The vLA81 has a few ‘hidden’ features which can only be fully unlocked with a 32K memory upgrade. If this is present then, as well as the standard 16K memory map, there are three other modes available.
They are all documented in the user manual but in summary they are:
RAM from 0x2000 (8192 dec) to 0x9FFF (40959 dec) – useful for HRG (high resolution graphics)
RAM from 0x4000 (16384 dec) to 0xBFFF (49151 dec) – the standard 32K expansion
RAM from 0x0000 to 0x7FFF (32767 dec) – a special mode which maps the ROM space to RAM
There have been several designs available to upgrade the ZX81 to 32K and I’ve always had the intention to provide a simple, inexpensive, design in kit form or fully built. I finally got around to it and here’s the result:
Couldn’t be simpler!
The design incorporates a thin PCB ‘shim’ between the 32K ram chip and the socket. The PCB does the simple job of rerouting a few pins to the right place and has no active components. Most of the ram chip’s existing pins are left intact. Four address wires are required to be connected to the ZX81 PCB as well as one short jumper under the board to connect A13 to the vLA81.
After the vLA1 was completed I set myself a challenge to replicate the functionality of some of the other Sinclair ULAs, firstly the ZX81 followed by the ZX Spectrum and finishing with the Toastrack ULA (as this would leverage most of the ZX Spectrum ULA design).
I knew that all of these had been already done before but as the inner workings had not been published in detail this meant that there would be plenty of design still required (the part I enjoy the most). Armed with Grant Searle’s fantastic ZX80/81 repository and Chris Smith’s wonderful expose of the Spectrum ULA, I started about a year ago on the vLA81.
A functioning vLA81 was completed in October 2018 and work started on the vLA82, by far the most complex of the three. A working vLA82 prototype was completed in March 2019 and work was begun on the vLA128. In parallel with the design, schematics were refined, PCBs finalised, and samples sent out for testing when they became available.
A working vLA128 was completed in May 2019. Over the time the other vLAs were progressing the vLA81 underwent its own evolution. As the Xilinx XC9572XL CPLD used in the vLA81 had unused resources I decided to add some extra functionality (see features below).
The complete vLA family (so far):
Feature summary:
vLA81
1. Fits within the same 40 pin DIL socket outline as the original ULA.
2. Narrow IC pins for easy plugin ULA replacement. In most cases the existing socket will be OK.
3. Provides the same functionality as the 2C210E Sinclair ULA.
4. Video level selectable between standard video (suitable for direct connection to a 75 ohm monitor input) and high sync level (suitable for the modulator input or some video mod boards).
5. Back porch on/off select.
6. Internal 32K memory support (High or Low for the different hires schemes).
7. Programmable user configuration gives access to:
Inverse or normal video and choice of black or white border.
50/60 Hz video standard.
Ability to map ram to the ROM space (write protect enable/disable – 32K internal ram required).
8. Configuration port socket provided on PCB base for external config switch option.
9. Tested with an extensive suite of ZX81 software (full list to follow).
10. Ideal for preserving the original ZX81 ULA.
11. Low power consumption.
vLA81 installed in an issue one ZX81
vLA82
1. Plugin ULA replacement. In most cases the existing socket will be OK.
2. A low profile and 40 pin DIL footprint means that the vLA82 is designed to fit in a ‘rubber key’ ZX Spectrum case without further modifications.
3. Provides the same functionality as the 6C001-7 Sinclair ULA.
4. Passes standard ULA benchmark tests (floatspy, Brendan’s Diagnostic ROM etc.).
5. Tested with an extensive suite of ZX Spectrum software (full list to follow).
7. Ideal for installing in an aftermarket Spectrum PCB without needing an original ULA.
6. Low power consumption.
vLA82 installed in an issue 4S Spectrum
vLA128
1. Fits within the same 48 pin socket outline as the original ULA.
2. Narrow IC pins for easy plugin ULA replacement.In most cases the existing socket will be OK.
3. Provides the same functionality as the Sinclair 7K010E-5/ Amstrad 40056.
4. Passes standard ULA benchmark tests (floatspy, Brendan’s Diagnostic ROM etc.).
5. Tested with an extensive suite of Spectrum 128/+2 (Grey) software (full list to follow).
7. Ideal for preserving the original Toastrack/+2 ULA.
6. Low power consumption.
vLA128 installed in a Toastrack (will also work fine in a Spectrum +2 Grey)
If any of these items would be of interest then please email to vdrivezx@gmail.com
vRetro Design 