From https://www.cryptomuseum.com/crypto/rs/pu104/index.htm :-

PU-104 was a key generator for perforated paper tape, made around 1986 by Rohde & Schwarz (R&S) in München (Munich, Germany). The device was able to create long sequences of truly random numbers and send them to external paper tape puncher via its built-in V.24 interface.

I'm trying to understand how random noise was sampled by the CPU. I don't think that they used an dedicated analogue to digital converter, but rather went directly from analogue to digital logic. I can't follow what happens after the noise enters the purple square in the following schematic extract:-


The extract is from the complete user manual at https://www.cryptomuseum.com/crypto/rs/pu104/files/pu104s2_manual.pdf. See pages 24-27 of the PDF file. The 'bottom' board photograph clearly shows two PM156 op-amps which I cannot locate on the schematics. They're not of a style I'm accustomed to.

It don't know why there are two schematics.

  • 2
    \$\begingroup\$ "directly from analogue to digital logic" <-- that is an ADC, just a 1-bit ADC. (don't laugh, these exist, even as separate components.) \$\endgroup\$ Commented Mar 10, 2019 at 15:31
  • \$\begingroup\$ page 29 defines D21–D24 to be SN75150AP line drivers, if that helps. An RS-232 line driver, to be exact. \$\endgroup\$ Commented Mar 10, 2019 at 15:36
  • \$\begingroup\$ V for transistors and diodes, that's interesting. Never seen that before. Probably short for a German word, since it's a German company. \$\endgroup\$
    – Hearth
    Commented Mar 10, 2019 at 16:19
  • \$\begingroup\$ @Hearth - the Vx naming convention is a holdover from the days of vacuum tubes (valves) which were used in equivalent transistor and diode applications. \$\endgroup\$ Commented Mar 10, 2019 at 16:22
  • \$\begingroup\$ @PeterSmith Ooh, that makes sense. I didn't realize the designator got carried over to semiconductor devices. \$\endgroup\$
    – Hearth
    Commented Mar 10, 2019 at 16:23

1 Answer 1


This doesn't look like a noise generating circuit.

According to the BOM on page 29, D21 to D24 are SN75150AP, so RS-232 line drivers, which need a dual +-12V supply. That's exactly what V1 and V2 are doing: being rough, regulated power supply. I'm not sure, but V3 and V4 being BZX79-C13 have a ~ 13V Zener voltage – these might simply be protection diodes.

Generally, your PDF is the manual and service manual to the V.24 serial interface card, not to the board that generates the randomness.

  • \$\begingroup\$ I see what you mean. That transistor/diode arrangement is the same as for producing Zener avalanche noise, which I've used and see all over the Interweb. I didn't expect R23/24 to be so low though. 39K perhaps, but not 390 Ohms so perhaps you're right. The implication is that the entire RNG circuit is missing from the PDF. \$\endgroup\$
    – Paul Uszak
    Commented Mar 10, 2019 at 16:51
  • 2
    \$\begingroup\$ as said, you're barking up the wrong PDF! You've got the service manual for the V.24 board, not for the RNG board. \$\endgroup\$ Commented Mar 10, 2019 at 16:57

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