Volume 2 · Number 3 · July 1982

Page 20 of 37

is probably the easiest. Check that you still have +5V on your Monitor and BASIC ROM and other RAM/​ROMs in the vicinity. (I didn’t when I did it.)

Next, obtain a 74LS156 and fit it an place of IC 44 (which is at present a 74LS155 of which the 74LS156 is an Open Collector version). This is necessary to allow correct power down operation. Fit 8 of 1K resistors between pin 3 and pin 4 of each Link block LKB1 – 8. This provides the Open Collector pull-up to the new battery backed, 5V supply, when it is fitted.

Now for the back-up supply. This is best done by reference to diagram 2 where the normal 5V supply can be obtained from the test pins near the bus connector and where the new 5V supply is connected to the new RAMs and pull-up resistors ONLY. (It is worth noting that another HM6116 could be fitted in place of the Monitor and Video ROM/​RAMs and, by the connection of the back-up supply and the addition of Write strobes, you would be able to change Monitors and Video/​Graphics at will and not loose their contents on power down, therefore mimicking the present ROMs. Write Inhibit switching is recommended as there can be no guarantee that spurious writes to the Cmos RAM will not occur during power up and power down. However, to date I have not experienced any problems in this respect and have tried to provoke spurious errors without “success”. To be certain, ALWAYS DISABLE WRITE BEFORE POWER UP OR POWER DOWN. I have not thought of a failsafe, software controlled Write Disable. If it could be incorporated, it would open a few more avenues for use.

Having now had this facility on my system for some while, I now wonder how I got by before it was fitted, and the time spent getting it right was well worth it.

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