THE GB TEST CART: WHAT YOUR GAME BOY CAN STILL TELL YOU
A Game Boy that switches on and runs a game looks healthy. Whether it actually is healthy is a question nobody can answer, because there is nothing to measure it against.
This cartridge answers it. It is the second one built for this collection, and unlike the first it plays nothing at all. It examines the machine it is plugged into and puts numbers on the screen. Press START below and have a look around.
How to use
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Arrow keys move, and page through text
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A open
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B back
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Enter hold to leave a test (START)
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Space hold for the explanation (SELECT)
The keys are named after the buttons on the console, so A is the A button and B is B. In the test screens the buttons are the thing being measured, which is why leaving one needs START held rather than B pressed. Every screen has an explanation behind SELECT, and it says what the numbers mean and how to read them.
LIMITS OF THE EMULATOR
The version above runs in a browser, and that changes what the numbers mean. Contact bounce is mechanical: a real rubber pad closes, opens and closes again within a few thousandths of a second, and a worn one bounces longer than a fresh one. An emulator switches a key in a single cycle, so every key reads one edge and 00.00 milliseconds, whatever the keyboard under your fingers is doing.
The link port behaves the same way. Some emulators never finish a transfer at all, because nothing on their side is there to finish it, and the screen then says stuck on a machine that is perfectly healthy. That is not a fault in the cartridge. It is the clearest possible proof that what these screens measure is hardware and not software.
THE GB TEST CART ON YOUR REAL GAME BOY (DMG-01)
The version above was built as a real Game Boy ROM. It can be downloaded free of charge and run on original hardware with a compatible flash cartridge, which is the only way the measurements mean anything.
A flash cartridge with a working battery is worth looking for. The log survives being switched off, and that record is what turns a single reading into an ageing curve. A link cable and a Game Boy Printer extend the cartridge further: the report prints on the thermal paper the printer was sold with, and a slip of paper in the box beats a photograph on a phone that nobody finds again.