Once the console has red light boot and is out of reset, with valid ASIC operation signals, the next step is the ASIC will try to read a game from the cartridge connector and proceed to boot and run the game code.
Here is every pin on the cartridge connector for reference.
Here is every pin of the Main RAM and MPU for reference.
Here is every pin on the dual ASICs for reference.
Here is every pin on the Main RAM for reference.
Here is every pin on the single ASIC for reference.
Here is every pin on the video RAM on the single ASIC for reference.
To read the data from the game cartridge the ASIC places the address information on address pins A0 to A15 for a 16 bit address. Then it clocks out the !Read pin 26 of the cartridge connector.
The address pins on the cartridge connector (pins 4-12, 25, 28-32, 33) look similar to this (each pin contains the address bit at a certain point in time as the ASIC asks to read different addresses in ROM):
The !Read pin is an active low signal meaning every time it goes low the game cartridge should output the data to the data pins D0 to 7 for a single byte (8 bits) of data stored in the memory of the cartridge ROM at the given address.
NOTE: Not all games use the !Read pin (for example Leaderboard Golf); some simply present the data on the data pins a set period after the address pins change value (usually in the range of nano-seconds) so as soon as the address lines change the data output on the data pins changes almost instantly afterwards.
If you have some games that always load, and others that do not, but work in another console, this may indicate your !Read pin on the cartridge connector is bad so the games that require a !Read signal do not load, but those that ignore it still work.
So long as the cartridge is receiving 5V at pin 2, and the !Read signal pulses low, the D0 to D7 pins should get loaded up with the data stored in the game by the IC inside the game cartridge.
You should see the !Read pin similar to this on pin 26 of the cartridge connector:
If you see no signals on the data pins even though you have a !Read signal, it is worth also taking off the front cover of a game and inserting it, then probing the pins directly on the ROM chip of the game to make sure the cartridge connector has good connection to the game. Here is an example of a bad cartridge connector making the !Read pin signal very low once it gets to the game ROM chip. Notice the voltage leve is less than 1V when it should be pulses of 5V like above.
You can try cleaning the cartridge connector and game connector with IPA, white vinegar and also fine sand paper all to try and remove dirt and grime and make a better connection.
The data pins on the cartridge connector (pins 12-15, 19-23) should be toggled with data when the cartridge is inserted showing the game cartridge is trying to load data so long as the !Read pin is toggled low or the game inserted does not use the !Read pin.
On the 1-ASIC models it is easy to tell. Here is with no game inserted so no activity on Data 0 pin 13 as an example, and with a game inserted. You can clearly see when the data is not being driven by the game and when it is:
However on 2-ASIC models it is harder to tell but it can still be done easily if you know what to look for. Notice without a game the hills instantly fall once they reach the top and there are no flat tops, as well as the pattern is identical. However when a game is inserted, the data changes and there are flat tops also.
Once a game has successfully loaded, there are several ways to detect the game is now running.
If you have a working screen and you see the game that is obvious. If you have a working audio board and there are no issues with the speaker, audio board or traces running to the audio board you can hear the game running and that is another sign of a working game boot.
However, for a more basic and solid proof the game is running, presuming you have possibly broken traces, bad audio boards and damaged LCDs, you can detect the game has loaded by using an oscilloscope to look for any of the following activity:
The IOReq (pin 37 on cartridge connector) starts pulsing low signals:
The only other obvious signal of the game successfully loading is sound output. Without relying on the real speaker and audio board, you can test the Sound Left (pin 37 on IC3 of 2-ASCI, pin 67 on 1-ASIC) and Sound Right (pin 38 on IC3 of 2-ASCI, pin 66 on 1-ASIC) for microphone level audio signals at a time division of around 10milliseconds gives a good visual:
For an easier visual of the sound the 4 digital channels of sound can be samples (pins 33-36 on IC3 of 2-ASIC, or pins 80, 86, 88, 90, 91, 94, 96, 97 on 1-ASIC). Again sample at around 10milliseconds.
If you see all address pins, data pins, !Read pins toggling data signals as expected, but you still do not get a game to load or see any IOReq or sound activity, it could be because any one of the address or data lines is damaged partially or fully between the ASIC, the cartridge connector, the MPU (2-ASIC only) or the Main RAM (IC4 on 2-ASIC, IC3 on 1-ASIC).
You should use an oscilloscope to test all the address and data pins, !Write, !Read, clocks and power rails on the Main RAM, cartridge connector, MPU (2-ASIC only) and ASIC(s) to make sure they are all receiving power, clock signals, reset signals, address and data signals and they all look the same.
For example if your cartridge connector shows data coming out of the Data0 pin correctly, but on the Main RAM or ASIC that Data0 pin does not appear at all, or does not show the same, it is a sign of a damaged trace between the connections of each chip and it can be repaired by jumper wire or similar.
NOTE: A singe break anywhere on any of the above mentioned pins to any one of the locations (RAM, MPU, cartridge or ASIC) will completely prevent a game from loading, so it is important to check every single one).
The first step to troubleshooting any issues above that do not match the expected behaviour is the look for breaks, shorts or corrosion on the traces and via's of the problematic pins.
Dark green patches where its normally light green on traces or vias (holes) are usually corrossion from capacitor or battery acid and cause broken lines.
Shorts between components or pads often occur from modification by the user (such as recapping, upgrades or mods) and should be looked for anywhere rework has been done.
If you have damaged/faulty MPU and shared RAM (or you remove them) then the !Write (pin 3) and Address pins (4-12, 25, 28-32, 33) of the cartridge do nothing. When you have only faulty Main RAM (or you remove it) the !Write (pin 3) and Address pins (4-12, 25, 28-32, 33) still try to read the game as normal. So if you get no !Write or Address activity but the ASIC is out of reset, it must be a communication issue with the MPU. NOTE: Only 2-ASIC models have the MPU so this issue will only be noticable on the 2-ASIC boards.