages or other graphics. Other major features added to the board include an audio amplifier for a small speaker (which can be mounted on the board), Alpha Joystick port compatible with most software, and ability to mount a modern clock oscillator. Have added about 10 new config jumpers/settings in support of new features. V1 is still fully supported including a separate build guide. Not forgetting Supermem bank RAM and internal FreHD
New Features
- Dual port video SRAM, reducing contention (snow) issues with shared CPU access.
- Onboard Audio amplifier utilising small class D module, with space for PCB speaker
- Alpha Joystick port with header to connect 9 Pin DB9 Connector
- Changed crystal oscillator to parallel resonant circuit based on 74HCU04 chip, which is pin compatible with a Full Can (DIP14) oscillator.
- New Video sync generation circuit, including Horizontal and Vertical Position.
- Improved support for RGBtoHDMI by exposing HSYNC on Video DIN socket
- Added Tim Halloran’s no-chip VBLANK modification, for improved video updating.
- Added Jumper (JP16) to allow main Reset button to function as full CPU reset.
- Can configure either RAM or ROM to occupy the 12kB to 14kB address space in memory.
- Added support for EEPROM in circuit writing (requires software support).
Changed Features
- The Video DIN socket now has Jumper (JP19) to disable external power
- Added PCB reset button (SW11) to allow full reset during diagnostics
- CPU speedup offer fast speed as 5.3 (default 3.55) mhz via jumper (JP17)
- Added Power LED indicator (D1) near to the main power switch.
- Fixed issue with TEST signal immediately disconnecting CPU from BUS – Credit Marcel Erz
- Fixed Issue with Bus control for interrupt Mode 0 and 2 where external device places data on bus
- Added (back) software character set control (Port FF Bit 7), as per JP board.
- Changed meaning of SW10-SW13. Now ON => Logic 1, and OFF => 0 (rather than reverse)
- RAM is configurable from $48k (default) down to 16k for compatability with external RAM.
Minor Improvements
- Moved main 10.6Mhz oscillator to central location, shorter signal paths.
- CPU clock speed selector flip-flop (Z63a) clocked at slowest rate.
- JP10 (and related circuit) used for video frequency was removed.
- Inputs of spare gates are now tied to GND or VCC, with easy trace cuts for future use.
- Added resistor pull-ups on address, data, and control busses, for CMOS stability.
- Added resistor pull-ups on keyboard, prevent issues when no keyboard attached.
- Provided ability to use spare IO pins (CAS and MUX) on IO expansion.
- Moved main power switch (SW1) and socket (J11) closer to other ports, better clearance
- Added several M3 mounting holes for internal expansion board mounting
- Improved (more modern) footprints for C19, C70, Q1, Q2, CR4.
- Replaced several discrete resistors with small resistor packs.
- Fixed multiple issues with small via’s and thermal reliefs on power rails
- Main power Capacitor C101 now has more space for horizontal mounting.
- Improved silkscreen for component identification, and configuration options
- Improved component identification in Bill of Materials
- Major improvement in the Schematic diagram quality.
- Many component designators have changed from V1, so some V1 docs may not be accurate
- Added testpoints to support 3 signals required by SuperMem
- Changed main power capacitor C11 to 470Uf after testing from MSly
- Changed main power switch to discharge C11 (using 100R R14) when switched off
- Keyboard resistor pack RN5 corrected to pull down keyboard data lines.
- Corrections to Bill of Materials, and silkscreen fixes and improvements.
You can find all the information, as well as downloads for the BOM and circuit board, at github.com/kiwisincebirth/TRS-80-MODEL-1-K/tree/main