8K Non-Volatile Memory Kit. Hunter, 1630 Forest Hills Dr., Okemos, MI 48864. Complete kit: $29.95 plus shipping; PC board and manual: $15.00.
The Non-Volatile memory (NVM) kit is a battery backup powered RAM board that is primarily designed to fill the transparent address area between 8K and 16K in the Sinclair ZX80/81. The key feature of this product is that with the onboard battery, the routines stored in NVM are retained even with ZX81 power supply unplugged.
The transparent address area is not affected by the Sinclair operating system. Commands such as NEW do not disturb this area. This area is the ideal place to store machine language routines instead of in REM statements, string variables, or above RAMTOP. Thus you can develop your own system utilities, machine language subroutines, alternate display file, etc. Since this is RAM, the routine can be easily modified. For versatility this is even better than an EPROM. The NVM can also be used at any of the 8K blocks between OK and 32K.
The complete kit comes with instructions, PC board, all components, lithium battery, and one HM611P-3 CMOS static RAM chip. The one chip provides 2K RAM. The space and sockets will accept full complement of four RAM chips. Construction takes a leisurely four hours to complete.
The lithium battery provides 10 months of life with one 2K chip on the board and 2+ months with four of the standard RAM chips. There is available from suppliers (at a premium price) a low power version of the 6116 which uses only 1/5 the power during idle.
The PC board is of very good quality and has several design touches that are nice extra features. A CPU reset pushbutton eliminates unplugging the power when crashes occur. There is a connection provided for a secondary backup battery, such as NiCads, which will further extend the life of the lithium battery. Extensive jumper connections are provided to handle the various options described in the documentation.
The instructions and documentation are strong points in this kit. The documentation is very complete and lucid. There is no trouble in following the construction instructions. The documentation includes parts list, schematic, lots of notes to cover the other options available, such as use of 2716/2732 EPROMs, EPROM/RAM mixture, or use with the ZX80. Finally, a sample program shows the use of a rapid secondary display file and utility programs for saving and reloading the NVM from backup tape.
I did have some initial problems with the contact between the NVM board and the Sinclair 16K RAM pack which piggybacks onto the NVM. The connector board simply needed to be cleaned to remove residual solder flux. Three was also a problem with defective retention of the memory. After much experimentation and modification, I found that this was occurring during the power transients and that the simple expedient of holding down the reset button while plugging/unplugging the 9V power cured the problem.
The edge connector to the computer is a wirewrap style with the wirewrap pins projecting through the NVM PC board to carry the signals to the 16K RAM pack. The small board, to which the 16K RAM pack connects, is placed between the wirewrap pins, the pins are bent inward and soldered to the board. This looks a little strange and awkward at first, but actually makes for a very sturdy assembly. The complete assembly of computer, RAM pack, and NVM makes a firmer connection with less wobble and crash problems than the original.
The ability to have utility routines available without having to do a tape reload at every session is a boon to a machine language programmer. This kit ranks quite high on the list of “must haves” for any serious machine language work.