Dans le mode d'emploi de la cartouche de test, il est indiqué ceci :

Connect a dumb terminal to the RS232 port of the unit under test (I.U.T.). You can use an Mega/ST running the VT52 terminal emulator program -- see the owner's manual for setting up VT52. The cable should connect pin 2 (serial out) of the I.U.T to pin 3 (serial in) of the terminal, and vice versa. Connect pin 7 (ground) to pin 7. The terminal should be set up for 9600 bps, 8 bits of data, 1 stop bit, no parity (this is the default condition for the VT52 emulator).

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The lowest 2 Kbytes of RAM is tested on power-up; if a location fails, the error will be printed to the RS232 device. It is assumed that if RAM is failing, the screen may not be readable and program execution will fail because there is no stack or system variables. The program will continue to test RAM and print errors, but no screen will be displayed (the screen may turn red). Repair RAM.

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If the display is unreadable, the RS232 terminal should be used. All messages are printed to the RS232 port as well as the screen.

(c'est extrait du Service manual du MegaST, section 3.4, téléchargeable ici : http://dev-docs.atariforge.org/files/MegaST_Service_Manual.pdf)

Donc apparemment on peut se brancher sur le port série pour voir les messages de test.