Re: Handshaking on USB serial devices

[Gene Heskett]

On Thursday 14 February 2008, Alan Cox wrote:
>> To make it clear: Even aside from the buffer in 2.6's pl2303.c, there's
>> a race: An in-flight write URB can fill all hardware buffers, making
>> unsafe what previously appeared to be a safe write.  I think it's
>> essential to delay submission of the URB on a stop-transmit condition.
>
>Hardware flow control *is* a race, and always will be. The remote end has
>a delay in signalling 'stop' there is a propogation delay and a response
>delay. This is why most implementations assert stop a bit *before* they
>run out.
>
>Given the size of transfers and the internal buffering one would hope the
>USB devices do their own flow control if told to properly.
>
>Alan

Apparently they do not Alan, the pl2303 in particular is a problem child, 
throwing several lost com errors a day when doing nothing more strenuous than 
talking to my belkin UPS from apcupsd, very small packets there, 20 bytes I 
believe at several second intervals.  I can cut those messages to about 
weekly by using an FDTI adaptor in its place, or I can stop them entirely if 
a pure rs232 connection is used.  Unforch, pure rs232 is going the way of the 
dodo bird, and the adapters in general fail such applications badly.  I want 
to build a new machine, but the first thing I'd have to get after a new video 
card is a serial card.  And there goes a slot I'll need for something else.  
My current home automation via heyu, the ups, and the Garmin GPS all require 
serial ports.

Another linux app, a route navigator program that uses the output of a GPS to 
display where you are, also has errors several times an hour when using a 
pl2303 to connect a low baud rate Garmin 12 with no flow control to it.  I 
just recently used it for a 3000 mile nearly month long trip.

So IMO, neither of the 2 readily available devices for usb-serial use is quite 
ready for mission critical usage.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
No extensible language will be universal.
		-- T. Cheatham
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