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DRAFT
Displays for the Masses
The goal was to prepare a plan for Project MAC and the AI Lab to
acquire a system of keyboard and display terminals large enough and cheap
enough to put one in every office in 545 Tech Square - about 130
terminals. The system is to be connected to three PDP-10's on the ninth
floor, and to MULTICS in building 39. Desirability of a direct
connection with the ARPA net was proposed later. The performance goal
was 20 or more 80 character lines of arbitrary characters plus graphics.
The committee to prepare this plan consisted of Tom Knight, John
McCarthy(chairman), and Jerry Saltzer. It was assisted by others from
MAC and AI. In addition, we believe we have consulted all interested
groups in MAC/AI.
An RFP (request for proposals) was prepared and sent to Data
Disc, Ramtek, and Owens-Illinois with a reply date of 1 November for
proposals. Proposals have been received from Data Disc and Ramtek, but
Owens-Illinois requested and received an extension until 15 November.
RFP's were also requested and sent to DEC, Evans and Sutherland, Systems
Concepts, and Imlac. Of these, Systems Concepts is known to be
interested in submitting a proposal. The others have not attempted to
arrange the meeting mentioned in the RFP.
Displays Page 2
After the RFP's were sent, we learned about the approach taken by
Peter Weiner of Yale. We visited them, and now recommend a slightly
modified version of their approach. Their approach is based on the fact
that complete TTL interfaced semiconductor memories of better than 450
ns. cycle, organized into blocks of 4x4k x 16 words can be obtained for
.86 cents per bit from at least two suppliers, AMS and Intel. Since our
purchase will be four times as large, we may get a better price.
This allows storing a 512 by 512 dot matrix raster for less than
$2200, without the maintenance problems and unexpandability of the Data
Disc system. It beats Ramtek by a factor of two in cost, and is much more
flexible than either, because the memory is directly and randomly
accessible to a mini-computer.
The proposed system consists of the following:
1. A PDP-11/45 computer to control the system.
2. Interfaces from the PDP-11 to the three PDP-10's, MULTICS and later
an imp or tip. Connection to the 370/165 run by IPC is also possible.
3. 1024K 16 bit words of IC memory. The Intel version bought by Yale
has 4k words on a card and for each four such cards, a control card, such
that 16k words form an independently addressible unit, producing up to
four 16 bit words in parallel. Our system would contain 64 such units,
Displays Page 3
so that 64 terminals could be active simultaneously.
4. A two port memory control / shift register card per console. One of
the ports reads out into a 64 bit shift register under control of a
centralized video timing generator. A 512 by 512 raster is presently
contemplated, but this is easily changed. The other port interfaces to a
large address space buss, driven by, among other things, the PDP-11.
Half of the PDP-11's 32k memory space refers to a selected display's 16k
words.
5. The 64 video signals go to a video crossbar switch which is presently
envisioned having 72 input lines and 150 output lines. This will allow
up to 150 terminals, 64 of which can be active at once. The other eight
inputs can be other standard video signals, or gray scale or color mixed
video produced from some group of the 64 standard channels.
6. From the video switch, a coaxial cable goes to each display, which is
an ordinary TV monitor. Possibly some small number of good gray scale
and color monitors are wanted.
7. With each monitor is a keyboard, not physically attached to the
monitor. The Stanford keyboards made by Microswitch (we have a sample)
are a leading candidate, but further investigation is necessary.
Displays Page 4
8. The keyboards are connected (possibly by multiwire ribbon cable,
possibly bit serial on coax) to a keyboard multiplexor, which in turn is
connected to the PDP-11.
This completes the description of the initial components of the
proposed system. Here are some remarks on its operation.
1. At first, characters and vectors will be written into display memory
by PDP-11 programs. Writing a character consists of copying a bit
pattern into display memory from the PDP-11's private memory. Because
shifting and masking is required, a PDP-11 program might require, in the
worst case, as much as 150 microseconds to write a character, giving the
system as a whole the ability to write only 7000 characters per second,
shared among 64 terminals. This averages to about 100 characters per
second, and will be adequate for reasonable, although not great service.
