BIOS Boot (Atari Jaguar)
The BIOS on the Atari Jaguar is stored in a M27C1001 1Mbit ROM (128k x 8bit), and is located to the right of the cartridge slot, labelled U35.
It is a 32 pin chip, and one of the first jobs of the system is to read the ROM into memory about 400ms after power is applied.
Detecting ROM Reading
Once you have checked your system has power and you get the power LED, the next step in the boot cycle is Tom/Jerry/CPU/RAM all working together to load the BIOS into memory.
The first step of this happens about 400ms after power is applied, and starts with the address 0 (pin 12 of BIOS) being toggled for data.
If you remove the BIOS and power on the system, with an oscilloscope on pin 12 of the BIOS, you should see this on boot happen once (so make sure to set a trigger).
If you see this, it is a good sign the Tom/Jerry/CPU are operational and the first attempt at reading the BIOS is happening.
Re-install the BIOS and power the system on again.
With the BIOS installed you should be able to probe all of the address and data pins for activity.
Make sure to be probing the BIOS during boot, as if the BIOS is corrupt or traces are bad, it will only attempt to load the BIOS once before going into idle and no activity on the pins.
As you can see from the table above, the BIOS should listen for Chip Enable !E (pin 22) and Output Enable !G (pin 24) to go low at the same time, and when they do, output data on the Q pins.
With the BIOS removed, you should see this activity once on boot on the !E and !G pin, similar to the A0 pin.
If you do not see this, then the Tom/Jerry/CPU are not requesting to load the BIOS and something is wrong with them or the power circuit.
Red Screen Boot
Once we know the general system is working, booting and attempting to read the BIOS, we can now observe any of the data pins (Q0-7) for their output during boot to observe Red Screen Boot.
The Q data pins are our clue to the next stage of system boot.
Have the BIOS installed, no game installed, and pins B34 and B35 bridged on the cartridge slot, to allow a red screen boot without a game.
If we hook up an oscilloscope to pin 19 (so data bit 5, Q5) of the BIOS, and observe it during boot, with a the Q5 data pin will toggle rapidly for around 0.5 to 1 second.
After that, you should reach red screen boot and the Q5 pin goes into an idle mode where the pin is high and pulses low for a few pulses once every 16ms (60Hz) to 20ms (50Hz) depending on region, which is at the start of each new frame.
Bad BIOS Boot
If the BIOS is corrupt then the Q5 data pin will toggle much longer than 1 second and either then go to 5V flat DC or continue forever to just toggle on and off as seen here.
Full Game Boot
You can see a successful boot here that is described below.
Presuming now your BIOS is ok, and the entire system (except video output/audio) is fully operational you should be able to observe the Q5 pin to see the console loading and playing a game without the need for a working Video/Audio output.
During boot the Q5 pin of the BIOS should be toggling for about half a second, a short pause then continue to toggle for another 2-4 seconds, before pausing and having a short pulse every 16ms (60Hz) to 20ms (50Hz) depending on region while the game intro plays.
Once the game loads the Q5 pin will then toggle forever while the game is playing as the BIOS is always read and processed for game play.