Serial HotSync patch for the Visor USB-Cradle
DISCLAIMER
Before you continue reading you should be aware that all information
provided
here comes without any warranty. The instructions given might be wrong
and therefore could cause damage to all involved material. The author
is
not responsible for any loss of data or any damage to your hardware.
You
should keep in mind that by applying the following patch you can cause
severe damage to your equipmet like destroying your Computer and/or
your
beloved Visor. This patch worked totally for the author's device but
might
fail for yours!
DO NOT MODIFY YOUR HARDWARE UNLESS YOU ARE ABSOLUTELY SURE ABOUT
WHAT
YOU ARE DOING! YOU ARE RESPONSIBLE YOURSELF FOR ANY DAMAGE!
Any modifications might very likely void the warranty by HandSpring,
Inc.
THE AUTHOR RECOMMENDS THAT YOU DO NOT MODIFY YOUR HARDWARE!
Continue at your own risk.
Introduction
When the HandSpring Visor was finally available in Germany, I could get
my hands on one of the first devices sold at local shops. Unfortunately
the bundled software only supports HotSync via USB under Win98. WinNT,
Win95 and Win2000 are not supported yet and HandSpring is currently
only
spending efforts to ramp up USB-HotSync for the latter one.
The situation on the Linux side of life is not much better as USB
support
is not working very well within the current stable kernels. After
spending
several nights trying to test the latest pre-2.4.0 alpha/beta kernels
all
I discovered was that my motherboard came with an USB chip whose
Linux-driver
(OHCI) is the last one which is causing severe trouble (see linux-usb.org
for the current status of Linux-USB).
A solution for all these problems became more likely when I read
through
the hardware docs at HandSpring.
There they describe the Visor's cradle connector rather detailed and
after
a short time, the idea of a serial patch for my USB-cradle evolved.
Background
Although being fully compatible to the original Palm Pilot, HandSpring
changed some details concerning the Visor's hardware. One of them is
the
cradle connector which is totally different compared to the Pilot's
one.
This is not suprising because the Visor offers HotSync via USB and the
related signals have to be fed into the cradle somehow. Fortunately the
Visor still offers a serial port at the cradle connector which allows
connections
to external serial devices and enables serial HotSync, too. HandSpring
is selling a serial cradle as well, but it has to be ordered
separately.
The serial port includes a stripped-down set of the normal RS-232
signals by providing only the data lines TXD and RXD. Thus no
hardware-handshake
is possible and therefore line transmissions speeds are very limited.
Nevertheless
this seems to be enough for HandSpring to implement a serial HotSync
protocol
from which this patch intends to benefit.
The Gory Details
All what is necessary to do serial HotSyncs using the original
USB-cradle
is to connect the TXD/RXD signals of the Visor with the appropriate
serial
lines at your computer and switch the Visor to serial HotSync mode. The
latter one is quite easy but the first one requires some soldering,
because
the signals at the Visor are using native TTL-levels and not the
required
levels for RS-232 interfaces. This is accomplished by this patch.
The heart of the schematic is a
MAX232
(level shifter) which does all the analog voltage stuff. It converts
the
TTL-levels from the Visor to the RS-232 levels while requirering only
one
single 5 V supply voltage. It is a nice circuit because it keeps all
the
+-9 V stuff out of our minds and lets us concentrate on the real
interesting
parts. The MAX232 is supplied by the computer's DTR and RTS lines which
are protected by two diodes against current flow back (the circuit must
not be powered with -9 V). It also needs four capacitors that are used
for voltage generation. There is an optional switch after the two
diodes
to switch off the whole circuit when it is not used (for cautious
people;
might reduce the stress to the signal lines).
To finally enable serial HotSync I had to fiddle around a little bit
because HandSpring does not document this topic and I have never seen
an
original serial cradle. The solution is to tie KBD* to GND during
HotSync.
You can do this permanently or you can use another switch to turn this
feature off. This reverts your cradle and thus the Visor back to USB
HotSync
mode and also allows you to interface the Visor with external serial
devices
like a modem (KDB* low instructs the Visor to interpret the incoming
data
having a special serial protocol used e.g. with external keyboards).
That's all for the theory. In order to integrate the circuit into a
cradle you can solder it on a small board that fits diagonally into the
empty part of the cradle. The wires to the cradle connector can be
soldered
directly to the cradle connector. The board in my cradle labels pin 1
and
8 of the connector so it was easy to find the correct pins. Seen from
the
solder side pin 1 of the connector is at the left. Double check this
with
HandSpring's hardware docs before running the first trial!
The type of serial connector for your computer depends on what
connector
your computer has at its rear. Old motherboards provide normally a 25
pin
D-SUB connector while modern ATX boards come with a 9 pin D-SUB
connector.
Note that there are 25 <-> 9 and 9 <-> 25 converters, so
this decision
does not restrict the usage to particular machines. Choose the most
suitable
version for your everyday work. More interesting is the gender: use a
female
one to directly connect it to the computer. A male one needs an
additional
null-modem cable but it can be used together with a normal serial cable
to connect the cradle to a modem.
A last tip: the two diodes fit perfectly inside a D-SUB package, so
you could solder them directly to the plug.
For further instructions on how to HotSync your Visor serially
consult
the documantation of the software you indent to use.
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