ET-3400 ROM - Testing the circuit

Now to test out the circuit. First thing I am going to check is to make sure my ET-3400 works normally before I remove the original MCM6830A mask ROM.

It boots up and resets to 'CPU UP.' on the display. I then examine a few memory locations and it's working normally.

The ROM is removed and the new circuit on the breadboard is plugged into the ROM socket via the 24 pin ribbon cable. To reduce the chance of smoking up any ICs on the breadboard I removed the 28C64B EEPROM and the 74LS20, connected up my multimeter to Vcc and GND to check the +5V rail.

Powered on and the +5V rail is good. I turned the backlight on my multimeter and it's a bit over exposed, but you can see 4.9444 if you squint. The ET-3400 display is showing '8.8.8.8.8.8.' as it should, since there is no monitor ROM of any kind fitted to run any code. So it's just displaying whatever is in memory. I also checked the power and ground connection points for the ICs on the breadboard and they are good too.


That gives me the confidence to install the 28C64B EEPROM and the 74LS20 and try and boot it up. I made sure to connect pin 2 (A12) of the EEPROM to +5V so it selects the upper part of the EEPROM where the stock code was burned.

Power up and reset results in a successful 'CPU UP.'. I examined a few memory locations including the original code at $FE3F..$FE49 and it checks out correctly.

Now it's time to test the patched ROM code. I connect pin 2 of the EEPROM (A12) to GND to select the patched ROM code and boot the ET-3400 up again and it greets me with  'CPU UP.' once again. I examine the patched code addresses in ROM and they are correct. Below you can see that $FE3F contains #$C6 which is correct in the patched version of ROM.


That's working fine, but I need to test out the alternate ways to enable the /CE and /OE line on the 28C64 next...


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