80-Bus News

  

May–June 1984, Volume 3, Issue 3











Page 15 of 51











15

16k decodes o1

74LS25 74LS21 1ORQ

A3

A2 to P6

bo CAS2

Block 2 cs

TALSTS to P7

74LS32 WALS25 at

74LS02

FIGURE 4

I built the additional circuitry on a 3 by 4 inch piece of Vero card which was mounted about 2 cm. above the main board; it was held at one end by wire-wrap pins which plugged into the empty EPROM sockets and was supported at the other end by two insulating pillars. These were held in place by fine insulated wire threaded through convenient holes in the boards. Connections to the various signal lines were then made by soldering to the IC socket pins on the underside of the main board. This made a reasonably neat arrangement of what is essentially a bodge job.

256k RAM chips are now becoming avaiable, but unfortunately they currently cost about £50 each, so even a single block would cost £400, which is beyond the reach of most of us. However, the price projection is that they will be under £10 by mid-1985. These chips require a full 8-bit refresh cycle and a little thought is required to toggle the A7 line at the appropriate times during refresh; a number of ingenious solutions have been suggested in various electronic journals. I wonder who will be the first to upgrade his RAM A board to half a megabyte?!

[1] INMC 80 News Vol 3, Feb-Apr 1981, page 16-17 see also 80-BUS News Jan-Mar 1982, page 12.

[2] 80-BUS News Vol. 1 Issue 3, July-Oct. 1982, page 21-23.

[3] D. Allen New Electronics Vol. 17 No. 5, 6th. March 1984, page 26.


This is an OCR’d version of the scanned page and likely contains recognition errors.











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