Hardware Corner: 30-Pin SIMMs

HARDWARE CORNER

Today I'll occupy you with SIMMs (Single Inline Memory Modules), which could be used as a cheap memory expansion for the Spectrum. Unfortunately, I cannot provide a ready-made solution since my hardware knowledge isn't as perfect as, for example, Fred Dürkes' or Frank Meurer's.

Usable are the so-called 30-pin SIMMs, as they were used in older PCs (up to 486). They have a data bus width of 8 bits, in contrast to 72-pin SIMMs which have 32 bits. Since these modules exist in sizes of 256Kb, 1Mb, and 4Mb, and the 4Mb modules cost just about 20 DM, I recommend using those. Some of the SIMMs have a parity bit, which can be recognized by an odd number of chips soldered onto the SIMM; however, these are not used and are superfluous on the Spectrum. SIMMs are nothing more than DRAM on the Spectrum, and therefore they also require a refresh signal (more on that could be read in SPC Info).

One could use them either like the banks of the Spectrum 128K, with 256 banks of 16 Kb each (the second Spectrum ULA generates a refresh), up to 256 banks of 32 Kb, like on the Spectrum 80K (8 Mb, so 2*4 Mb SIMMs maximum), or like in the MB-02 as 256 banks of 16Kb each paged in instead of the ROM. With 16-bit addressing, one could even give the Spectrum 2 Gb of RAM, but that would be slightly exaggerated, because at 4–8 Mb you can still load everything from disks, beyond that you need a hard drive or CD-ROM drive.

Here is the 30-pin SIMM pinout from the Hardware Book from the I-Net (which I corrected a bit — as soon as I get onto the I-Net, I'm going to have a word with the guy who maintains the Hardware Book, apparently he can't even count!) (Pin 1 is located at the notch on the left):

Pin  Name   Meaning
 1   VCC    +5 Volt supply
 2   /CAS   Column Address Strobe
 3   DQ     Data 0
 4   A      Address 0
 5   A      Address 1
 6   DQ     Data 1
 7   A      Address 2
 8   A      Address 3
 9   GND    Ground (0 Volt)
10   DQ     Data 2
11   A      Address 4
12   A      Address 5
13   DQ     Data 3
14   A      Address 6
15   A      Address 7
16   DQ     Data 4
17   A      Address 8 (N/C on 256Kb SIMMs)
18   A      Address 9 (N/C on 256Kb SIMMs)
19   A10    Address 10 (N/C on 256Kb and 1 Mb SIMMs)
20   DQ     Data 5
21   /WE    Write Enable
22   GND    Ground, same as Pin 9
23   DQ     Data 6
24   A11    Address 11 (N/C on 256Kb and 1 Mb SIMMs)
25   DQ     Data 7
26   QP     Data Parity Out (No connection on SIMMs without Parity)
27   /RAS   Row Address Strobe
28   /CASP  Parity-Column Address Strobe (see Pin 26)
29   DP     Data Parity In (see Pin 26)
30   VCC    +5 Volt

So 25 of the 30 pins are relevant for the Specci, because parity does not need to be connected even if it is present. Whoever sends me a schematic for a 4 Mb memory expansion first wins 25 freely selectable original games from my collection! And next time, the schematic for an EPROM burner for the MB02 printer port will follow.

Tiger