No, not a refugee from page 3 (we got told off by readers last time we let a naughty word onto these hallowed pages), but to draw your attention to the fact that we all make mistakes. Last issue, we published a bit about adapting the series 1 memory card to carry the 8K Basic ROM, and went on to describe how to put the remaining three EPROM sockets back into use. Well the Basic ROM bit works, but the EPROM bit doesn’t!!!

Having fitted the Basic ROM to the card, and written it up for the INMC, in a most un-natural fit of enthusiasm, ! modified the board for the EPROMS, ‘zapped’ in a couple of EPROMS, gave it a quick go and wrote it up, but because of the closeness of the copy date, I didn’t get round to actually testing it thoroughly. Later, (over Christmas), horror of horrors, fit some EPROMS and the Basic does funny things. Close examination of the circuit reveals that enable of IC24 (pin 1) comes on when the Basic ROM comes on, so with EPROMS fitted they are enabled along with the Basic ROM resulting in rubbish on the bus. The strange thing is though, that in the conflict between the EPROMs and the ROM, the ROM usually won, so Basic didn’t just crash as you’d expect, but became unreliable.

At first sight this problem looks very difficult, requiring a three pole switch (very hard to find) to switch the IC24 enable along with the Basic enable, and the decodes. However, look at the circuit again, it won’t take long to realize that the enable line to IC26 (which we already use for the Basic ROM) could just as well be used to enable IC24 instead of the signal currently used.

So, now to the correct version, refer back to the previous article, and notice that a 4K7 pull up resistor (on the right hand connector) is used to disable the Basic ROM when not in use, leave this where it is, but reverse the connections to the other side of the switch. Viz: IC26 pin 19 is connected to the pole of the switch, and IC27 pin 20 is connected to the left hand connector. Further, another 4K7 pull up resistor has to be fitted to IC27 pin 20 to pull the Basic ROM ‘off’ when the switch is over the other way. Carefully cut the track to IC24 pin 1 (adjacent to the pin), and connect pin 1 to the right hand connector of the switch, leaving the existing 4K7 pull up resistor in place to turn 1024 ‘off’ when the switch is over to the Basic.

All this seems to work satifactorily, but we have heard of a few cases of difficulty (even without EPROMs on board), a ‘Stop Press’ was added to the previous article about fitting a 100n capacitor across the back of the ROM to decouple the supply lines, I have found that 2u2 bead tant. is better. You wouldn’t believe the ‘crud’ the Basic ROM generates on the supply line as it’s enabled, if you’ve got a 50MHz scope to hand, have a look at it across pins 12 and 24. A further aid to quieting down and speeding up the system is to decouple each EPROM socket with 10n, and to pull the data outputs of the ROM and EPROM block high with 10K resistors.