Easy Internet For Retro Computers With The PiModem

Retro computers are great, but what really makes a computer special is how many other computers it can talk to. It’s all about the network! Often, getting these vintage rigs online requires a significant investment in dusty old network cards from eBay and hunting down long-corrupted driver discs to lace everything together. A more modern alternative is to use something like PiModem to do the job instead.

PiModem consists of using a Raspberry Pi Zero W to emulate a serial modem, providing older systems with a link to the outside world. This involves setting up the Pi to use its hardware serial port to communicate with the computer in question. A level shifter is usually required, as well as a small hack to enable hardware flow control where necessary. It’s then a simple matter of using tcpser and pppd so you can talk to telnet BBSs and the wider Internet at large.

It’s a tidy hack that makes getting an old machine online much cheaper and easier than using hardware of the era. We’ve seen similar work before, too!

34 thoughts on “Easy Internet For Retro Computers With The PiModem

          1. Hmmm, last night I determined that I could build a large shift register from parts on hand, but that it would consume an A4/Letter amount of board area and probably 10 or 15 watts. Also it would have had a total propagation delay you could measure with a stopwatch, so I decided that discretion is the better part of valour and I’ll order the damn thing.

          2. I agree; the most efficient parts to use are those you already have on hand, which is not necessarily about the $$$.

            I recently wanted to add a BlueTooth Audio receiver to my “mechanical inputs [only]” audio amplifier. I needed a two-channel analog switch. I had “in stock” 74HC4066, CD4066 and several others. I also had a few ANALOG DEVICES AD7592’s (DC: 8335), which requires +/- 15 volt supply. I opted to use the AD7592 b/c it had two (stereo) SPDT “switches”. The BT module has a MUTE output that directly drives the select input of the analog switches. The most difficult part of the design was coming up with low-parts-count power supply to generate +15, -15 and +5 volt rails … all from the internal +/-25 volt DC supply. Luckily, I also had some [ late 70’s vintage ] LM7815 and LM7915 3-pin linear voltage regulators … and an LM309H [DC: 7425 ] 5 volt regulator in a TO-5 can. Interestingly, the 1uF/35V tantaum capacitors [ same 70/80’s era ] ALL popped at around 18 volts! They were likely “16V” mislabeled as “35V”. Again, all parts I have had “in stock” … for several decades.

            On the PImodem; Whatever works, yes?!

            I have a few various 300 baud MODEM I.C.’s from the 80’s as well. How about [ retro computer ] [ 300 baud MODEM ] [ simulated TelCo line ] [ 300 baud MODEM ] [ PI INTERNET interface ] for a true retro “dial-up 300 baud MODEM” ? :)

            Peace and blessings,

  1. I would a Fidonet or AX25 radio modem (for example baofeng)
    not for hamradio but normal PMR. I need simple ssh adhoc communication

    This is needed. Why not create a hat for raspberry pi for emergency adhoc communication (and mobile power)

    1. Maybe using cheap SDR, and the Pi’s ability to transmit radio would help there? One would need a Ham licence though. Question is: would there be enough people to use it, and pay for the equipment, in a given area? I bet one could deliver a reasonable system for less than $100, assuming the end user already had a screen.

        1. it’s abysmally slow, in significant part due to handshakes that cause your transcievers to switch from receive to transmit and back again a lot, because modern protocols were not designed to run on links with that kind of dynamic. i mean, you could tune the tcp protocol parameters to work, but even if you eliminate various overhead types, 9600 baud, which is the fastest modulation on UHF is still something people just won’t put up with these days.

          just search ax25 telnet and ax25 irc on the internet to get a feeling of what we are talking about.

          encryption is a touchy feely subject.

          while some countries, like the us apply the qualifier of encryption “for the purpose of obscuring”, i doubt anyone has the money and energy to legally fight that particularity in court, should some bureaucrat decide he needs to pick a fight. besides, a lot of other countries’ legislature does not make that distinction.

          therefore, any sort of encryption: ssh, https, secure scuttlebutt (which also encrypts messages), etc… are out for anything gaining any semblance of popularity, and in this case, notoriety.

          what you decide to do in your own backyard backwater island of a QTH will, however, be unlikely to attract the attention of the authorities, and will most likely only get you a slap on the wrist if you say you are really sorry and will stop, as long as you don’t do stupid things like interfere with frequencies that other people pay for.

