Apparent noise on unused serial cable

Derry

In my application, the Javelin module is mounted in a board with a four-pin header attached to the programming port pins. I have been experimenting with the board using a serial cable of about ten feet with a DB9 at one end and a four-pin socket for the header at the other. I plug the DB9 into my computer. I noticed an odd thing: If I get the program running, and then unplug the cable from the computer, the program stops. When I push the reboot button, nothing happens. When I unplug the (unused) cable from the board, everything works again -- immediately, in fact, since it reboots itself. I am guessing that the unterminated cable is picking up noise and transmitting it to the Javelin. Questions:

1. Is my hypothesis is right?

2. In the real installation, the pigtail from the board will be about a foot long, not ten feet. Is that likely to prevent the noise, or reduce it to the point where it isn't a problem?

3. Is there some kind of termination I can plug into the DB9 to prevent problems when the computer isn't connected? Would a ferrite bead help? Any other ideas?

Thanks,

Derry

Peter Verkaik

Yes, I have had that experience too with a 3meter cable (which is about 10 feet).

I since then always unplug the cable at the program connector.

It is the ATN line that picks up noise and keeps the javelin in reset.

Try it with a 1 feet cable and see if the javelin runs.·If it doesn't

you could try to mount a resistor between ATN and GND at the loose

cable end. I suggest a 3k3 value. That should leave enough voltage

so the javelin can still be reset by pulsing the DTR line (which connects to ATN).

Question: do you have the two 0.1uF capacitors on the ATN line as in the

javelin demoboard schematic?

regards peter

·

Derry

Thanks; I'll try the resistor. Regarding the capacitors: Hmmm, I can't remember if those are there or not. I will look and install those too if they are missing. Good suggestions.

Derry