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'[EE] USB to RS232 converter continued'
2005\09\26@203132 by olin piclist

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I made up a little DB-9 M-F straight thru extension but with the handshaking
lines looped back on the F end.  Specifically pins 1, 4, and 6 were
connected together, and pins 7 and 8 were connected together.  Pins 2, 3,
and 5 were wired accross to the M connector.

The USB serial port worked with an EasyProg when connected via the loopback
adapter, but did not work when connected directly.  This was on a desktop PC
running Windows XP Home.  This seems to prove there is an error in the
driver.  My software uses the standard Windows interface to the COM port and
specifically disables RTS/CTS and any other kind of flow control.  It works
fine on all real Windows COM ports I've ever tried, and I can't see a good
reason why the USB serial port couldn't also implement this part of the
interface spec.  But for whatever reason it doesn't.

Tomorrow I'll try again on a Windows 2000 system.  The wrong driver may have
gotten loaded.

I'll also publish a description of how to modify my devices to add loopback
so that such USB serial ports work directly.

Another point is that the EasyProg took longer to perform the same operation
with the USB serial port.  Programming a full 16F876 with readback at two
voltage levels took 55.5 seconds when using a built in serial port, and took
64.4 seconds with the USB serial port.  There is a lot of handshaking in the
protocol, which may have an effect on this.  The protocol was designed so
that the host could be one command ahead of the programmer.  With a real
serial port that allows a continuous stream of bytes from the host up to the
speed the programmer can handle, with the programmer immediately able to
perform the next command when done with the previous.  The USB serial port
may have more latency when constantly switching between reading and writing.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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