6-16
•For current samples, there are two forms for the second byte. The two forms are signaled by the
most significan bit of the second byte. If the byte is clear, the second byte represents a course
representation and contains information on bits 14 – 8 in modified two’s complement format. If
the bit is set, the second byte represents a fine representation and contains information on bits
14 – 5 in a modified two’s complement format. Values in the fine representation carry three
more bits of information. Values in the coarse representation are weighted by a factor of 8.
Values in the coarse representation are modified in that they have been rounded up based on the
value of bit 7, stored in the lower byte and represent values from +64*8 to –64*8.
Values in the fine representation are modified in that they have been rounded up based on the
value of bit 4, stored in the lower byte and represent values from +64 to –64.
The second byte is used by the meter to calculate the High Speed RMS value used for waveform
capture. The two ranges allow the samples to cover a wide range, with greater precision on
smaller signals than larger ones, while the rounding centers the scales around the true zero of the
full range of the A/D. The full 15 - bit current value can be recovered from these two bytes using
the following steps:
1. If the most significant bit of the second byte is set, skip to step 6.
2. If bit 7, stored in the first byte, is 0, skip to step 4.
3. Decrement the 7 - bit two’s complement value of bits 14 - 8, stored in the second byte.
4. Combine bits 14- 8 of the second byte with bits 7 - 0 of the first byte.
5. Skip to step 10.
6. If bit 4, stored in the first byte, is 0, skip to step 8.
7. Decrement the 7 - bit two’s complement value of bits 11 - 5, stored in the second byte.
8. Combine bits 11 - 5 of the second byte with bits 4 - 0 of the first byte.
9. Sign extend the 12 - bit value by three bits (copy bit 11 to bits 14 - 12) to produce a 15 - bit
two’s complement value.
10. The resulting 15 - bit value is the two’s complement value of the sample.
11. Sign extend the 15 - bit value by one bit (copy bit 14 to bit 15) to produce a standard 16 - bit
integer.
Since the range of the A/D is +2.5 – -2.5, one bit of the coarse representation represents 0.039063V,
one bit of the fine representation represents 0.004883V and one bit of the 15 - bit value represents
0.000153V.
Table 6.8 on the following pages gives several examples.