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Old 7th Jul 2019, 8:54 pm   #134
john_newcombe
Hexode
 
Join Date: Apr 2019
Location: Skipton, North Yorkshire, UK.
Posts: 252
Default Re: Gemini 80-Bus System

I have been able to spend some more time with the test code provided by Neil and have a few results. Before starting I changed C44 in the reset circuitry as suggested by John, following this, the code now runs from power up. I did retest the original ROM and a second that I now have with no success.

So with the test code running I had a probe around with the scope. This is what I found.

VIDEO RAM IC9
-------------
/CS was permanently low, this is correct as it is tied to ground.
/WE shows negative going pulses at around 33Khz

Code:
e.g.
__________________
 |    |    |    |
/OE shows positive going pulses at the same frequency

Code:
e.g.
|    |    |    |
------------------
CHAR GEN ROM/RAM
----------------
/CS Low on the ROM and high on the RAM this is as I would have expected.

CRTC
----
/CS shows negative going pulses at around 33Khz as per the VIDEO RAM
/OE shows negative going pulses at around 667Khz
/WE shows pairs of negative pulses at around 250Khz

Code:
e.g.
__________________  
||   ||   ||   ||
WORKSPACE RAM

/CS shows negative going pulses at around 33Khz
Code:
e.g.
__________________
 |    |    |    |

I did notice that the DOT clock is all over the place and noticed that IC25 Pin 9 shows the same negative 33Kz pulses. Pin 9 seems to select either the crystal controlled clock or the VFO. I am thinking that this is something to do with the status port access of the test program. If that is the case from what I can see, which is quite limited, it all seems to be working as expected, or am I missing something?
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