ASR 33 Teletype Information
The ASR33 is a printing terminal and a program storage device (paper tape)
used with PDP-8 systems. This model of Teletype terminal was made from
about 1965 to 1976. They were quite common on earlier machines or
minimally configured machines. The ASR 33 uses rolls of
paper on which it prints 72 characters at 10 characters per inch.
The ASR 33 teletype can print 64 characters which only allowed for
UPPER CASE LETTERS, numbers, and symbols. The paper tape reader and
punch can handle 8 bit binary data.
The tape punch uses 1" wide oiled paper tape rolls. Oil is added to the paper to lubricate the punch. The communication to the host computer uses 20 mA current loop. The early PDP-8 serial interfaces only supported current loop. In the 8/E timeframe the serial interfaces supported both current loop and RS-232 which uses voltage levels for sending data. The last PDP-8's only supported RS-232.
The ASR 33 transfers data at 10 characters per second, 110 baud. This isn't too bad for interactive use but is slow for printing large listings or loading large programs. It would take close to 7 minutes to load 2000 words of the PDP-8 memory from the paper tape. That made the high speed reader/punch or a disk drive popular options.
The ASR 33 was derived from the Teletype corporation Baudot terminals which were used for telecommunications, newswire reports, and radio teletype. The Baudot units used 5 bit codes. The ASR 33 used 8 bit codes to be compatible with computers although it is upper case only. During this time computers were standardizing on multiples of 8 bits for the word size. The PDP-8 with its 12 bit word was the end of the earlier era when many different word sizes were used.
The ASR 33 has very little electronics. The keyboard, printing, and reader/punch encode and decode the serial data electromechanically. Early electronics were expensive so were used sparingly. The first Teletypes were made in 1902, well before the transistor was invented in 1947. Since it is mechanical it requires periodic maintenance. The manual calls out about 500 spots to oil or grease plus many adjustment points. The one shown in these pictures mainly needed oiling.
This type of printing terminal was made obsolete by faster dot matrix printing terminals such as the DECwriter II more DECwriter II and CRT terminals such as the DEC VT family VT52 and VT100 more VT100.
The teletype was quite noisy during operation, loud enough to need to speak up to be clearly heard when talking while it was printing. The videos below will let you see and hear what it was like using one.
The unit in most of the pictures was made in about 1972 and the older one about 1970. In 1974 the ASR 33 teletype sold for $755 to $1220 depending on options.
Teletypes used with PDP-8's were normally configured with mark parity keyboard. Mark parity is highest bit is always 1. Some early PDP-8 software only worked with mark parity. Later software ignores the parity bit.
DEC also made a modification to the teletype which used a reader run signal from the PDP-8 to control the paper tape reader. See LT33 mod
For more information see History of the Teletype corporation Teletype timeline Another ASR 33 page and another
Instructions for operating ASR 33 with a PDP-8 (scanned PDF)
Documents related to ASR-33 Teletype including ASR-33 manual
teletypeparts@aol.com has parts for ASR 32 and 33 Teletypes and also does restoration of them.
| Data Transfer Rate | 10 characters per second, 110 baud |
| Encoding | ASCII |
| Printing | Upper case only 10 characters per inch 72 characters per line |
| Printing Paper | 8.44" by 4.5" diameter roll |
| Paper Tape | 1" by 1000 foot roll 8 level 10 bytes per inch |
| Operating Environment | Temperature: 40 to 110 F
Relative Humidity: 2 to 95 % Altitude: 0 to 10,000 feet |
| Power | less than 4 amps at 115V 60Hz. |
| Size on stand, not including paper holder | 22" x 18 1/2" x 34"(Width x depth x height) |
| Weight | 75 pounds on stand with paper |
| Cost | $1850 and $120 installation from DEC in 1974 $37 per Month for maintenance contract |
Teletype VideosAs seen on TV. National Geographic came over to film my Teletype for the Titanic: The Final Secrets show. You can see it from 16:07 to 16:16 in the video. I now have an IMBD entry
The following picture links also have descriptions of what is shown in the pictures.
The following are pictures of the entire ASR 33 Teletype
Front view ( 31K)
rear view ( 16K)
The following seven pictures are of my older portable Teletype with sound cover FORTRAN print sample ( 28K)
Front view ( 22K)
Inside cover ( 31K)
Front view without sound cover ( 28K)
Rear view ( 30K)
Inside front unrestored ( 58K)
Inside rear ( 49K)
Reader Power ( 37K)
The following are pictures of the main teletypewriter set
Front ( 38K)
Rear ( 60K)
Left ( 58K)
Right ( 54K)
Closeup of selector cam ( 59K)
Closeup of typing unit ( 59K)
The following are pictures of the typing unit
Front ( 41K)
Rear ( 40K)
Left ( 48K)
Right ( 47K)
Top ( 47K)
Bottom ( 39K)
The following are pictures of the paper tape punch
Front ( 48K)
Rear ( 54K)
The following are pictures of the paper tape reader
Front ( 25K)
Left ( 41K)
Right ( 44K)
Rear ( 35K)
Top ( 36K)
The following are pictures of the keyboard
Top ( 31K)
Top no cover ( 38K)
Closeup ( 46K)
Rear ( 37K)
Bottom ( 34K)
left ( 42K)
right ( 49K)
The following are pictures of the call control unit (power supply)
Front ( 24K)
Left ( 22K)
Right ( 23K)
Rear ( 25K)
Top ( 21K)
The base ( 32K)
Chad bucket with chad ( 34K)
National Geographic filming ( 46K)
Feel free to contact me, David Gesswein djg@pdp8online.com with any questions, comments on the web site, or if you have related equipment, documentation, software etc. you are willing to part with. I am interested in anything PDP-8 related, computers, peripherals used with them, DEC or third party, or documentation.
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