3421A CAL RAM

[Boeingflieger]

In case anyone is still interested how to calculate the parity for the 3468 and the 3421A calibration RAM after all these years.

The 3468 and the 3421A use a two dimensional parity scheme which can correct single-bit errors and detect double-bit errors to protect each calibration entry.

To calculate the parity bits, the calibration data is arranged in 2 columns of 6 nibbles each. The first column ist holding nibbles 0 to 5, the second column nibbles 6 to B.

Code:


                            │ E: d 1101 │ F: 7 01░░ │ 1
                                  ²↑↑↑↑       ²↑↑     │
│ 0: 0 0000 │ 6: 6 0110 │ 1 ┈┈┈┈┈┈┈┘│││        ││     │
│ 1: 0 0000 │ 7: f 1111 │ 1 ┈┈┈┈┈┈┈┈┘││        ││     │
│ 2: 0 0000 │ 8: 4 0100 │ 0 ┈┈┈┈┈┈┈┈┈┘│        ││     │
│ 3: 0 0000 │ 9: 3 0011 │ 1 ┈┈┈┈┈┈┈┈┈┈┘        ││     │
│ 4: 0 0000 │ A: d 1101 │ 0 ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┘│     │
│ 5: 0 0000 │ B: c 1100 │ 1 ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┘     │
      ¹↓↓↓↓       ¹↓↓↓↓                               │
│ C: f 1111 │ D: 0 0000 │ 1 ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┐┌┈┈┈┘
                                                ³↓↓⁴
                            │ E: d 1101 │ F: 7 0111 │

1. Compute the parity of each column and make the parity odd.
   Parity goes into nibble C for the first column and into nibble D for the second.
2. Compute the parity of each row and make the parity odd.
   Parity goes into nibble E and the two most significant bits of nibble F.
3. Compute the parity of the column parity (nibble C and D) and make the parity odd.
   Parity bit goes into nibble F bit 1.
4. Compute the parity of the row parity (nibble E and the two most significant bits of nibble F)
   and make the parity odd. Parity bit goes into nibble F bit 0.
   

CG