June 30, 2021 at 7:52 pm #956
If it were me I would modify the code that switches the rom to also drive a 1 bit latch on the expansion board, obviating the need for an extra wire. The latch could easily be included in the glue logic.June 30, 2021 at 8:36 pm #957
Hi John, the point is that either mainboard rom or mainboard ram occupies the PG memory map. Always! As standard, theres nowhere to open a gap for a paged area.
The rom area (ie the lower 16k or 32k depending on the A14 jumper) is always occupied, either by mainboard rom or mainboard ram depending on the state of the uart /op2 pin, which isnt on the expansion bus. A further point is that rom is paged in by /op2 only for read, its never paged in for write – the PG effectively has a separate ‘read memory map’ and a ‘write memory map’ 🙂
This enables the rom to copy itself to itself and then page out and continue running the same monitor code in ram as though nothing had happened!
None of this is difficult to change, but this was the point I was making in saying that the PG hardware would need to be modified to enable paging.
For me, as an experimenters computer, the Playground is perfect as it is!
PhilJune 30, 2021 at 8:48 pm #958
I certainly agree with your last sentence!
JohnJuly 1, 2021 at 12:58 pm #964
Your freebie I/O board was posted today John
PhilJuly 1, 2021 at 4:22 pm #966
Where did you buy the INS8255N? Or do you have a spare one you could sell me?
JohnJuly 1, 2021 at 7:34 pm #968
The ones I used are 40+ year-old ones John
These look genuine, and reasonably priced:
Edit: only one left!July 6, 2021 at 11:27 am #985
The board has arrived, Phil. Thanks very much.
Now just waiting for the components.
JohnJuly 6, 2021 at 1:22 pm #986
Re the ebay 8255 listing linked above, someone bought two, was that you John?
I left the last one for a few days to give others a chance but I just bought it just to ensure it has a good home. Bit expensive at £11 delivered but hey ho.
Would anyone be interested in a ready-made & tested I/O board? It would be at cost.
£11 for the 8255, £1 for the HCT10, £2 for the PCB, £2 for the socket row (I only use genuine Harwins from RS) and £1.50 for the turned-pin sockets – so £17.50 plus postage? Wow I didnt realise how that adds up, no wonder they’re not popular!
PhilJuly 6, 2021 at 2:56 pm #987
Yes Phil. It was me that bought two. I like to buy a spare in case of any problems in future.
JohnJuly 30, 2021 at 5:04 pm #1055rolandParticipant
Hi, I got an 8255 clone from old cccp via ebay/Bulgaria. From some old army stock. Working like a charm, With cccp stamped on plastic
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