The Cambridge Ring was one of the first local area networks (LANs).
It operated with a line rate of 10 Mbps using a slot size of 38 bits.
Two address bytes and two bytes of data were carried in each slot.
This view shows the ring cards commonly in use to make a station when
I became a PhD student at the Computer Laboratory in 1984. At this
time, there were three rings in operation, connected using bridges,
providing network service for a total of 100 or so hosts.
The three cards in this view are the repeater, the station and
the Z80 tiny server. A fourth slot in the backplane was often fitted
with an 8-port serial card to create a terminal concentrator. Other stations
were connected to a pair of VAX machines and to a bank of LSI-4 machines.
Later, further stations were connected to VME backplanes and 68k processors.
The 68k processors became the main processor bank.
Cambridge Ring Repeater Card
The repeater card took its power from the ring and could work in
isolation without a station card. In the absence of a station card,
it simply retimed the data whereas with a station card the data
was routed through the station for filling or emptying slots.
Cambridge Ring Station Card
The station card provided the means to send and receive data. The
station would place data in an empty slot and the slot would circulate
until it arrived at the receiver. There the receiver would have a number
of options which it would flag in a response field. It could take out
the data or not, thus providing flow control back to the transmitter.
The transmitter would always empty the slot and pass it on free
for another station to use in a fair way.
Unlike the ATM systems that followed, the 16 bits of data per cell
were too small to efficiently contain upper-layer addressing
information, such as port numbers, so the stations had to use source
addresses to demultiplex incoming blocks, and only one flow, or
segmentation and reassembly operation, between a pair of hosts could be
supported at one time. Indeed, to avoid the host device driver
from having to perform multiplexed reassembly, a hardware autoselect
system could be used so that a new transmitter received the busy
response from the destination if the destination was mid-block
from another source.
Cambridge Ring Z80 Tiny Server Card
This Z80 with 64Kbytes of RAM provided a cheap and simple host
that could be used for various management issues in the Cambridge
Distributed System. For instance, it might act as a resource
manager that keeps track of which servers are in use with which
terminal sessions.
My contribution to the cards shown on this page was the small chip
piggy-backed in the centre of the tiny server. I fitted this gate to
prevent the tiny-server making a spurious I/O cycle access into my own
attached hardware during interrupt acknowledge cycles.
Imagine asking a modern programmer to get all of the program code
and data buffers for an 8 port terminal concentrator into 64 Kbyte
address map!
Cambridge Ring Monitor Station
This view shows the monitor and logging station for one of the three rings
operational in 1985. The monitor station is screwed to the wall of the
Titan Room. The three main sections are the repeater, station and tiny
server. Each was wire-wrapped. A serial cable connects to a printer where the system log
was printed.
Cambridge Ring Station Inside CAP Computer
The CAP computer was an experimental computer
with capability-based access control implemented in hardware.
This view of the CAP shows one of the covers open, with
a wire-wrapped Cambridge Ring station mounted inside the cover.
Indeed, the whole of CAP was wire-wrapped. See R. Needham and
R. Walker "The Cambridge CAP computer and its protection system" in
ACM Symposium on OS Principles, pages 1--10, 1977.
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