PC Cradle Pilot
DB9-Female DB9-Male 10-Pin
---------- -------- -------
1. DCD 1. DCD
2. RxD ------- 2. RxD 6. TxD
3. TxD ------- 3. TxD 8. RxD
4. DTR --+---- 4. DTR 3. GPI
5. Gnd --|---- 5. Gnd 1. Gnd
6. DSR --+ 6. DSR 10. GPO
7. RTS ------- 7. RTS 5. CTS
8. CTS ------- 8. CTS 7. RTS
9. RI 9. RI
PC Cradle Pilot
DB25-Female DB9-Male 10-Pin
----------- -------- -------
2. TxD ------- 3. TxD 8. RxD
3. RxD ------- 2. RxD 6. TxD
4. RTS ------- 7. RTS 5. CTS
5. CTS ------- 8. CTS 7. RTS
7. Gnd ------- 5. Gnd 1. Gnd
6. DSR --+ 6. DSR 10. GPO
|
20. DTR --+---- 4. DTR 3. GPI
|
The key here is on the PC end you jumper DSR back to DTR.
Instead of the PC getting its DSR signal from the Pilot's GPO
pin, it just gets it right back from its own DTR, which is necessarily
asserted. See the information below about GPO leakage. I used 24 Ga insulated wire of about 1 inch length, stripped
only a few millimeters at the end. Short wires with normal insulating
jacket will end up providing some of the rigidity between the
two connectors. The DB9-M connectors had standard solder ears
on the back. You can make a housing from heat shrink tubing over the ears
of the DB-9 connectors, or build the whole thing in a DB-9 -
DB-9 enclosure (if you can still find one!). The 25-pin PC Plug Adapter wiring to the left is courtesy of Andrew Lattis. See his explanation below. |
I don't have a open 9 pin port so I had to use the 25-9 pin
adapter that came with my palm pilot. This plus the drain fixer
was alot of stuff for one port so I made an all-in-one fix. Used
a 9 pin male connector and a 25 pin female connector, along with
a "MultiPurpose Hood and Adapter Housing" made for 25
pins ( Radio Shack Cat no. 276-1520A ) which put it all in one
nice neat package. They may have that type of hood for 9 pin's
too but I didn't think to look. (JRH: I haven't seen them in
years.)
3Com Tech support asserts that the Pilot's battery can be drained
while in the cradle because, "Leaving the unit in the cradle
keeps the serial port connection open. This could use the battery
life quicker than normal." While it is true that the cradle
does drain current out the serial port, it is not because the
port is "open" and it is certainly not the case that
leaving it in or out of the cradle affects whether the serial
port is "open" or "closed."
The Pilot's serial output is driven by a Max3222 RS-232 driver
chip. The chip has an on-board DC/DC power supply that steps up
the 3.3V of the Pilot's power supply to +/- 6 V for reliable drive
of RS-232 systems. This is true for all of the signals except
DTR and DSR. DTR goes into GPI, which drives the base input of
a transistor switch, which, according to Luis
Coutinho, goes to a General Purpose Input pin 32 of the Dragonball
CPU. DSR is driven by a transistor switch which comes from a General
Purpose Output pin 18 of the CPU.
When HotSync is active, the CPU turns on the Max3222 DC/DC
converter through a control pin (U1 22 TOUT2 to Max3222 pin 22).
This could be thought of as the serial port being "open".
Simple Term does the same thing with its "Online" and
"Offline" buttons. When online, you can see +6 V or
-6 V on all the serial terminals. When HotSync is completed, the
CPU turns off the Max3222 DC/DC converter through the same control
pin. The voltage goes away completely on all the pins. Well, all
of them except GPO.
When the serial port is "open," the GPO is asserted
to +6 V. When closed, however, the GPO pin falls and remains at
+3.3 V. I don't know why this is. I think it may be a leakage
through the base-collector circuit from the Dragonball CPU. The
end result is that because DSR is driven by GPO, which is not
on the Max3222 driver, it doesn't turn off completely, and the
DSR input on the PC continues to drain current out of the Pilot.
That's the bad news. The good news is that there is ONLY ONE
signal that is causing the drain! The solution is simple: disconnect
the DSR input to the PC. I've wired it over to the DTR output
of the PC to satisfy the DSR signal. Simple as that.
Now, you could just disconnect DSR. In fact, in experiments,
I've determined that HotSync Manager ver 2.1 doesn't need to see
DSR in order to operate. Just the same, I think it's a better
practice to tie it off to DTR. Who knows what future version of
HSM might decide to start looking at DSR to fix some bug with
HSM popping open at untoward times because of noise on the line
or something.
One improvement might be to wire this whole circuit into your
HotSync Cradle itself! In that case, looking at the pin connection
diagram, you'd just desolder the GPO cable wire (from pin 10 on
the connector at the base of the Pilot) and solder it to to the
GPI pin (pin 3). I haven't tried it but it should work just fine.
That way you don't need extra dongles in the wire. Of course,
it will void your warranty on the HotSync cradle, so the dongle
approach lets you preserve original factory equipment. It also
lets you still have the option of using your HotSync Cradle with
an external modem, which definitely will look for the signal.
Update: There is no wire on pin 3 of the Cradle (GPI). This
would be expected to come from the computer's DTR, but it looks
like the Pilot isn't interested in sensing whether the computer
is actually connected. I had recommended you short the removed
DSR wire from pin 10 to pin 3. Since there is no wire on pin 3,
this will not do any good. Based on my experiments and others,
just leave the wire from pin 10 disconnected and it should work.
Or, if you want to avoid electrical noise, wire it to Gnd (pin
1).
Terence Conklin of Conklin Systems describes the Cradle
Modification in fine detail. Check out his site. Special thanks
to Terence for motivating this research.
