After my question on the parallel port, I must ask you something about the serial port. Why I cannot see on my scope the data signal if I put my scope directly to the pins. To see it, I must connect the serial port on an other device and then, put the scope on the line.
Thanks for all yours valuable input!!! Fred
You have to jumper DSR to DTR and RTS to CTS, otherwise the computer thinks that what it's connected to isn't ready to receive. Do a google search on "null modem".
Good Luck! Rich
--- Connect your scope ground to SIGNAL GROUND in the serial port connector and then connect the probe input to either RX DATA or TX DATA. Center your trace and set the vertical sensitivity to something like 5 volts per box. Set your sweep speed for a convenient display of the data rate you're expecting and set the sync to auto.
That should get you started, and then you can fiddle with the settings all you want. Be aware that you'll be looking at data constantly changing in time, so unless you're sending (or receiving) a repeating pattern and you can get the sweep speed and sync set up just right you won't see a stable display.
-- John Fields Professional Circuit Designer
Most terminal programs allow you to specify what (if any) handshaking you want to use (even Hyperterminal, the piece of trash MS includes for free with Windoze) so one can avoid worrying about whether or not a proper null modem-type cable is needed.
I'm no experts on serial ports, or electronics in general for that matter but here goes. As I understand it the serial data is actually transmitted via a circuit. When it's left open circuit (not plugged in) a detection circuit knows about this and so the line doesn't get raised or lowered.
When the circuit is fully connected up, the circuit is complete and the line can be raised and lowered. I can't remember offhand but I think that the line needs to be held in a naturally low state so that it can be driven high.
If this sounds like a laymans explaination, that's because my understanding of this is very crude.
Regards
Colin Dawson
Key thing missing here, is to go into device manager, select the port, select the setup options, and 'advanced', and set 'flow control', to 'none'. If this is not done, the port will usually default to using hardware flow control, and data won't be sent, unless the right flow control lines are 'made'. With this done, you should be able to type data in a terminal program, and see the serial data on the scope.
Best Wishes
I had to do the null modem wiring on my win95/98 mcachines lto echo bytes to the com port, but on XP the extra wiring wasn't needed.
In some situations like my page below, using a terminal program isn't an option. The bottom link shows the simple loopback wiring I used on the DB-9 plug for use with my win95/98 machines.
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