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Re: USB "null modem" cables and related Linux driver questions




On Thu, Jun 07, 2018 at 08:07:15AM +0100, Tixy wrote:
On Wed, 2018-06-06 at 22:26 -0600, Joe Pfeiffer wrote:
Richard Owlett <rowlett@xxxxxxxxxxx> writes:

> I have two computers with USB ports.
> I wish them to communicate as simply as mid-20th-century computers
> did.
> Then we used RS232-C with a null modem &/or  appropriate software
> software at both ends.
>
> The underlying problem is that both ends egotistically expect to be
> *MASTER*.
>
> The hardware problem is solvable
> [e.g. http://www.ftdichip.com/Products/Cables/USBtoUSB.htm].

Given how FTDI does things, I'd be really surprised if this didn't
meet
your desires.  Have you actually tried it?  Are you sure there is no
driver in the kernel?

There is a driver in Linux because, from what the datasheet says, that
cable is two of FTDIs standard USB-to-serial chips wired together. I.e.
it's equivalent to getting 2 USB to serial cables and connecting them
with a null modem cable.

Yes, that's right. I have no idea why someone would want to do that, but that's exactly what it is.

There are also usb transfer cables which are basically a proprietary FIFO buffer with a usb master on both sides, which allow higher speed networking. (Which at least seems more useful than going through a RS-232 conversion, but much more limited than an actual network. In some cases where you want more than gigabit speeds and will never have more than two computers it might be useful. I'm not sure how many of them can connect machines with different OSs, either. This is the "laplink" style connection.)

If you want a true, standardized, network session over your peripheral connection, then get firewire. (This is what not having a bus master allows.) It'll be old hardware, because it turns out that nobody actually wanted to pay more so that every device could be a master, and so firewire died. But once upon a time, this was one of its big advantages over usb.

Mike Stone