TRS-80 Model 1k - Hardware Replacement Main board - Evolution (Rev K)
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)
- Drop in (mostly) replacement main board for existing TRS-80 Model 1
- Modernised using larger density (memory) components to reduce component count
- Onboard some common features, that required a piggyback board's or piggyback IC's
- Provides some level of additional internal expansion, or ability to evolve the board itself.
- Remains faithful to the use of traditional thru-hole parts, and discrete logic IC's
- Is not compatible with 32Kb external RAM expansion, external RAM will need to be disabled
- Is not compatible with Model 1 unregulated power supply, requires regulated 5V power
- Is not a faithful recreation of the original Model 1, other projects exist for this.
The board started with the Japanese board as a base, because it is slightly upgraded over the US board with some notable improvements that carry over:
- The Video sync has been upgraded with improved stability, and 50Hz support
- The cassette input has minor improvements in the A/D design.
Unfortunately this means the Japanese Service manual has be used ss a base for the basic schematic and component ID's as they are quite different than the US Model 1.
The Japanese "specific" features have been removed to align this back to being a US Model 1. This includes the keyboard and Japanese Kana character set support.
This system is intended to be used (almost) exclusively with CMOS technology utilizing 74HCT (CMOS TTL compatible) parts. Noting traditional 74LS components should work, however this has not been tested
A double speed (3.56 Mhz) clock speedup mod has been applied. A jumper (J18) allows the clock to be slowed to normal. This could be by external switch, or any future circuit potentially under software control. See U1, U2 in schematic. Note: If the cassette motor is turned on the speed will automatically be slowed.
The use of a modern CMOS Z-80 CPU is preferable
Uses a standard 27xxx EPROM (or 28256 EEPROM) rather than mask ROM. These are more readily available and easier to program. The two ROM's (Z42, Z43) have been replaced with a single 28 pin EPROM (U42) supporting 2x128 thru 2x512 chips. The board has jumpers to configure the ROM type, and/or 16kB page used for larger ROMS
An additional set of jumpers allows the ROM to provide an addition 1 or 2kBytes in the memory address above the ROM (0x3000 - 0x37FF), allowing a total of 12,13, or 14 kBytes of ROM. This can be useful for ROM extensions, commonly used on some Model 1 peripherals, or the Model 3.
The 4116 16kb DRAM has been replaced with a single static RAM chip (AS6C1008) U15 providing 48KB of RAM without the need for an external expansion interface or other add on card. Note this chip is 128 kBytes in size, but only 48kB is being used. No provision for RAM paging has been made.
If you intend to connect an expansion interface then the RAM in the interface will have to be disabled to prevent conflict.
All character generation (both alpha characters and graphics) has been replaced with a single character set that includes all 256 characters defined in ROM (U37). Each character defines the full 6x12 pixel matrix that is generated, and requires 16 bytes (only 12 are used) per character, for a total of 4k bytes for the entire character set.
Multiple character sets can be defined (depending on ROM size), the character set chosen is configured by Switches SW1. Since characters sets can control all lines of the raster true lower case descender's can be defined i.e. Gendon3. The fonts are compatible with Glens Stuff TRS-80 Model 1 Clone.
Normal font ROM's are not compatible.
Composite video output is via standard a RCA connector or the original DIN plug, you can solder either onto the board. The RCA connector is provided to be compatible with a larger range of external monitors, potentially more reliable and less subject to noise.
The main board has an internal 40pin Header (J20) identical to and located just behind the main 40 pin expansion port. This is primarily designed for an internal expansion board which sits inside the case. The primary advantage being compactness. A optional second connector provides +5V in a four pin header (J21) for powering a internal expansion board.
Routing an external 40 pin ribbon cable to the connector is also possible, bypassing the 'unreliable' card edge connector.
Also provided for expansion are:
- Pin Header (J13) for internal audio amplifier, taken from cassette output
- Pin Header (J14) for internal reset signal, mirroring the external reset switch
- Pin Headers (J15,J16) are for internal 5V power for any other devices
- Pin Headers (J17) for internal (un-switched) +5V (both pins) power, for any devices requiring constant power
NOTE: The DRAM multiplexing signals CAS, and MUX are no longer generated, and have be removed on the main expansion connector.
A prototyping area is self explanatory, it has 12 chip supporting 14 or 16 pin power and locations for decoupling capacitors. This was included to add minor new features without the fragility of piggy back boards, or IC's
Replacing the RAM means the computer does not requires 12V and -5V power, only 5V is required. This has driven the transformation of the power system for the computer by moving the power regulation off the PCB itself, and instead requires a external 5V DC regulated power supply. The external TRS-80 power brick is no longer supported. Power connection is by the of a standard barrel jack replacing the DIN connector.
This leaves substantial PCB real-estate, which is available for future use and evolution of this board
The Power (and Reset) buttons have been superseded with more available standardized components. The original component footprints have still been left so can still install the original parts.
Additional changes have been made too numerous to mention please see Changelog for details
See the separate Status and Future
See the separate Builders Guide for complete technical documentation
- The Board Folk Ver 1.1 Japanese board without which this project would not be possible.
- Glens stuff TRS-80 Model 1 clone, of which some inspiration was taken.
- VCF Forums for some help and guidence