Analog modem emulator?

Yes. The bit-banger is fairly usless for serial input, although it's fine for output. I suspect it was mainly used for driving a serial printer (I first came across it with a Qume Sprint 5 hooked up to it), and Apple used ETX/ACK protocol _because_ you then know when the peripheral is possibly going to send something (unlike XON/XOFF when the peripheral could send said characters at any time). The 6850-based card is going to be a lot better for input, but it's a more complicated and therefore expensive (I guess) card. But more suiltable if you want to hang a terminal off the Apple (e.g. for running the P-system [1]). [1] As you doubtless know, the Apple P-system checks for a card in slot 3, and if it finds one, uses it (with the firmware drivers on that card) for text I/O. Most people put an 80 column card there, the firmware of which used the Apple keyboard for input, but there's no reason why you can't put a serial card linked to a dumb terminal there. The manual gives examples of how to set this up. The bit-banger card also supports current loop operation, and the manual gives instructions for linking it to an ASR33 (!). -tony