Date   : Thu, 07 Aug 2003 12:36:11 +0100 (BST)
From   : Jules Richardson <julesrichardsonuk@...>
Subject: Re: Lathe ROM modules... / Watford VPO ROM

Hi,

> Sounds interesting any chance of a few pictures. What is the message
> when you type *help ?

*help doesn't seem to give me anything - I just get the standard stuff from the
OS ROM. I tried with/without the PAL module, and also tried in another Master
just to be sure.

pics are at: http://www.moosenet.demon.co.uk/temp/bbc/rom


I traced out the schematic for the module:

  Data lines aren't touched at all and just pass between the BBC and the ROM.
  A0 - A12 are also passed through to the ROM from the BBC as-is, but with
A5-A12 also entering the PAL device. 
  A13 is disconnected between the BBC and the ROM, and rerouted through the
PAL. 
  A14 from the BBC is ignored, with A14 to the ROM under control of the PAL.
  *OE and *CE are connected between the BBC and the ROM, but also pass into the
PAL.

The EPROM itself is a 27256 device.

So it looks like this PAL is just controlling the A13 and A14 lines on the ROM,
based on the input from the A5-A12 lines. That's unfortunately a different
animal to your module, which seems to modify the actual data based on address
line input :-(

Got me curious as to what I have though! 

cheers

Jules

====
Backward conditioning: putting saliva in a dog's mouth in an attempt to make
a bell ring.

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