On Mon, 19 May 2008 01:56:09 GMT, "powerseed via ElectronicsKB.com"
u43512@uwe put finger to keyboard and composed:

Q: Do I have to be concerned about plug polarity with this 12V AC power
supply?

No. There is no polarity.

I am proceeding to troubleshoot the communication side now that the tablet
appears functional. Unfortunately when I plug in my serial to USB connection
per my previous post, no COM2 port activity is observed when I move the puck
and push buttons.

Switches: Firstly I rechecked my tablet switch settings to confirm they
matched the spec given with my Virtual Tablet software.

Interfacing Hardwa As I read it from their manual, the Summagraphics
tablet uses a three-wire, null modem interface with loop back handshaking.
They recommend a standard DB25(male) to DB9(female) converter/adapter cable.
I use a DB25/DE9 converter plug into the female DB25 table port A instead of
a cable. I checked male/female continuity on this adapter plug to insure it
was a null modem configuration...2 on DB25 to 3 on DE9, and 3 on DB25 to 2 on
DE9. The DE9 female of the converter plug is connected to the male DE9 of a
Keyspan serial port adapter which is USB cabled to my Compaq laptop.

Pin 2 of the 9-pin connector is an input (Rx) to the PC, pin 3 is an
output (Tx).

Output Voltages: With my crappo multi-meter I first checked the DC voltage of
the DB25 female serial port A on the tablet, particularly, the TX (2)/RX(3)
pins compared to signal ground (7).

Pin 2 to Pin 7: variable 0.7 to 1.4 volts DC

That's the voltage (floating) which one would expect on an input pin.

Pin 3 to Pin 7: -5.8 volts DC

That's what you would expect on an output pin.

So it appears that your Tx and Rx pins are correctly wired. Your
tablet is configured as a DCE (data communication equipment), not a
DTE (data terminal equipment).

Q: Does this imply the tablet is streaming data out?

You need to measure the pins when the puck is off the surface, and
then again when the puck is in proximity.

COM port assignment: The Keyspan USB/DB9 male serial adapter has DB9 pin-outs
analogous to a DB9 socket on a PC (23=RX, 3=TX, 5+GND, 4&6=DTR/DSR,
7&8=RTS/CTS, 9=ring). One first installs the Keyspan driver software and then
the adapter is connected to the PC via USB cable. The doc says that upon
plugging in the Keyspan, it will take the first open port after those that
have been assigned or set aside. It auto-installs at COM5. Problem here is
that the Virtual Tablet software only allows connections via COM1 through
COM4. Device Manager only shows COM1 taken for the modem.

Using a Keyspan utility program, I reassigned the Keyspan to COM2 and Device
Manager recognizes it at COM2 as available. Keyspan diagnostic software has a
port test function and it passed “transmit only” and “internal loop back”
tests. I assume this means its functioning properly in COM2. I ‘ll have to
get a DE9 serial loopback connector before I can run the “external loopback”
test.

Just short pins 2 and 3 (with a screwdriver?) and access the port
using a terminal program such as HyperTerminal. Disable local echo and
then type any key. If the port is working you should see your typing
being echoed on your screen.

When I plug the keyspan into the tablet, power up the tablet and then boot
the PC, the Keyspan port mapping diagnostic show the adapter on COM2 and
“Busy”.

Maybe the RTS/CTS/DTR pins are in the wrong state. You should measure
them with your multimeter. An RS232 breakout box might be handy.

Another diagnostic provides status reports (driver event and data
monitoring). It shows streaming driver events and no data activity. The
Tablet Manager software was also set to COM2 and it’s status page also shows
no data transmission activity.


At this point I am stymied but, per your recommendation, I did get an old PC
laptop that has a real DE9 COM port. I need to pick up an adapter cable
before I can test the tablet in this manner with the Summagraphics MGIIItest
software.

The Keyspan doc doesn’t show any way to set comm params such as baud, parity,
stop bits, etc. on the computer end. Since the tablet uses two stop bits
would this screw up flow if the Keyspan is operating on a one stop bit
default???

Probably.

Sorry for the data dump but I wanted to give you all my observations in hope
that you may have some further insights to assist me. Thanks to all!

Bill Curry

I wrote the following COM port testing utility to help me test the
various RS232C signals:
http://www.users.on.net/~fzabkar/comstest.exe

It should also help you determine how your cable is wired, ie which
pins are looped back to others. The program runs under DOS and only
works with real COM ports.

- Franc Zabkar
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