Date   : Wed, 14 Feb 2007 22:17:59 +0000
From   : splodge@... (Richard Gellman)
Subject: BBC Master 128 problem

*obscured by circuit diagrams from advanced ref. manual*

Hi,

I've looked at the circuit diagram, and you'll be pleased to know this 
is a *very* simple 1-wire hack..

First, move LK18 or LK19 (depending on which socket you stuck the GoMMC 
in) back to its "Sideways RAM enable" position (east).

This will leave a pin exposed. That pin goes solely to the _OE (output 
enable) line of the chip socket. You need to connect that pin to pin a2 
of your chosen cartridge socket (side "a" is the side nearest the back 
of the machine, pins are numbered left to right starting from 1, and do 
not alternate between sides, e.g. a2 and b2 face each other).

--- technical bit, not essential but good to know --

The entire sideways ram select is based on connecting the memory 
controller's bank select to either the relevant OE line on the socket, 
or to the DRAM CAS lines on the sideways banks (in accordance with the 
4Mhz clock). The OE line is tied to +5V by a 4k7 resistor, and the pin 
left exposed on the link is a tap into this OE line. By connecting it to 
pin a2 on the cartridge slot you re-assign the OE line on that socket to 
be enabled by the cartridge OE line which works in an identical manner, 
but for sockets 0-1/2-3 rather than 4-5/6-7.

-- end of technical bit --

NOTE: Plugging a ROM cartridge into the hijacked cartridge slot would be 
*very bad*. At best it will confuse the system and make it hang/crash.

-- Richard

adam colley wrote:
> Hihi
>
> On 14/02/07, Mike <profpep@...> wrote:
>   
>>> How would I go about connecting the cartridge port to the eprom socket
>>> under the 128K ROM so my GoMMC doesn't eat half my sideways ram?
>>>
>>>       
>> The cartridge appears in the sideways RAM space, so it might be possible to
>> put the GoMMC into a catridge, I would have thought. You can put RAM in a
>> cartidge - I have a Peartree cartridge that does just that.
>>     
>
> Ahh, but what I want is the GoMMC inside the machine but occupying
> something outside banks 4-7 -.o
>
>