Ricochet/PalmPilot Modem Connection

A novel, slim connection using a HotSync Cable (with a DB-9)
and a custom Null Modem / Gender Changer 




      
This is the slimline Null Modem / Gender Changer for connecting
      the PalmPilot to the Ricochet modem. It should work with any
      Ricochet model that has a standard cable to plug into a PC (with
      a DB9-Female connector on the end).


      
OK, so you can just order that custom Ricochet-SE/PalmPilot
      cable direct from Ricochet. But if you have a HotSync Cable (or
      cradle!) already, and you're handy with a soldering iron, you
      can make this Null Modem with about $3 of parts.


      
The picture to the left shows the layout of the connection.
      The custom Null Modem appears in yellow. It connects a standard
      Ricochet PC cable directly with a standard 3Com PalmPilot HotSync
      Cable. No bulky DB25 connectors. No DB25-DB9 adapters. No extra
      gender changers. Just slim cables and one interconnect block.
      Click the picture for a larger view.


      
One additional advantage of this custom Modem Eliminator design
      is the freedom from assertion conflicts on DCD which you would
      suffer from most Null Modems. See below for more information.
    

  

  

    
      


         WIRING DIAGRAM
   --------------------------

   DB9-Male     DB9-Male  or (DB25-Male)
   --------     --------      ---------
    1. DCD                       
    2. RxD ----- 3. TxD   or   2. TxD
    3. TxD ----- 2. RxD   or   3. RxD
    4. DTR ----- 6. DSR   or   6. DSR
    5. Gnd ----- 5. Gnd   or   7. Gnd
    6. DSR ----- 4. DTR   or  20. DTR
    7. RTS ----- 8. CTS   or   5. CTS
    8. CTS ----- 7. RTS   or   4. RTS
    9. RI                        
  

    		
    
      
   


      
This is the wiring diagram. Notice that we cross over TxD
      / RxD, DTR / DSR, and RTS / CTS. We don't do anything with DCD.
      See the explanation of DCD conflicts below.


      
The DB25 pinouts are given as a convenience to those wishing
      to make a 9-25 Null Modem for using a HotSync Cable or Cradle
      with a standard external modem (which has a DB25-F connector
      on it).


      
Note that both DB9 connectors are MALE (with pins, not holes).
      This provides the Gender Changer aspect which you need to connect
      these type of cables. Standard Null Modems go male-to-female,
      which would require additional adapters.


      
I used 24 Ga insulated wire of about 1 inch length, stripped
      only a few millimeters at each end. Short wires with standard
      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.
    

  








DCD Conflicts



Most Null Modems (notably, the Radio Shack one) have DCD "driven"
by DTR from the same or other side. Either way, this is a problem
with this application. This is because DTR also drives DSR, which
is fine if you are connecting two terminal devices (like one PC
to another). However, in this case, you are doing the opposite.
You are connecting two "modem" devices together. (The
HotSync cable's DB9-F end is the "modem" configuration).
A terminal (or "DTE") device DRIVES DTR and SENSES DSR
and DCD. A "modem" (or "DCE") device DRIVES
DSR and DCD and SENSES DTR. The problem with the standard Null
Modem is that since DSR and DCD are wired together, when you drive
them both with a "modem" (DCE) device, you have ample
opportunity for big current drains if the device drives them in
opposite directions! 





Since the Pilot doesn't listen for DCD at all, we can just
omit it from the design and leave it disconnected. 











Physical Considerations



See the pictures below for detail on how the heatshrink jacket
works in conjunction with the short wires to make a nice, rigid
casing. Sorry I didn't photograph the wires themselves. Use your
imagination.



 





Pick a diameter of heatshrink tubing that will just fit over
the outer flanges of a DB9 connector. Cut enough length to extend
just to the furthest extents of the metal casing around the pins.
The shrink factor will curl it back a bit. Don't worry; the DB9-M
goes over the outside of the mating DB9-F's of the cables so the
heatshrink won't interfere.



 





This is the detail of the physical connection. Notice how the
heatshrink tubing forms a rigid casing. It stands up easily to
the force of plugging both connectors into it. I don't have threaded
hex nuts for screwing down the locking bolts but it stays together
with no problem.









Please Email me if you have any questions. Good luck.






Photos taken with Ricoh RDC-2 Digital Camera with
macro capability.









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John R. Haggis



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Last updated: 8/4/1999