Try this equation:
((Signed(A)*256)+B)*(1/128)
Using your FC 80 hex response…
((Signed(252)*256)+128)*(1/128)
((4*256)+128)*(1/128)
(1024+128)*(1/128)
896*(1/128)
7
For 8bit Signed(A) results…
0xFF = 1
0xFE = 2
0xFD = 3
0xFC = 4
.
.
.
0x80 = 128
0x7F = 127
0x7E = 126
0x7D = 125
0x7C = 124
.
.
.
0x00 = 0
Stated another way…
0000 0100 (0x04) FOR +4
1111 1111 (0xFF)
==== ==== XOR FOR ONE’S COMPLIMENT
1111 1011 (0xFB)
0000 0001 (0x01)
==== ==== +1 FOR TWO’S COMPLIMENT
1111 1100 (0xFC) FOR 4
XOR:
… 0  1 
…………….
0  0  1 
…………….
1  1  0 
…………….
