[aprssig] Garmin GPS SERIAL VS USB

Scott Miller scott at opentrac.org
Tue May 31 14:05:51 EDT 2005

And since everyone keeps asking me, no, just because a TNC has a USB port
doesn't mean it'll connect to a USB peripheral.  USB is a very host-centric
protocol and it takes a special host controller to connect to peripherals.
The uTNT and TNC-X both use serial to USB converter chips and don't have any
control over the bus at all.  Devices like the TNC7multi and my unfinished
DC1 are USB functions, not hosts, and don't have the hardware to handle host

Even if you could handle the enumeration and polling and everything, all of
these USB devices have their own ways of transferring the data.  Without the
drivers, all you've got is a set of endpoints.  Could be bulk, isochronos,
interrupt, or control endpoints.  There's no generic way of knowing which
endpoint does what.  Even within the constraints of the USB communications
device class there's a whole lot of ground to cover.

The biggest misconception about USB is that it's somehow just like RS-232
only faster and with a different connector.  Some people see 'serial' in the
name and think that's synonymous with RS-232.  The fact that you can buy
RS-232 to USB adapter cables probably reinforces that view - people don't
realize that there's a relatively complicated chunk of silicon in that cable
that's doing all the work, and drivers on the computer to handle the other
end.  USB is serial in the same sense that Ethernet is serial - it sends
bits one at a time over a differential pair.  The protocol is far more
complicated than the physical interface.

I've heard there are some lightweight host controllers hitting the market,
so maybe in the future it'll be practical to build adapters for specific
devices.  Don't expect to see anything smaller than a PDA with a major
platform OS working as a generic host, though.

----- Original Message ----- 
From: "Wyatt Foard" <wyattfoard at comcast.net>
To: <timbercutter at yahoo.com>; "TAPR APRS Mailing List"
<aprssig at lists.tapr.org>
Sent: Tuesday, May 31, 2005 10:38 AM
Subject: Re: [aprssig] Garmin GPS SERIAL VS USB

> I've had some discussions with design engineers at Garmin and from what 
> I've heard RS-232 is on its way out. Most new designs are being done with 
> only a USB interface. Some higher end models will likely continue to 
> include the port, but the lower cost consumer receivers will be USB only.
> Wyatt - KR5WF

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