ZX81/TS1000/TS1500 Cleanup: Part 3 (TS1000 RAM upgrade)
22Finally did the RAM upgrade on the Timex Sinclair TS1000 - replaced the original 2KB RAM with a 32KB RAM (62256). However, not yet sure if the full 32KB is accessible - 16KB is not an issue. If you want a simple guide, this isn't it ...
As I'm using the vLA81 it seems that I only need to connect the additional A11/12/13/14 pins to the corresponding Z80 address bus pins. I also connected pin 28 (Vcc) to +5V and !WR to the corresponding pin 8 on the ULA. Note that you also need to cut tracks on the board as well.
Isolated pins:
Pin 1 (A14)
Pin 2 (A12)
Pin 23 (A11)
Pin 26 (A13)
Pin 28 (Vcc)
Rewired pins:
Pin 1 to A14
Pin 2 to A12
Pin 23 to A11
Pin 26 to A13
Pin 27 to !WR (ULA pin 8)
Pin 28 to Vcc
For the 32KB of RAM to be available from BASIC it seems you need to modify RAMTOP as the maximum the system expects is 16KB. This is easily done by poking (or POKEing) the value 192 into memory location 16389. By default it has value 128:
PRINT PEEK 16389
POKE 16389,192
But I couldn't see any difference, such as creating a large array like DIM A(4000), taking at least 20000 bytes, when using the vLA81 and internal 32KB expansion.
However this seemed to work when using the ZXpand+, which also includes 32KB.
Apart from this, I replaced all the other IC sockets, both electrolytic capacitors (axial to radial) and all the ceramic capacitors with MLCCs.
Internal RAM upgrade:
http://www.zx81.de/english/32k-rame.htm
ZX81 Memory Check program:
https://www.sinclairzxworld.com/viewtopic.php?t=2664
vLA81 ... excellent replacement ULA:
https://www.vretrodesign.com
ZXpand+ Manual:
https://github.com/charlierobson/ZXpand-Vitamins/wiki/ZXpand---Online-Manual
Other ZX81/TS1000/TS1500 videos:
https://www.youtube.com/playlist?list=PL5pwokf56wMLVboogvR4xuENeiSlZQJcW