With some further reading and research (and the kind help of Heinz Lycklama and Jon
Steinart) I’ve found that my understanding of early loop networks at Bell Labs confused
several different systems. As far as I can currently tell there were at least 4 different
loop networks developed around 1970 at Murray Hill.
1. The first one is the “Newhall Loop” (paper published in 1969). This loop used twisted
pair cabling, ran at about 3Mhz and used variable sized messages. It seems to have used
some sort of token to coordinate between hosts. This might have been the network that Ken
Thompson recalled as having been in operation when he arrived at the labs in 1966.
2. The second one appears to have been the “Pierce Loop”, as described in 3 BSTJ papers
submitted in 1970/71. This one was coax based, used T1 compatible frames and was used to
connect H516 computers with various bits of equipment. It seems to have had a very short
life span. Part of my confusion was that the term Pierce Loop also appears to have been
used in a generic sense to denote loop networks with fixed-sized frames.
3. The third one is the “Weller Loop” (paper published in 1971). This loop used coax
cabling, ran at 3.3Mhz and used fixed 35 bit frames/cells. Each cell carried one address
byte and two data bytes. One participant on the loop was the controller and effectively
polled the other stations. In its 1971 form it appears to have been for the H516’s only
and was referred to as a “Serial I/O bus”. This is what Jon Steinhart was talking about.
The Weller loop was later redesigned (memo written in 1973) to interface with PDP-11’s as
well. Heinz Lycklama used this loop in 1974 to connect several systems running (rump) Unix
- see his paper about peripheral Unix here:
This Serial I/O bus remained in use for several years at least.
4. The fourth and best known one is the “Spider Loop” (memo published in 1974, but
operational from 1972). Twisted pair cabling, using T1 compatible frames. In use until
about 1978. Main uses appear to have been the ‘nfs’ file store and the ‘npr’ remote
My conclusion from all that is that in 1974 Unix had access to two networks, Spider and
the Serial I/O bus. For both, first experiments would have been in 1973. It is hard to be
sure which one came first. If I had to venture a guess today, I’d say that Spider
connected to Unix several months before the Weller loop (even though the 1st generation
Weller loop preceded Spider). Maybe the conclusion is that both happened more or less in
parallel: Weller was also one of the designers of the Spider hardware.