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Re: [gpsd-dev] another git head problem

From: Dave Platt
Subject: Re: [gpsd-dev] another git head problem
Date: Thu, 10 Oct 2013 14:30:59 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.0

On 10/10/2013 02:22 PM, Eric S. Raymond wrote:

> (Yes, that ttyACM0 is a USB port - the TCX0 pretends it'a a USB
> modem for some odd reason.)

Probably because it's easier / standardized, I'd guess.

The ttyACM* name means that the USB device is enumerating with a
"communications device class" endpoint.  This is a standard
"serial communications, stream of characters" protocol that
can be used by modems, cellphones, or anything else that wants
to behave sorta-like-a-UART.  If I recall correctly (quite
possibly not) the CDC endpoint type omits the hardware-specific
signalling aspects of a UART e.g. hardware flow control,
carrier detect, data terminal ready, etc. - it's simply
providing a bidirectional byte-stream connection, using native
USB flow control in both directions.

The alternative is to actually present a serial-port endpoint,
and as far as I know, all of these use vendor-proprietary USB
command sets and require that the operating system have a
driver for that vendor's chipset.  FTDI and Prolific are the
most common, there are some TI parts as well and probably
others... and driver problems with all of these are a constant

Using the CDC endpoint type avoids much of this, since CDC devices
all use the same protocol (in theory... and as we know, theory and
practice are equivalent only in theory and rarely in practice :-)

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