[aprssig] Com port SHARING vs Com Port JOINING

Stephen H. Smith wa8lmf2 at aol.com
Mon Jan 14 22:27:35 EST 2008

Claude Head III wrote:
> I seem to remember comments on this SIG some time ago about a COM port 
> sharing utility, whereby a single COM port could be used by two (or 
> more) applications at the same time.
> My desire here is to use a Garmin GPS receiver to feed data to a 
> DeLorme Street Atlas program and at the same time feed the GPS data to 
> a program that would keep the computer (a Dell Latitude laptop) time 
> correct. I have found an inexpensive program that will set the 
> computer's time using GPS data, but it does not share access to the 
> COM port.

The various replies to this question are confusing two different issues:

1)    Joining two applications (running at the same time on the same 
PC)  that each normally communicate with hardware via com ports.  For 
example, MixW operating as a software TNC outputs to a virtual serial 
port.  You then want to connect the virtual TNC to an application, such 
as UIview that expects to connect to a real TNC connected to a serial 
port.   Port bridge utilities such as the MixW serial port bridge create 
a pair of simulated serial ports cross-connected (i.e. virtual null 
modem).  Each application is then set to use one of the virtual ports. 

The net effect is the same as connecting two REAL serial ports on the 
same PC back-to-back with a null-modem cable, without wasting the 
precious physical ports on data that never leaves the machine.

2)    Spitting a single data stream from an external hardware device 
such as a GPS into multiple virtual serial ports so that several mapping 
(or APRS) programs can use the GPS data at the same time.   The 
Comfoolery application can do this but is rather kludgy since you have 
to run TWO programs back-to-back to implement the splitter; then run 
multiple applications at the same time to use the ports.

The best application for splitting ports is GPSgate from Franson Software. 


This utility can split the incoming data stream from either a physical 
serial port or a virtual com port created by a USB-to-serial dongle into 
almost any number of virtual com ports that other programs can then use. 
It can also convert to/from TCP/IP so that a com port source can be 
distributed over a LAN via TCP/IP. On other PCs on the lan, copies of 
GPSgate can then convert TCP/IP back to virtual com ports.  

In addition, GPSgate can convert Garmin proprietary binary format coming 
in on USB,  to standard ASCII NMEA on multiple virtual com ports!   This 
allows you to use just about any Garmin GPS, even those without NMEA 
and/or serial outputs, with almost any program that can use standard 
serial NMEA data. 

GPSgate is available in two versions. A lite "Express" version that 
spits a single input, real or virtual, into just two virtual outputs is 
USD $13.  The "Standard" version that can split multiple source ports 
into any number of outputs is USD $40.    If you "split" a virtual 
comport into just one new port, you can create the equivalent of the 
1-port-to-1-port  bridge utilities described in 1) above. 

You can run multiple instances of GPSgate Standard on a single machine. 
For example, you could have one instance splitting a GPS from a physical 
port for use with multiple mapping programs.  At the same time, a second 
instance could be joining a soundcard soft TNC to an APRS application 
like UIview or APRSplus. 

Real world applications that I have used:

I routinely split the single physical COM port on my Panasonic Toughbook 
mobile laptop to feed GPS data into UIview, Visual GPS, MapPoint, 
Delorme Topo USA, and Precision Mapping at the same time.  GPSgate is 
set to load automatically each time the computer boots.  

Using a second instance of GPSgate on the same laptop, I have 
intercepted one output of the first instance and converted to TCP/IP. I 
then distributed the IP NMEA stream via ad-hoc WiFi to a second copy of 
GPSgate running on a second laptop in the back seat of the car, where 
several more mapping apps were running. 


Stephen H. Smith    wa8lmf (at) aol.com
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