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

http://store.hp.com/UKStore/Merch/Product.aspx?id=Z5F90EA&opt=ABU&sel=NTB

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).

https://en.wikipedia.org/wiki/Intel_8251

https://en.wikipedia.org/wiki/Intel_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).

http://www.ti.com/lit/ds/symlink/pc16552d.pdf

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):

http://uk.farnell.com/texas-instruments/tl16c554aipn/async-comm-element-1mbps-lqfp/dp/2335690

and the 2-channel 1-LPT version in PLCC68 for £7.30:

http://uk.farnell.com/texas-instruments/tl16c552afn/async-comm-element-1mbps-plcc/dp/2335689

Bought an MSM81C55 chip. I may not use it, but it will be there if I want to experiment.

2019-11-10

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.

2020-04-25

Fitted:

U1 80C85 CPU socket
U3 ROM socket
U4 32K RAM socket
U5 GAL16V8 socket
U6 74HCT573

2020-04-25

Fitted:

Fit links for 32K EEPROM
Fit 5V inlet

2020-06-06 weekend

Program GAL 
Program EEPROM
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.

To do:

Replace 10 MHz crystal with 9.8304 MHz.
Modify hardware and firmware to use 16C552 chip
Build 16C552 board with serial and parallel ports.