USB question

connecting two USB devices, at least one device must be able to run in the host mode?

Yes. (Even with "on-the-go" one device acts as a host.)

Yep, there is the concept of a device being a mini host if you want to make a device that is usually a slave into a master, but (I think) in this mode it can only connect to one other device.

tim

-- Jeff Nibler

Rob and Tim, thank you both for your replies. One follow-up question... assume some strange external device has a serial port connector. That serial port has an adapter connected to it which converts it to USB. That device is then a USB compatible device (client-mode of course) correct? Meaning, if you had a computer with USB host capability and a USB hub, you could connect several of those external devices to your computer via USB, and your computer would be able to read both of them correct?

Thanks again!

Yes!

Short answer: Yes.

Longer answer: There are may USB->Serial dongles on the market. These all require some sort of device driver on the host, thus one particular model might (or might not) have Mac or Linux (or...) support.

In theory, you should be able to have several of these gadgets connected at once. In practice, the device drivers supplied with some of these makes that impossible (although the situation is better these days).

Also, as compared to a "normal" serial port, the latency of control actions (flow control, for example) is usually quite high (there's a lot of buffering between the application and the serial port). So a device that drops DTR (or CTS, or whatever) and expects that data will stop flowing in one or two bytes may be sorely disappointed, and may generate an overrun or lose data. Likewise, if you're application is doing a lot of very time specific control of the various control signals, you're going to have problems.

In short (and as always), before you claim these capabilities to a customer, you ought test them, with the particular device and serial port dongle in question.

Thanks Robert!

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