[Opendispense] Questions - new memory map

Harry McNally harrymc@decisions-and-designs.com.au
Mon Aug 11 07:04:35 2003


On Mon, 11 Aug 2003 12:53:23 +0800 Adrian Chadd <adrian@ucc.gu.uwa.edu.au>
wrote:

> On Mon, Aug 11, 2003, Bernard Blackham wrote:
> 
> > We also have the problem of the RW' line requiring to be low (R) to
> > access the EPROM and hence riser board. We'd need to hack the board
> > and cut some tracks to get around this, which wouldn't be so nice.
> > 
> > I'd be happy for somebody more versed in computer architecture to
> > point out some other ways :)
> > 
> > As to which UART to choose, are there any pros/cons between them? If
> > I start writing some code to chat with a 16550A UART, will it be
> > overkill?
> 
> If you can make it small enough, no.
> 
> Is the CPU socketed? How is it mounted on the board?
> You do realise there's nothing _really_ wrong with soldering a bunch
> of little wires to make up the rest of the riser board interface..
> (ie R/W, A15, etc.) I don't see why you need to cut tracks.

Hi Bernard and Adrian

I had a cunning plan to install a 32 pin machine pin socket to replace
the 28 pin EPROM socket. We could then run four more signals up on to the
mezzanine board through the extra pins but the mezzanine can still be
removed and replaced easily (even restore the original EPROM if necessary).

If you look at the scan of the top of the board (top.PDF or top.JPG) which
I think is on CVS there, the following mods were what I had in mind:

- Remove the EPROM and socket.

- Remove C29 and R81. Wick out the holes and reinstall these parts on the
  other side of the board. Trim back excess lead so it is flush with the
  top copper prior to resoldering.

- Trim the legs on pins 15, 16, 17, and 18 on a 32 pin machine pin
  socket.

- Solder wire wrap pigtails on the legless socket buckets.

- Insert a cut down 40 pin WrapID (tm) wire wrap marker on the machine pin
  socket as an insulating spacer.

- Install the socket as normal on the remaining 28 pins.

- Terminate the four wire wrap pitails on signals needed for read and
  write and full addressing (A15) onto the PCB.

- For now, reinstall EPROM.

If you look at 1 o'clock to the EPROM there is an unused mounting hole.
We can use this to support the mezzinine board with a threaded nylon
standoff and nylon screws.

I've used this trick before and it was a tidy solution. It almost looked
like a bought one :) 

If you like, we could do a rework session and snap a few images of the
steps for the web records.

cu
Harry

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