[time-nuts] FASTTRAX GPS

Al Wolfe alw.k9si at gmail.com
Thu Jul 3 17:07:01 EDT 2014


    Back in the 1980's I was tasked with installing about 100 terminals 
(VT100 type) to a mainframe computer. These all ran at 9600 baud, RS232. 
Some of the terminals were almost 1000 feet wire distance from the switch. 
We used some Beldon multiple twisted pair to interconnect these devices with 
telco 66 punch blocks in line as well. It worked flawlessly for years before 
individual PC's took over.

    It was only later that I ran across the specs for RS232 that specified 
RS232 as only rated for fifty feet. Oops! Well, it worked for us.

    Anyhow, it appears that the issue here is not the RS232 signals but the 
one second timing pulse width being very short. If I was dealing with this I 
would try a 555 timer as a pulse stretcher. The 555 can operate safely up 
with to 12 volts or more Vcc. It can put out a very robust pulse at that 
voltage. There are cookbook 555 applications on the web for setting up a 
555. Of course the 555 is 30+ years old now and there are probably better 
devices available but the 555 is known to me. The pulse width can be a few 
microsecond to many minutes with this chip depending on the timing 
components, a resistor and a capacitor. The output almost instantly goes to 
nearly the Vcc level when triggered by a negative going trigger. If the GPS 
unit outputs a positive one second timing pulse then a simple one NPN 
transistor inverter should work just fine.

    Set up this way you could shove a robust positive pulse down a piece of 
rusty barbed wire few hundred feet. (Or CAT5 even)

Al, retired, mostly 




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