I bagged nine OKI 80C85AH chips for just £8 on eBay. Rated 5 MHz clock on data sheet.
Original intent was to base architecture on the Tandy Model 100, early laptop which ran for 20 hours on four AA cells.
Four NiMH cells = 600 to 2850 mAh at 50mA drain, x 1.2 volts = 0.72 to 3.42 Watt-hours.
This laptop claims 10.5 hours from a 32.5 WH battery
So a model 100 should run 9.5 to 45 times longer!
The Model 100 uses an 81C55 for parallel I/O and a 6402 UART for serial I/O. I had six of the latter, but none of the former.
The Model 200 uses an 82C51A for serial I/O. This is not the chip used in the IBM PC (i.e. the 8250).
The 81C55 used the timer for baud rate control. If I use a UART with internal baud rate generation (the 8250) then I only need parallel I/O for the keyboard. A 74HCT574 and 74HCT245 should do. Later UARTs had 16-byte buffers (16C550) and printer ports (16C552).
The tape interface can be safely abandoned. Solid state storage is widely available.
Farnell sell the 4-channel, no LPT version in LQFP (0.5mm pitch):
and the 2-channel 1-LPT version in PLCC68 for £7.30:
Bought an MSM81C55 chip. I may not use it, but it will be there if I want to experiment.
Bought a ready-made PCB: http://www.malinov.com/Home/sergeys-projects/minimax8085
This will avoid a lot of wiring work. Since it is a ready-made computer already, I need to improve it.
The 8251 is primitive and there is no user I/O for this board. So I shall use an 16C552 chip, which has two serial ports and a parallel I/O port.
U1 80C85 CPU socket
U3 ROM socket
U4 32K RAM socket
U5 GAL16V8 socket
Fit links for 32K EEPROM
Fit 5V inlet
Fit 10MHz crystal (near enough ideal 9.8304 MHz to work)
Order null-modem adaptor for neater solution in future
Create null-modem adaptor to get going this weekend
Check board works with 8251 serial port and unmodified software
Success, it runs Tiny BASIC, at 19200 baud. I will replace that with the dual monitor-BASIC ROM.
Replace 10 MHz crystal with 9.8304 MHz.
Modify hardware and firmware to use 16C552 chip
Build 16C552 board with serial and parallel ports.