The difference between "0" and "8" is the MSB of the 4-bit input to the 7447. The MSB in REV- boards is tied to ground. The MSB in REV-B boards is connected to the BLANKING signal through W16. Your board is a REV-B with W16 installed and therefore the MSB is connected to the BLANKING signal.

01_board.jpg
02_blanking.jpg
03_digit_input_msb.jpg

Check the ~BLANKING signal. There is a test point just above U20-2 that is connected to U20-2. It should be LOW when you see "8" and HIGH if/when you see "0".

Alternatively you can test 1J21-2 or 1J22-2. If you test at this point then it should be HIGH when you see "8" and LOW if/when you see "0".

If the signal matches the state then you likely have an issue with the blanking circuit. The Leon does not test the blanking functionality. The blanking circuitry keys to the constant strobing of the display (tied to bit 2 of the strobe input). If the processor is executing correctly and the software is correct then the software needs to continually strobe the display and will cycle through all 16 strobes (4 bits).

If you are using a 27128 then double the actual 2764 image. Since W2 is installed on your board you will need duplicate the image as the A13 address is active. If you remove W2 and install W3 then you can place the 2764 image in the "upper" half of the available storage as the A13 address line is tied to VCC with W3 installed.

If you are using a 27256 then quadruple the actual 2764 image - double the double. If you remove W2 and install W3 then you can place the 2764 image in the 4/4 "slot" of the available storage. The 27256 has $8000 bytes of storage. The 2764 image is $2000 bytes. Place the image at offset $6000 in the available $8000 bytes of storage. You will need to do this as A13 (pin 26) and A14 (pin 27) are tied to VCC with W3 installed.

For High Speed:

• U27 should be 27256
• U26 should be 2764 but can be 27128 (double) or 27256 (quadruple)