64k RAM

The 64k RAM module uses 2 65222 static RAM chips for up to 64k of RAM in the RC2014.  Jumpers allow you to set it to only 32k, 48k, 56k or 60k if you wish so you can use 32k, 16k, 8k or 4k of ROM, and a page pin allows for the lower 32k to be disabled.

64kRAM

If only 32k is required, only IC1 (74LS32), IC2 (62256) and IC4 (74LS04) need to be fitted and all the jumper pins can be ignored.  This works the same as the 32K RAM Module; Address line A15 goes through a NOT gate on a 74LS04 to the /CE pin, thus mapping this memory to address 0x8000 – 0xFFFF. This is the upper 32k.  The /WE and /OE lines are controlled by ORing the /MREQ with the /WR or /RD from the Z80

For any other configuration, IC3 (62256) and IC5 (74LS32) and all the other jumpers need to be fitted.  If address line 15 goes high, then /CE will go high, thus mapping this to address 0x0000 – 0x7FFF. This is the lower 32k. The /WE and /OE lines are controlled in the same way as the upper 32k.

Because the lower 32k may well clash with addresses in ROM, address lines A12, A13 and A14 go through some OR gates and jumpers.  These can be used to force the /CE pin high if a particular lower address space is used.  For example, if using the 8k MSBASIC ROM, any memory requests that have A13 and A14 low (0x0000 – 0x1FFF) will cause /CE to go high, thus disabling the lowest 8k of memory.  Or, to put it another way, /CE will only go low on IC3 if an address between 0x2000 and 0x7FFF is being called.

Click the pdf below for full schematic

64k RAM Schematic

Jumper settings for the start address are as follows;

0x0000 0x1000 0x2000 0x4000
0== ==0 0== ==0
0== ==0 ==0  ==0
0== ==0  ==0 0==
==0 ==0 ==0 ==0

 

The lowest jumper is for /CE override, to allow the lower RAM to be paged out.  Normally this would be set high with a link soldered by the bus pins – however, if the jumper is in the 0== position, and the link removed, the lower RAM can be disabled by pulling the page pin low.

64kRAM Photo

Buy one here [Tindie link coming soon]

Bill of materials

64k RAM

  • 39 pin RA Header
  • 2 x 28 pin wide DIL socket
  • 3 x 14 pin narrow DIL socket
  • 2 x 74LS32
  • 1 x 74LS04
  • 2 x 62256 RAM
  • 3 x 100nf capacitors
  • 4 x 3 pin header
  • 4 x jumper

 

32k RAM

  • 39 pin RA Header
  • 1 x 28 pin wide DIL socket
  • 2 x 14 pin narrow DIL socket
  • 1 x 74LS32
  • 1 x 74LS04
  • 1 x 62256 RAM
  • 3 x 100nf capacitors