30 Aug
2002
30 Aug
'02
1:32 p.m.
--- Tony Duell <ard(a)p850ug1.demon.co.uk> wrote:
> > How exactly is your board designed?
> The UART is local to the 68000 which sits on the OMNIBUS....
In which case, assuming you have some spare space in
the 68K's memory map
(and I can't believe that you don't)
Plenty of spare memory space. The basic COMBOARD design is the top two
address bits select local RAM vs shared memory vs I/O vs ROM. The RAM
area is 4MB of space with 32KB (2114s) or 128KB (4116s) or 512KB (41256)
or 2MB of RAM (44256s). All the DRAM models use a 74S409 for refresh.
There is a COM 5025 or Zilog Z8530 for Bisync/HDLC comms, and on most
models, a parallel port for a dataproducts or (with external printer
adapter) DEC LA180. The original model uses the second port of the 6821
for an external programmers console - blinkenlights for debugging.
The shared memory (in all the later models) is big fun - the 68K has up
to 22 bits of what it thinks is local memory. The shared memory circuit,
when enabled by a bit in the CSR, turns those memory accesses into DMA
cycles on the host bus. The code that moves buffers on and off the board,
looks just like a memory-to-memory block copy. The shared memory circuit
holds the 68000 off until the DMA cycle repeats. Timeouts are possible
but a sign of very bad things.
As an aside, I have used this feature, in conjunction with a Fluke 9010
and 68K pod to test RAM chips on Qbus boards.
then the easiest solution is to wire up the 8042 to
the 68K's bus. Look
at the PC/AT schematics in the TechRef to see what's needed
I'll do that when I get to the hard design phase. I still have to figure
out how I'm going to make a bunch of TTL cause a databreak cycle.
Alternatively you could wire up an LK201 to the UART,
but you really need
both halves of the UART to communicate with the LK201. It is _very_
useful to be able to send it commands...
Ah. Wasn't sure about that. I guess the uVAX2000 _does_ dedicate an
entire serial port to just the keyboard when in workstation mode.
Thanks for all the advice.
-ethan
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