Whether performance will be improved by increasing this depends on the
following presently unanswered questions:
a. How active will the users be? How much do they just sit?
How much time do they spend doing input that requires the display
of just 1/3 characters per second? How much time do they spend
page turning which might like a 2400 character page twice a
second? As people get more used to reading documents on the
system, page turning usage will increase.
b. What other bottlenecks are there? At present, the PDP-10's
and MULTICS massage each character outputted so many times, that
it is not clear these machines would put out more than 7000
Displays Page 5
characters per second even if the display system were infinitely
fast. If (when) increased speed is wanted, however, a simple
block transfer shifter/masker can be put on the memory buss,
making characters at least three times as fast as direct
programming, and more than one could be added.
2. The vector generator situation is very similar. PDP-11 programs will
be provided for writing vectors onto the raster memory, but this will
probably limit some applications. A vector generator under control of
the PDP-11 can easily be added. Besides all this, it is possible to add
PDP-11 or other processors to the memory buss so that a channel might
have the full attention of a processor.
3. Although super-display capablility was not part of the charge for
this committee, it turns out that 1000 line TV's can be supported in this
system by providing additional video timing generation and making it
possible to take four 16k blocks so as to have a 1024K raster. The shift
registers will be fast enough for this. The 25 megahertz signals
required for 1024 line TV present some electronic problems, however, so
that experiment is required before a large scale attempt is made to pipe
1024 line TV throughout the building. Cheaper memories will make this
increasingly attractive.
4. On the other hand, memory requirements can be reduced for character
only displays by designing suitable on the fly character generators.
Displays Page 6
This will permit more channels to be allocated to 1024 line use, but is
not proposed as part of the initial system.
Displays Page 7
Furniture
Each display in an office consists of the keyboard and monitor,
each having a power and a signal connection. The keyboard power
connection is a wall mounted power supply. The ones Stanford uses cost
$28 apiece. We recommend also the use of metal tables with wings costing
about $20 and a display rack sitting on the table that holds the display
at a convenient viewing angle. The rack cost about $15.
Displays Page 8
Procurement
1. The longest procurement item is the PDP-11/45 for which a (revocable)
letter of intent has been placed. Delivery on the 11/45 is eight months,
but an interim smaller machine may be obtained.
2. The next longest, curiously enough, seems to be keyboards, with 12-14
week delivery.
3. The approximate order of construction is:
1. Semiconductor memory interface
2. video timing generator
3. teletype multiplexor
4. Attachment of a few terminals
5. At least one main computer interface
At this point, host systems programs can start being debugged.
6. Remaining computer interfaces
The interface to MULTICS requires a high speed data line between building
39 and Tech Square. Once these are in, a 64 terminal system can be
operational.
7. Building wiring
8. Video switch
Other construction can wait until the system is operational and we see
where the shoe pinches.
Displays Page 9
A related project should be to have something like the Xerox LDX
printer on each floor so that hard copy from all computers can be
obtained.
Displays Page 10
Cost Estimate
1024K 16 bit words 450 ns. memory 146K
(based on Yale purchase)
150 keyboards at $205 30.75K
(based on quote from Microswitch)
150 monitors at $190
(based on quote from Ball Brothers) 28.5K
PDP-11/45 with 16k memory 26K
(based on quote from DEC)
Coax at 200 ft per console at 4.5 cents/ft 1.5K
(we mention this since some people propose
elaborate schemes to reduce cable costs)
Video switch 20K
(based on Stanford experience)
Memory buss and video generator 15K
Keyboard multiplexor 10K
Computer interfaces 25K
(5K per PDP-10 and 10K for MULTICS)
Furniture and power supplies 8K
←←←←←←←
Total not counting labor and in house design 287K
Contingencies 20K 20K
←←←←←←←
307K