[Coco] RAM SIZE INPUT mystery

[Roger Taylor]

Is any of this associated with the Alternate Color Set mode?




At 03:50 AM 9/27/2008, you wrote:
>Can someone shed some light on this...
>
>In the CoCo3 Service manual on page 10 it shows Bit 2 of $FF22 as 
>"RAM SIZE INPUT".
>
>Then, on page 9 it shows bit 6 of $FF02 as "KEYBOARD COLUMN 7 / RAM 
>SIZE OUTPUT".
>
>I had a look at the CoCo3 schematic diagram on page 103 and 
>investigated these PIA addresses. It shows Pin 12 (PB2) of IC4 
>(68B21) as being connected via a 4.7K resistor to 5V. This indicates 
>that the "RAM SIZE INPUT" is set to high (1).
>
>It also has a link that takes it down to pin 16 if IC5 (LDC81001) 
>which is indeed column 7 of the keyboard matrix. This matrix 
>includes the F2 key.
>
>I cannot see how this arrangement determines RAM size. The CoCo 1 
>and 2 had a similar pin but it actually was wired to a jumper which 
>you set to High or Low to select 16K/32K of RAM. This one in the 
>CoCo3 is connected to the keyboard and would appear that a key (F2?) 
>be held down to send the "RAM SIZE INPUT" low.
>
>In the Super Extended Color Basic Unravelled book, it states that 
>this RAM SIZE INPUT pin is unused.
>
>I find it strange that such a pin would be provided and documented 
>as something it is not. And why is it connected to the keyboard? Why 
>or when is it necessary to possibly hold a key on this keyboard 
>column to send an input to the RAM SIZE INPUT pin of the PIA?
>
>Can any shed some light into the logic behind this?
>
>Nick Marentes
>
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