UART JagLink (Atari Jaguar)
The Jaguar GameDrive as well as games such as Doom use the UART pins on the Jerry ASIC.
The UART pins come from the Jerry to the game slot.
Pin 52 (UARTI) is the UART Input that Jerry is expecting to receive data from the game.
Pin 56 (UARTO) is the UART Output that Jerry is expected to reply to the game when it sends a valid UART Input stream.
Make sure R130 is 1K and not damaged otherwise communication from the game cart will not reach Jerry.
The best test is to measure with your multimeter in resistance mode from pin 52 of Jerry, directly to B30 of the game slot for 1K resistance (due to R130).
Then measure pin 56 of Jerry to B31 of the game slot, for 0R (or in continuity mode for a beep).
Inspect the cartridge slot that both of those pins are also good, clean and will make contact with the game.
Jerry has an internal pull-up to 5V on the UARTO (pin 56).
If you do not measure 5V on this pin when turned on, you can try adding a 30k or higher resistor from B31 of the game slot, to 5V, to fix a potentially bad pull-up circuit inside of Jerry.
The simplest way to test UART communication is to use a Jaguar GameDrive.
Insert the GameDrive into the system, connect your oscilloscope up to the Jerry UARTI pin 52 (B30 on game slot), set a trigger at 2V, voltage to 10V (so you can see the 5V signals fully), and time to 2ms divisions.
Turn on the console and you should capture the input data coming from the GameDrive. Firstly a slow low pulse, then a second or two later a data pulse, followed by another data pulse if it is fully working (after the Jerry replies).
Next turn the console off and probe UART0 pin 56 (B31 on game slot) in the same fashion. Turn on the console and monitor for data.
About 2 seconds after launch, you should see a reply from Jerry to the GameDrive similar to this.
If you see a solid 5V line instead of a data reply, yet you saw the UARTI sending the data pulse (usually only the first one, as if it doesn't get a reply it wont send the second), then typically the Jerry ASIC is bad.
Bad Jerry UARTO
If you get the no reply on UARTO, then typically what happens is the GameDrive will show the first screen showing Press OPTION.
Then the writing will disappear and this screen will stay on (instead of going into the game menu).
When UART is working, and Jerry replies, the GameDrive would load into this menu.