As was mentioned, DECtape formats are very much the same at the hardware
level, but the block size does vary.
Block header format, all controllers except Type 55x, in 18b words:
word 0 - 0
word 1 - forward block number
words 2,3 - 0
word 4 - reverse checksum (077)
Block trailer format, all controllers except Type 550/555, in 18b words:
word 0 - forward checksum (one's complement of XOR of each 6b data frame)
words 1,2 - 0
word 3 - reverse block number (complement obverse of block # if read
forward)
word 4 - 0
The ringer is the Type 55x controllers, which was used in the PDP-1, 4,
5, 6, 7. The checksum format varied from unit to unit. Some used parity
checking (XOR); others (like the 7) relied on software and used one's
complement arithmetic. The reverse checksum was always 0777777 (-0).
Further, the first five Type 550's had only four word headers.
So "modern" (TC series) DECtapes are all interchangeable. However,
ancient DECtapes from before 1966 will require unique handling.
/Bob
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