I started off looking at the TRS80 but discovered a superior clone called the LNW80. The TRS80 only had monochrome text display, the LNW80 had pixel graphics and colour. So it seems a good idea to use the LNW80 circuit as a start.
Circuit Diagrams available on the net as scan
of hand-drawn circuit.
http://pilot.ucdavis.edu/trs80/tr03000.html
by Chipp Davis
Full technical manual and better quality circuit diagrams at:
http://www.xs4all.nl/~fjkraan/comp/lnw/index.html
by Fred van Kraan
The TRS80 should
be relatively easy, since it was entirely implemented in TTL
chips and off-the-shelf LSI.
The LNW80 is a
little more complex but much of the colour modulation circuitry
can be discarded
CPU
This a Z80, and the LNW80 technical manual reports it runs at 16MHz / 9 = 1.777 (recurring).
The LNW80 can be switched between normal (1.77 MHz) and fast (4MHz) operation.
Video
Video is 64x16 text, or 128x48 block graphics (3x2 blocks in a
character cell), in 1K video RAM.
The machine included a monitor that was essentially a TV with the
RF sections stripped out. If so, then the video signal should be
easy to adapt for the composite video input of a modern TV. If
the display rate may be fixed at NTSC frame rate of 60Hz, this
may only suit NTSC televisions.
The LNW80 has additional graphics and colour.
A 50Hz frame rate modification would diverge from the original standard but would allow PAL TVs to be used.
Keyboard (Click for details)
I/O
The 1771 FDC is single-density only.
Serial
The 1602 UART is used.
Audio
None.
Misc
Both machines have the fairly unusual feature of having buffers between the CPU bus signals and the main bus signals. This allows the CPU to be isolated from the rest of the machine so that another processor can take over the machine - for example for testing. This may actually be the primary reason, because the signal that tri-states the CPU signals is called /TEST. The buffers are not essential for doing this - asserting the /BUSRQ signal should have the same result - but the buffers do allow the CPU to drive the signals over a longer distance as might be the case if a lot of expansion boards were fitted.
There is also further buffering to drive the memory devices. There are sockets for six 2K ROMs. If replaced by a single 16K ROM, the memory address buffers could be discarded.
The TRS80 had eight 4K x 1-bit DRAM chips fitted by default. The LNW80 used 16K x 1-bit chips.
The UART and FDC were not part of the motherboard, and resided on a system expansion board.