One problem that some users face, is that of the 4116 DRAM (VRAM) chips failing
. These are 16k bit DRAM chips that store a single bit across an address range of 16k. So you need eight of them to store data as bytes.Further, they require a -5V bias supply, a +12V main operating supply as well as a +5V interface supply
.They are obsolete, have not been manufactured for many years and most of the spares out there are NOS (New Old Stock). The only other spares are used chips. As supplies are used up, so the price goes up
.Over on the Facebook group page, Dave wanted to know if another type of memory could be used, like a modern SRAM chip.
Alas, the TMS9929 VDP communicates with the 4116 DRAM (VRAM) chips completely differently to how a CPU communicates. So although it is possible for the DRAM chips to be replaced with a SRAM chip, the circuitry that needs adding needs more support chips than you would think
. The design would require a daughter board big enough to take all the new chips. Add to that the need to remove all the "old" 4116 DRAM chips and the need to either add sockets in place of the 4116 DRAM chips (making sure the daughter board pins line up) or make many soldered connections means it is not an easy task .MTX users are lucky in that the power supply circuitry for the MTX machines normally is very reliable. Unlike ZX Spectrum owners, where a failure of the inverter on the Spectrum PCB can kill a number of the 4116 DRAM chips.
If only one or maybe two 4116 DRAM chips has died, the cheapest solution is still to replace them with working 4116 DRAM chip(s).
But in future, this may no longer be practical
.There are a number of possible solutions with a number of variations to each.
- Someone design a replacement for the VDP that either has internal memory (RAM) or which uses a modern 32k Byte SRAM chip.
- Replace the old obsolete TMS9929 with the later Yamaha V9958 which can use a wider range of DRAM chips (which are also obsolete) but which may be easier to obtain. It is software compatible with the TMS9918A and V9938 and a bonus is that it has a linear RGB output.
- Design a board to replace the existing 4116 DRAM chips with a design that uses a 32k byte SRAM. This board will need a lot of extra logic chips to interface between the VDP and the SRAM chip.
.However, this was a done on a new design, not on an existing platform. It will require some effort to make it work in a MTX computer.
So say thank you to the nice people who worked on the The N8VEM Home Brew Computer Project
.The ECB Sprites, Color Graphics, and Sound (SCG) board uses a TMS9918 Video Display Processor and a 32k SRAM to generate a colour display for use on a colour VDU/monitor/SCART TV with composite video input etc.
More detailed information about the SCG card here
However, the interesting document to download is this one SRAM Replacement for TMS99x8 VRAM (click to download)
The circuit schematic for the SCG board is here (click to download)
If I am reading all this right, it should work for a TMS9929 chip as well.
Mark
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