Need help with an odd design construct

[Jim Brain]

Having picked up one of these little TI Compact Computer-40 (cc40) units 
over the Summer, I thought I'd work on reverse engineering a RAM 
cartridge for the unit.

As shown on this forum:

http://atariage.com/forums/topic/255728-the-compact-computer-40-cc40/?p=3890516

The design *appears* to havea  "floating" ground when powered off 
battery alone.  The cartrridge edge connector has 2 grounds.  I didn't 
think anything about them at first, assuming they were connected to each 
other.

Hwoever, someone with a RAM cart is helping me reverse engineer, and it 
appears pin 1 ground is connected to the RAM GND pin, but is connected 
via a 6K8 resistor to the 3V battery ground, which is connected to pin 
27 ground line.

I *assume* this means that, once the cart is pulled, the battery voltage 
sits somewhere in the middle of the 5V swing the RAM needs to see, but I 
can;t figure out how one calculates the voltage divider value for the 
inherent resistance of an SRAM, as the 3V is sent through a germanium 
diode (bringing the effective Vcc of the SRAM to 2.7V), and then SRAM 
ground is sent through the 6K8 to battery ground.

Obviously, reverse engineering being fraught with errors, we could be 
wrong, but assuming not, what is going on in this circuit, and how does 
one calculate the effective potential of the GND pin of the SRAM?

Pics:

http://www.go4retro.com/downloads/CC40RAM/

Jim

-- 
Jim Brain
brain at jbrain.com
www.jbrain.com