From: William Levak (wlevak_at_sdf.lonestar.org)
Date: 2007-01-24 04:16:01
On Tue, 23 Jan 2007, William Levak wrote:
>
> After thinking it over, I have decided that part of my previous answer is
> wrong.
>
> CS3 is A11, but it is implemented by being connected to either ground or +5V.
> If you want to use a 4K EPROM you will have to disconnect the trace for CS3
> on the circuit board and jumper it to A11. Otherwise, use 2K EPROMs and
> ignore CS3.
After going over the schematics again, I find that it is a little more
complicated than I thought. Not all the ROM sockets are wired the same.
For ROMs that are the upper half of a 4K block, A11 is on CS1. For ROMs
that are in the lower half, A11 is on CS3. There is an A11 line on all
the sockets, so you can use 4K EPROMs. For $E000-E7FF only a 2K EPROM is
required. You may be able to use A11 as a chip select.
For the character ROM, none of the chip selects are actually used.
Phi 2 probably is not required, but might be required to be connected to
Output Enable on the EPROM.
I have the schematic for a 2114 to 6550 RAM adapter that we used to use.
Besides the address logic, the read/write line is combined with phi 2 to
produce the read/write for the 2114. A 74LS139 is used with the 6550
read/write on pin 1, phi 2 on pin 3 and the 2114 read/write output on pin
5. Pin 3 is selected to ground.
wlevak@sdf.lonestar.org
SDF Public Access UNIX System - http://sdf.lonestar.org
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