Schematic diagrams

The Visor serial interface

The cradle interface is described in the Handspring technical specifications. Contrary to what is said in this document, the serial interface is not TTL compatible when the Visor is inserted in a USB cradle since the 0 logic level is -3V and the 1 logic level is 0V.
The serial interface is reduced to a strict minimum : Transmit data (TxD) and Receive data (RxD). Unlike the standard RS-232 interface, it has no flow control signals.

Schematic

The schematic below is inspired from a schematic posted at http://get.to/engineer a page that describes how to build a serial cable. The original diagram has been modified since the interface doesn't work when the visor is inserted into its cradle (non TTL levels).

Click on the picture for the full-size diagram

How it works (Wow, it works :))

IC1 is a TTL <-> RS-232 transceiver. It is 5V powered and has its own charge-pump converters to to generate the +10V and -10V voltages required to meet the RS-232 specifications. The MAX203 is the best choice for minimum component count and minimum size design since no external capacitor is required for power conversion. Data sheets and free samples can be obtained from MAXIM at http://www.maxim-ic.com.
The board power (+5V) is drawn from the PC via its RS-232 control lines (RTS and DTR) and regulated by IC2, a common +5V voltage regulator.
R1, R2 and Q1 is a shift level circuitry that converts the TxD signal from the Visor form the range (-3V, 0V) to a TTL compatible signal. R3 and R4 do the inverse job (TTL to (0V, -3V)) for the RxD signal. Negative power is draw from the V- line (-10V) of IC1.
Pin 2 of the cradle is grounded to indicate that incoming data should be expected on the serial RXD pin. See Handspring specifications for additionnal information.
The serial cable is a NULL modem cable. As the Visor provides no flow control signals, RTS and CTS are connected together, as well as DSR, DTR and DCD. Thus, your Visor will be regarded by the PC as a device allways ready to communicate.

Part list

Reference	Part value
R1, R3	10k
R2	100k
R4	20k
C1	47uF
C2	100nF
D1, D2	1N4148 or equivalent
D3	1N4004
Q1	BC547 (or any small signal NPN transistor)
IC1	MAX203
IC2	LM7805 or equivalent
	9 Pin female sub-D connector

Note : J1 and J2 can be any connectors. In my (minimum cost) design, I did not use any connectors and I soldered the cables on the PCB.

Tests

The table below summarises the results of my tests.

Make	OS	USB	RS-232
Homebrew (PII 300)	W98	Fail	OK at highest speed available
Homebrew (Athlon 850)	W98	OK	OK at highest speed available
HP Vectra VL (PII 266)	W95	Fail	OK up to 19.2 kbit/s

Do not connect the USB and serial interface simultaneously. None of them would work properly. However, if the serial interface is not connected, the USB interface is still fully operational.