# Need help with an odd design construct

Jim Brain brain at jbrain.com
Tue Nov 14 17:20:55 CST 2017

```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: