[TUHS] updated: loop networks at Bell Labs

Heinz Lycklama heinz at osta.com
Fri Jun 19 11:01:16 AEST 2020

A later memo in 1978 provides a Figure that shows how the I/O Loop
was used to support multiple satellite processors. See here:


On 6/16/2020 3:41 PM, Paul Ruizendaal wrote:
> 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:
> https://www.tuhs.org/Archive/Documentation/TechReports/Heinz_Tech_Memos/TM-75-1352-2_Emulation_of_UNIX_on_Peripheral_Processors_19750109.pdf
> 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 printing service.
> 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.

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