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Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →To connect two retro computers by dial-up, you need two modems and a local telephone signaling path—usually a telephone-line simulator or a PBX with analog telephone adapters (ATAs). To give an older computer access to a BBS or selected network services without a second physical modem, use a serial Hayes-modem emulator such as tcpser instead. These are different builds: choose based on whether you want the call, handshake, and PPP session, or just the modem-style interface.
Choose the kind of dial-up network you want
| Option | What it recreates | Main hardware | Main setup burden | Key limitation |
|---|---|---|---|---|
| Telephone-line simulator and physical modems | A local call, modem handshake, and potentially a PPP session | Two modems, a server, simulator, and phone cords | Dial-in server software and a client modem profile | Rate and stability depend on the modem and simulator combination. |
| PBX and VoIP ATAs | A phone-network call path between analog modem ports | PBX host/software, ATA phone ports, and two modems | SIP accounts, PBX routing, and ATA setup | Reliable modem-tone behavior is not established for every ATA, codec, or VoIP service. |
| Serial tcpser bridge | A Hayes modem command interface to a BBS or network endpoint | Host, serial/RS-232 interface, and cable | Serial wiring and emulator configuration | It does not create a physical phone-line call between modems. |
Build a two-modem connection with a line simulator
This is the most direct option if the goal is to hear the modems dial and negotiate, then connect an old computer to a dial-in server. A local telephone-line simulator supplies the signaling path between the modems, so you do not need an active outside landline. Jeff Geerling’s 2026 guide uses a Raspberry Pi 3, 4, or 5 (or equivalent Debian host), a USB hardware dial-up modem on the server, a modem in the retro computer, and a two-way simulator. Geerling notes, “POTS, or the Plain Old Telephone System, is tricky to emulate.”
Connect the hardware
- Install a serial-compatible USB hardware modem on the Debian server and confirm that the operating system recognizes it as a serial device.
- Connect the server modem and the retro computer’s modem to the simulator’s two phone ports using suitable phone cords. Check the labels: the devices’ line and phone jacks may serve different purposes.
- Set up the server to answer incoming modem calls and hand the connection to PPP, then configure the retro computer’s dial-up client for the server’s credentials and network settings.
The DLE-200B line simulator and StarTech.com USB 56K modem are examples used in Geerling’s setup, not universal compatibility endorsements. Check the current product revision, regional availability, driver support, and connection requirements before choosing hardware.
Set expectations for speed
Do not equate a modem’s “56K” label with a 56 kbps connection in a complete local analog-to-analog setup. The companion geerlingguy/pi-isp project repository describes its DLE-200B configuration as reaching up to 28.8 kbps and suggests a DLE-300 for up to 33.6 kbps. Geerling’s 2026 guide reports that his setup connects at 33.6 kbps most of the time, with occasional synchronization problems. These figures describe particular configurations; they are not guaranteed results for other hardware.
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Use a PBX and ATAs as the call path
A second physical-modem design routes calls through a local VoIP PBX. In the Asterisk and Linksys PAP2T example described by Raspberry Pi’s 2019 article, the server modem and retro computer modem each connect to an ATA phone port. The PBX routes between the two SIP accounts, providing a local call path without a dedicated line simulator.
This option adds SIP account, PBX routing, and ATA configuration to the project. The article describes the architecture but does not establish reliable modem-tone behavior across current ATAs, codecs, firmware, and VoIP services. Test the exact equipment locally; do not assume a consumer VoIP service will carry modem calls reliably.
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Use tcpser when you need a virtual modem
If you want a vintage computer to dial a BBS or network service, rather than call a second physical modem, a serial bridge can provide the simpler route. The Raspberry Pi Magazine guide describes a Raspberry Pi running tcpser, connected to a vintage computer’s RS-232 port through a MAX3232 transceiver. tcpser presents Hayes-style commands such as ATA, ATH0, and ATDT with a host and port. It is a modem-command emulator and network bridge, not a phone line or two-modem circuit. See the Raspberry Pi Magazine serial dial-up guide and the tcpser project.
Before wiring, identify the target computer’s connector and pinout. Some vintage machines use nonstandard wiring, and TX/RX polarity matters. Choose the transceiver and cable for the machine’s actual serial port rather than assuming a generic adapter will work.
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Configure the dial-in server and PPP
In the Raspberry Pi ISP example, mgetty listens for an incoming call, negotiates with the remote modem, and hands the established connection to PPP. PPP then authenticates the remote computer and configures its network connection. The project provides an Ansible playbook for a Debian host; its example client instructions are for Mac OS 9, so the client setup will differ across operating systems and eras.
The project documents dial / dialpi as example default credentials. Change them before use and configure the client with the credentials and connection details you actually set on the server. Follow the project’s repository instructions for its playbook and configuration rather than assuming those example values are universal.
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Troubleshoot in the order the connection is built
- Check modem visibility. Confirm that the server detects the USB modem as a serial device. If it does not, troubleshoot the modem, USB connection, and host support before investigating call negotiation.
- Check the signaling path. Verify the simulator or ATA phone-port connections, the phone cords, and which jack is the line connection. Confirm that the destination modem answers when called.
- Inspect call and PPP logs. If the call reaches the server but the network session does not come up, check the
mgettyand PPP logs during negotiation to identify whether the failure occurs before or after handoff. - Reduce the maximum modem rate if synchronization is unstable. Geerling reports adjusting the modem’s
AT+MSmaximum-speed setting to address occasional sync problems. Lowering the ceiling may improve reliability, but reduces the maximum possible throughput.
Keep old computers behind deliberate security boundaries
The Pi ISP project installs Macproxy Classic by default to make some modern web content more usable in older browsers. Its documented example service address is 192.168.32.1, port 5001; these are project defaults, not universal settings. The project also warns that Internet security is the user’s responsibility and cautions against leaving old computers connected for long periods, especially while browsing arbitrary websites. Use non-default credentials, expose only the services the build requires, and do not treat an old operating system or browser as safe for unrestricted public-Internet access.
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