Re: MK14 schematic revisions
Update!
Well, theres good news and bad news....
Good news is the the lack of bus activity I was seeing was because I missed out a track between the SC/MP and the crystal (oops!) and with this fixed all seems to be working well.... except the display is backwards! The keys also map to the wrong functions so I suspect I have something wrong around the 7445 chip.
This does tell me however that the TI 74LS157N multiplexers I bought from Farnell seem to work fine in this application, I am using them with 1977 7408j's to drive the display, so it did occur to me that possibly the older 7408s might have higher input capacitance which holds the signal long enough for the 74LS 157s to latch it. This is pure speculation as I have never seen any plausible explanation of why some 74LS157s might work when others dont.
It also shows my PROMS that so many people got programmed correctly for me are fine, which is another relief! The '8' key seems to working as the 'mem' key and I was able to stem through the first few bytes of the ROM and reading backwards the values are the same as the monitor listing.
It looks like when I have worked out what tracks need cutting and re-wiring I might need at some point get a new board made with all the corrections as I already have 5 "bodge" wires and am l,ooking at adding at least another 10...!
With a 9v power supply the regulator is getting quite hot, so I might have to look inti getting a bigger heatsink for it at the same time.
Slothie
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