Radial Serial Protocol was the brainchild of Eric Peters in the DEC
Research Group, who later went on to found Avid, the video editing
company. It was intended to be a "junior brother" of MSCP. The
resemblance is most obvious in the use of command/response packets,
rather than registers (or pseudo-registers) for programming. However,
the two protocols were developed in parallel, by independent groups.
MSCP was hammered out by the DEC Storage team doing the HSC50 in
Colorado Springs; RSP went directly from the Research group to TU58
development.
Because I had been the development supervisor for the HSC50 and UDA50
feasibility prototypes in storage advanced development, I consulted with
Eric on RSP before I moved over to microprocessor design in 1978. I
don't have any memos from that era. Various web documents mention that
RSP was unreliable because it lacked low-level flow control, meaning
that there was no way to block character transmission (in either
direction) once it started, even if the receiving UART was being overrun
due to heavy I/O traffic at higher interrupt priorities. MRSP solved
this particular issue.
/Bob Supnik
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