ZX81 16K RAM Pack Features Sixteen Kilobytes That Lived and Died by the Edge Connector

Sinclair shipped the ZX81 in 1981 with a Z80, an 8K ROM, and one kilobyte of static RAM. After the screen buffer and system variables took their share, a BASIC listing had only a few hundred bytes left to stand in. A black plastic box that slid onto the rear edge connector promised sixteen times that space. For a lot of first-time owners, that pack was the line between a curiosity and a machine you could actually write on.

Happy Little Diodes opens one and discovers 15 chips rather than a clean row of memory. Two boards sit side by side, connected by a ribbon cable that performs a decent job of holding them together. One of the boards contains eight 4116 DRAM chips as well as a standalone power circuit – a rather simple piece of hardware. The other appears to be a collection of components, including four 74LS157 multiplexers, a 74LS393 counter, a 74LS32 OR gate, and a 74LS00 NAND. Those 4116 parts were inexpensive, with each storing only a small amount of data in a tiny capacitor. They wanted +12 volts and -5 volts, which the ZX81 never actually placed on its connector; it was great of Sinclair to come in with a blocking oscillator based on a single ZTX750 transistor and a little coil to fix that. Result: 9 volts in, +12, and -5 volts out, all nicely kept in place by a 12-volt Zener and a quench diode that prevents the entire thing from ringing. A memory cartridge that generates its own rails is a strange object even now.

Better yet, DRAM only lasts if each row is touched every two milliseconds. A Z80 would handle this with its own refresh register, but the ZX81’s register is too busy counting down the TV’s scan line. That’s why the kit includes its own 74LS393 to do the job. Meanwhile, RAS latches down address bits A0 through A6, while CAS grabs hold of A7 through A13. During the refresh half-cycle, the multiplexers switch to the external counter, a nice quick flip of the switch. Finally, to tie everything together, some short RC delays are used to set the edges. Once that pack is fully plugged in, the RAMCS line eliminates the internal 1K, transforming the map into a single, clean 16K block following the ROM. People were understandably pleased when they conducted a basic RAMTOP check and discovered a proper machine capable of running a program.

That circuitry is odd because it lacks clamps, screws, and other supports. It all hangs from a standard edge connector. A bump on the desk, a lean on the case, or even a poke on the membrane keyboard might easily lose contact for a second, resulting in the loss of all your work. People got around this by tying rubber bands around the stack, applying Blu-Tack to the junction, and even erecting wooden cradles, but there was still the risk of a stray elbow knocking the connection away. To make matters worse, the added load from the pack caused the ZX81’s five-volt regulator to overheat, forcing some stores and hobbyists to go out and acquire some heatsinks and a diode or two simply to keep things going.

Later on, the official memory packs simplified things up by combining the 74-series logic into a single custom chip, which was then used on a single board. Meanwhile, third-party boxes managed to fit in 32K and 64K, with only around 56K remaining usable after accounting for the ROM’s location on the map. The larger extensions still used the same flimsy connector, so you’d be concerned about going across the room and knocking everything free for a second. For those who learnt to write in 1K, which was about all you could squeeze in most of the time, sixteen kilobytes felt enormous, as long as you stayed put.
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