Mar1e the Elf now has an EPROM!


I hardly dare post my humble efforts in the same forum as Todd's, but here goes...

I've typed UT4 into a 2732 and wired Mar1e's second 24 pin socket for the EPROM. A 74HC139 selects the 6116 at 0000 and the 2732 at 8000. The other outputs for 4000 and C000 aren't used yet (but see below).

I've written a simple program to point R4 at 8000, make R4 the data pointer and wait for IN to be pressed and released, then output the contents pointed to by the DP to the hex display, go back and wait for the next press. I watched almost with disbelief as the bytes of UT4 came up one after another. 

Next job is to reinstate the serial port (disconnected during the mods) and see if it will talk to the Psion Organiser. 

I haven't fitted, or even wired the sockets for, the DL1414 displays yet. I'm thinking of memory-mapping these; one chip select going to each of the 4000 and C000 select outputs of the 74HC139 and the two address lines going to the address bus. Allocating a whole 16k block to a device with four memory locations, one for each character, might be a bit wasteful but if it works I'll be happy. I don't know yet whether the address bus can drive the inputs so I might wire them all low and just use one character to prove the rest of the circuit, that being the 139 to !CE and the buffered data bus to the data inputs. 

Todd, I am in awe. 

John, Nottingham

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