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BEU - Bus Expansion Unit

When Timex released its Technical Manual for the TS2068, we learned how to add memory to and switch be­tween its three internal memory banks. This was won­derful stuff, and it's given us many excellent TS2068 products long after the "profane world" thought that the Timex computer was dead. Still, if we think back, we may remember that Timex originally promised 256 banks. In addition to the Home, Dock, and EXROM banks (which we we'll call the "standard banks"), there would also have been the provision for special enhancements that we'll refer to as "expanslon banks".

Wes Brzozowski

How to build BEU?

After reading Wes articles, I could say the easiest way to make BEU bankswitching is to hire 8951 CPU and control 8 BEU units -- Dock-like slots (36pin), where ordinal Dock cartridges are plugged in. Some next 8 pseudo-units can be created to use extra features of 8051, as serial interface.

There must be an input latch (74LS574) to remember recent "memory port write" (storing A13..A15 and D0..D3) for the 8051 to process.

The 8051 output part to manage several cartridge slots can be 8×4b latches (two 74LS670 - for each chunk a separate 4b latch), where bits Q0..Q2 select slot and bit Q3 tells if it should be activated. Latches are written by 8051, according commands. Latch outputs go to two decoders (74LS138 -- one for /MREQ, second for /IORQ) and select one cartridge slot.

Another buffer (74LS670) can be used to give Z80 possibility to "memory port read". The 8051 fills all 4 registers after processing the command.

To do is some logic to manage /BE and option of another such unit. Also, as all the cartridge-units can generate an interrupt, there's a need to manage them.


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Files for download

wes-1.html 35.0kB (2010-04-14 12:50)
"The Mystery of the Missing 253" - articles by Wes Brzozowski, 1986. How the BEU would work being finished. Thanks to Eric Johnson for scans.

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© 2009-12-28 21:50 Jarek Adamski, http://8bit.yarek.pl