          1. I agree with what you say, I suppose one would have to have a very specific use case – and even then, the system would probably be more problematic technically and legally than it is worth. Would you want to be caught with a secure radio/software somewhere where an authoritarian regime take you for a terrorist or subversive?

            As an erstwhile network tech, I like entertaining hacky impractical ideas for fun – in the real world I’ve helped stop projects e.g. point to multipoint 5ghz as a replacement for ISP links in a MAN, as it just wouldn’t do what was being promised.

            If laws were not a problem, peer to peer (or community led) comms is a very interesting topic – I could imagine it of being use in some developing countries, but even then proper infrastructure would always be better – people want modern connections and content, why wouldn’t they?

          2. @Rog


            traffic from a MAN through a half duplex shared medium? even if the central point is duplex, the edges of the network would uplink to it through a frequency they would presumably clog for their neighbors….. arbitrating that would require clever timeslot arbitration…..

            and then choose a medium that goes bonkers when it RAINS? or if there’s trees in the way?

            for that you’d have to sectorize the antennas and basically build a cell tower network…. i know it’s being done on 2.1GHz, and that’s close enough to 2.4GHz….. but on 5?

            what on earth would justify basically reinventing something that amounts to a mobile telephone network, on a less robust frequency, in competition with existing infrastructure that has far greater size, both physically, and monetary, to absorb the costs?

          3. @Rog

            sorry for the tangent. peer to peer is not haphazardly trivial to manage in a shared medium for transmission, let alone one where the broadcast range for a single node can be from a couple hundred meters to tens of kilometers, depending on antenna placement alone.

            wifi devices usually have a more predictable range, due to the limitations of power, antenna compromises and poor object penetration.

            Packet radio lives on a single frequency, but has separate wide digipeaters that need to be carefully coordinated in their placement, lest they flood the frequency for miles and miles with packets, and routing is…. lackluster by modern networking standards, but accepted because the infomration network is expected to be very localized.

            Then, i forget the name of another network that uses full duplex links between nodes with yagis on vhf/uhf, where on a frequency pair there are only two nodes, and they store/forward through the network, but that REALLY brings placement and frequency coordination to a whole new level almost entirely impractical for large scales….

            then there’s NewPacket Radio, optimized for packet radio over a dedicated repeater with timeslot allocations and everything, although i guess one could peer-2-peer it.

            neither of which makes anything else than an interesting experiment, and in my opinion is not really and truly robust.

            not to mention that with the exception of aprs over packet radio, none of the other solutions allow for easy location changes, but in the case of aprs that comes at the cost of only reaching…. couple of percent of the theoretical network capacity, due to collisions, retransmissions, and digipeating.

            DMR seems to be the more robust of the interesting digital ham radio networks, in that it allows for mobility around a repeater, but even that has ABYSMAL baud rates, not suitable for anything other than its vocoder based codec, short messages, and MAYBE a low-res picture.

            Winlink seems interesting, though i haven’t played around with it as a decentralised network, i only used its gateways and central servers to transmit mail, i am unfamiliar in judging it as a cypherpunk postapocalyptical networking replacement

  2. this is really confusing. Is he using a modem? hooking some circuits up to a phone line? VoIP? It’s amazing how he goes on and on about any and all tangential stuff without addressing this…

  3. Considering that the Pi is probably 10x more powerful as a computer than the computer it is now serving, I really don’t see the point beyond the proof-of-concept.

    1. People enjoy using vintage computers today not because they are computers but because they are vintage.

      That said, a project like that worth every penny spent, even if you pay $35 for for a Pi Zero like I do in the country I live.

  4. I’ve used a Pi several times for a connection to my TRS-80 Model 102. But I looked at the page he wrote for doing that, ah, I suspect that a lot of people will find themselves lost in something.

  5. “Retro computers are great, but what really makes a computer special is how many other computers it can talk to”. – No, what makes retro computers great is that they can do what they do without needing to talk to another computer :)

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