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[aprssig] usb-serial

Stephen H. Smith wa8lmf2 at aol.com
Mon Dec 1 20:44:00 UTC 2008

Ben Lindner wrote:
> Hi All,
> I have just  got a usb-serial  cable working under XP that has been 
> sitting around for many years and has never worked under XP before so 
> did some searching for drivers and found some and it now works 100%. I 
> have tested it with a GPS, all good.
> /My question is, has anyone used such a cable ( USB-SERIAL ) with a 
> TNC without any problems. /I would be interested to hear any thoughts 
> or suggestions

Again,  these devices vary wildly in how faithfully they emulate a real 
serial port. 

It can depend on:

1)    The version of Windows you are using.   [2K - XP - Vista    USB 
support in earlier versions of Windows is so inexorable it's not even 
worth the bother in my opinion.]

2)    The patch level of the version  of Windows you are using.

3)    The application you are using.  [Does it care about serial port 
handshaking lines (RTS, CTS, DSR, DTR, etc) being asserted correctly or 
does it just ignore them?  Does the app use the serial interface in 
unconventional ways (such as wiggling the handshake lines for PTT, CW 
keying, etc independently of the data flow on the actual TXD and RXD 
data lines)?   Or does it expect the port to operate at non-standard 
word lengths (such as 5-bit Baudot RTTY) or odd (by today's standards) 
baud rates such as 110 (ASCII RTTY) or 45 (Baudot RTTY)  ?   ]

4)    The device on the other end of the cable.   [Does it have a full 
multi-conductor handshaking interface that requires correct assertion of 
the lines, or just a simple three-wire TXD/RXD/COMMON serial hookup? 

For example, the Kenwood TH-D7 has a simple three-wire inteface while 
the D700 has a full 9-pin serial port interface that can respond to 
handshaking.   The Kenwood radio memory programming utiltiy for the D700 
gets hung by wrong handshakes, while UIview ignores them on the same 
connection to the same radio.]

5)    The version level of the driver for the particular chip in the 
USB<-->serial cable you are using.   These converter cables are actually 
active devices with a dedicated controller chip, dependent on software 
drivers,  molded inside the DB9 housing -- they are not just a DB9 
socket at the end of a wire. 

[Newest is not always the greatest!  I have found that the latest 
drivers for the Prolific Tech chip set widely used in USB-interfaced GPS 
devices and some USB-to-serial cables works perfectly on one laptop and 
totally locks up another.  These are different machines, but running the 
same patch level of a clean install of Windows XP SP3.   I actually had 
to "down-grade the Prolific drivers two versions to get Prolific-based 
devices to work reliably on the one machine.  The difference may lie in 
the particular USB root chipset INSIDE the the computer, that the 
drivers have to interact with to reach the controller in the cable.] 

Over time and hundreds of posts on ham-related mailing lists, the 
USB<-->serial conversion products with consistently the most favorable 
comments, the most stable & bug-free drivers,  and virtually no 
complaints are the ones made by Edgeport.  

That being said, I  *AM*  using two generic unbranded cables based on 
the FDTI chipset in my APRS webserver to connect a KAM dual-port TNC and 
a KPC3+.  They have been cranking away for years now with no problems at 
all.  [I have found devices based on the FDTI chipset in general to be 
less problematic than those based on Prolific Tech chipsets.   The key 
is to ignore any drivers included on a CD-ROM with the device, and 
instead download the latest versions from the FDTI website.   
[Frequently, the Taiwanese and Chinese mfrs of these devices just 
include a copy of the drivers that came on a developer's CD years ago -- 
often alpha or beta versions of the drivers!]


Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      14400    [Think bottom of the 2M band]
Home Page:          http://wa8lmf.com  --OR--   http://wa8lmf.net

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