Decimal Point Format (SPICE series)

[teenix]

The display digit LEDs being on or off are determined by the 4 bit nibbles in Registers A and B, which are decoded inside the CPU IC. These nibbles are sent through the display decoder as each digit is to be displayed and probably latched and then decoded as the next 4 bits enter.

Register A nibbles determine the digit 0 - 9, E, r and o.
Register B nibble determine the digit sign, DP or Comma.

If register B nibble bit 0 = 1, then a DP or Comma should display
If register B nibble bit 1 = 1, then a DP will display
If register B nibble bit 1 = 0, then a Comma will display

The FX pin on the CPU IC is connected to the power supply VCC rail via the link on the power supply board. If the link is in, FX = Logic 1. If the link is out, FX = Logic 0.

if FX = Logic 1, then the DP or Comma display will happen as above.
If FX = Logic 0, then bit 1 of the B nibble will be inverted, changing the DP for a Comma and vice versa.

I guess a simple 2 input XOR gate is connected to bit 1 of the display decoder latch and the FX input.

cheers

Tony