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Serial port

F

Fred

Jan 1, 1970
0
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
 
R

Rich Grise

Jan 1, 1970
0
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.

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
 
C

Colin Dawson

Jan 1, 1970
0
Fred said:
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

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
www.cjdawson.com
 
J

John Fields

Jan 1, 1970
0
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.

---
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.
 
R

Roger Hamlett

Jan 1, 1970
0
John Fields said:
---
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.
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
 
S

Si Ballenger

Jan 1, 1970
0
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".

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.
 
J

Joel Kolstad

Jan 1, 1970
0
Si Ballenger said:
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.

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.
 
S

Si Ballenger

Jan 1, 1970
0
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.

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.

http://www.geocities.com/zoomkat/ezservo1.htm
http://www.passmark.com/support/loopback.htm
 
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