MTX512 Video RAM

Memotech MTX computers use a dedicated Video Display Processor (VDP), either a Texas Instruments TMS9929A (PAL versions) or TMS9918 (NTSC versions), with 16kb of dedicated Video RAM (VRAM), in the form of 8 x TMS4116, or equivalent, 16k x 1 bit RAMs. An overview of the VDP hardware can be found on this page, and full circuit diagrams can be found in the MTX User and Service manuals.

The VDP requires only a +5V supply but the VRAM requires +5V, +12V and -5V. The TMS 4116 datasheet states that -5V must be applied before, or at the same time as, the other voltages and removed last. Failure to follow this this sequence will impact the long term reliability of the RAM. Similarly, a DC supply failure can cause one or more of the RAMs to fail prematurely.

Whether or not this is the cause, it does appear that VRAM failures are commonly found to be the source of the problem when investigating faulty MTX display output. 4116 RAMs are also used in many other computers and video games from the era, including the lower 16kb of RAM in the ZX Spectrum where they also seem to be a common cause of failure.

Should your MTX develop a VRAM fault, you obviously need to obtain 4116, or equivalent, RAM chips - these are obsolete and not too easy to come by. There are still some available, from what I have seen, mainly from ebay.com in the US.

The VDP manual advises that the VDP will work with TMS4116 RAM that has an access time of 150 or 200ns. Like the majority of RAM chips - particularly the more recent ones, TI used a suffix on the chip ID to denote the access speed, e.g., "-15" for 150ns or "-20" for 200ns. Unfortunately, not all manufacturers of 4116 type RAM followed this convention and it may not be obvious whether the chips that you may find are compatible.

All of the MTXs that I own use DRAMs manufactured by ITT - labelled "4116 3N", contrary to what you might expect, this does not denote 300ns access time RAM - they are 200ns. When replacing the VRAMs, you should use chips at least as "fast" as those originally installed and it is probably not a good idea to mix RAM of different speeds on one computer. (The other 4 digits shown in the photo are the manufacturing date code, typically having two digits for year and week of manufacture.)

I have collated data from a number of manufacturers below - if you know of any more, please let me know, the only ones that I have actually tried are shown in green text. Full specification data for these components can be found on my Datasheets page.

RAM manufactured by other than TI may have subtle internal timing differences, but it is assumed that RAM with similar quoted access times will be suitable

After I had produced the table above, I found this useful page about 4116 class DRAMs on the minuszerodegrees website, this site also has a very good table of 4116 type RAM.