The PET Transfer Protocol (PTP), also known as Punter or Old Punter, was developed ca. 1980 by Steve Punter for use with his PETBBS and BBS64 bulletin board system (BBS) software. The "PET" in the name comes from the Commodore PET computer.
Compared to other contemporary protocols, PTP was slower than YMODEM and ZMODEM but faster and more reliable than XMODEM.
The earliest version of Punter supported only 7-bit transfers and used a back-correction algorithm involving two checksums for failsafes. One of the two checksums was additive, and the other was Boolean in nature (executing EOR instructions), making for an easy to understand algorithm for other programmers to understand and emulate. Having two checksums - both of them being 16 bits wide - made it significantly more accurate than the single-byte checksum used by XMODEM, its major competitor in the early 1980s. Regardless of the potential for errors to creep in, in comparison to the YMODEM protocol of the late 1980s, which was arguably superior, it has been widely used on Commodore PET and Commodore 64 based bulletin boards.
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