5MHz Upgrade hacks

Can we speed up the Model T? Apparently!

I have been investigating various ways to make the Model T run … faster.

Initially, I started with fixed 5MHz upgrades. These are practical if you never ever want to run at 2.5MHz. But, since 5MHz operation effectively renders the cassette port unusable, a switchable design might also be nice. Switchable means that you can control the speed from BASIC. In the M100/T102, port /Y1 is free to use. So, for switchable implementations

  Power up default is 2.5 MHz
  OUT144,0   sets to 2.5MHz
  OUT144,1   sets to 5MHz

Current Upgrades Status

Computer model	Fixed 5MHz	Switchable 5MHz
T102	V1.1 PCBs, instructions available	Pending new PCBs
M100	V1.1 PCBs, instructions pending	Pending new PCBs
M100 (UK)	V1.1 PCBs, instructions pending	Pending new PCBs
M100+NSC800	V1.1 PCBs, instructions pending	Pending new PCBs
T200	not yet investigated	not yet investigated
PC-8201	not yet investigated	not yet investigated
KC-85	like M100?	like M100?

As I test and more-or-less land on a reasonable upgrade process, I will post instruction files here If you have any questions, please get in touch with me at Twospruces at --the google mail service.

What are the tradeoffs?

When installed, your laptop will be running at 2x clock speed. However, the system clock remains unchanged.

  1.  Processor, memory, LCD, I/O. BCR are all running at 5MHz
  2.  The serial port, modem and printer ports are unchanged
  3.  Cassette port will not work correctly.  It will be 2x faster. (makes the case for a switchable solution).
  4.  5MHz draws more current, (2.5M = about 60mA, 5M = about 100mA), so shorter battery life.
      (but you are working faster!)

The switchable solution is a nice way to go. Switch up when needed, switch down when necessary.

What about compatibility with REX#/REXCPM?

REX# and REXCPM from November 2021 are built and tested compatible with 5MHz. In fact my test station runs at 5MHz. If you have an earlier REX#/REXCPM, contact me to discuss what to do.

What do you need?

  Some hardware skills - soldering, PCB modification etc.
  a clock divider circuit board
  a small number of parts (depending on the solution)
  a 9.803 HC49 crystal

Instructions

Take a look at the instruction files below. Please don't try this if you are not confident of success! I welcome feedback and comments. Especially if you find errors or improvements.

M100: I have a solution but it isn't simple enough yet. I have a small PCB coming that I hope will simplify the hack. M100 is more involved because the 8kB RAM module (stock) is too slow, and they cannot be sped up. In addition, the main ROM is super-slow, and simply cannot run at 5MHz. So, you need to replace the main ROM with a 27C256 + adapter board. The new PCB that is coming, is a special main ROM adapter board that also includes an SRAM to replace the slow stock SRAMs.

So, a bit more involved.

Fixed 5MHz Upgrades

Computer model	Version	Upgrade file
T102	Fixed 2x upgrade V1	T102 5MHz mod V1 Nov 2021.pdf
T102	Fixed 2x upgrade V1.1	T102 5MHz mod V1.1 Nov 2021.pdf

Switchable 5MHz Upgrades

I now have a working solution for a switched implementation. I will work on the instructions and post it when complete. Like the other upgrades I will have a board file at OSHPARK for those keen enough to try.

Photos

T102 with a fixed 5MHz board

T102 with a switchable 5MHz board

FAQ

As this is tried I'm sure questions and comments will arise. We can capture useful information here. Thanks.

coming soon - M100 fixed upgrade, software switchable solutions