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PDP-9 - S/N 183


The PDP-9 computer was the successor to the PDP-1, PDP-4 and PDP-7 line of 18 bit computers manufactured by Digital Equipment. There were only 445 PDP-9 machines manufactured. The PDP-9 has as standard 8192 word of 1 us cycle time core memory. Memory is expandable to 32768 words. The PDP-9 is microcoded. It has a ROPE memory that has 64 locations. Each location is 36 bits wide. The microcode even include a very simple bootstrap to read directly from paper tape with a single press of a key on the frontpanel. It uses a single accumulator register for all operations. There is also a MultiplierQuotient (MQ) register used to implement multiply and divide. The entire machine is utilizing discrete components mounted on small boards, either single width or dual width.


Our PDP-9 has been used at Studsvik nuclear testing facility in Sweden. After Studsvik our machine ended up in a high school were we got it in the 80ies. From the very comprehensive list of DEC 18 bit systems installed we get the original delivery. The serial number is 183 as seen from this badge on the system rack.

System as deliverd

 Option Description Serial Number Delivery Date
 KC09A   Main PDP-9 CPU 148 03/68
 KD09A   I/O subsystem of the PDP-9 220 03/68
 KE09A Extended Arithmetic Element Option (EAE) 241 05/68
 KG09A Memory Extension Control  335 07/72
 MC70B 8k by 18 bits Memory System 239 03/68
 MC70B 8k by 18 bits Memory System 801 07/72
 ME09B Memory Extension, Memory Parity, Memory protection chassis 288 07/72
 DA09B Input / Output Bus Adapter 3 03/68
 DR09B 18-bit output Relay Buffer 8 03/68
 AA01A 3 Independent 12 bit D/A converters 108 03/68
 AA01A 3 Independent 12 bit D/A converters 179 07/69
 AA01A 3 Independent 12 bit D/A converters 230 07/70
 AA01A 3 Independent 12 bit D/A converters 241 07/70
 AF01B Single 12 bit A/D converter 118 03/68
 PC09 Paper tape reader and punch 556 03/68
 TC02 DecTape Controle 209 07/72
 TU55 DecTape transport 1889 07/72
 TU55     DecTape transport 1922 07/72

What we can get is that the first delivery was made in march 1968 when the basic processor, KC09 (central processor), KD09 (I/O logic) and MC70 (8 kword memory), paper tape reader / punch (PC09), Input/output buffer DA09, Output relay buffer DR09A, 12 bit A/D converter (AF01) and one D/A converter (AA01).

A few months later the KE09A option was added. KE09A is the extended arithmetic option that allow for faster maths operations. The KE09A options is just 45 more modules added top the backplane. In 1969 the system was upgraded with a two DEC tape units (TU55) and a DecTape controller (TC02). The same year another D/A converter was added. In 1970 two more D/A converters (AA01A) were added. In 1972 a major upgrade took place where the memory was increased from 8192 words to 16384 words (MC70). To handle this the Memory extension option was needed which was equipped with the KG09A modules.

PDP-9 s/n 183
Our PDP-9

After it has been in use at AB Atomenergi it ended up in a school in Hudiksvall. We got the computer from them in the middle of 1980ies. At that time it consisted of the main CPU, one memory cabinet, the A/D and D/A cabinet and a Cabinet which held the two TU55 drives and the TC02 controller. Directly after acquiring this machine we managed to run some test programs to exercise the CPU and memory successfully. Unfortunately we never managed to make the TC02 and TU55 drives work as supposed. We could have the software move them back and forth but there were no data read into memory.

PDP-9 front panel

A close up to the right. A very impressing looking front panel.

TU55 closeup
A closeup of the TU55 drives. The two reals are direct driven by two induction motors. The tape format includes ten tack track which are combined into five + five redundant tracks. One track is used for keeping a clock pulse to clock the other tracks. Then there is a track to indicate what kind of information is present on the data tracks. A project has been started to read TU55 tapes using a micro controller based solution.

The A/D converter is a special module located in a separate chassis. This is the frontpanel.

A/D converter

At Rhode Island Computer Museum they also have a PDP-9 which they are working on to restore. RICM PDP-9