Subject: Re: CASIO SF to PC interface
Date:     Mon, 11 Aug 1997 17:46:40 GMT
From: (Gareth Blades)
To: fravd@***antiSPAM***

On Mon, 11 Aug 1997 18:49:46 +0200, Francisco Vila Doncel <fravd@***antiSPAM***> wrote:

>Hi, I'm trying to connect a CASIO SF-4300B diary to an PC.
>I have four questions for you.
>First, you say, in your document:
>that SF-4300 does not work with interfaces 1 and 2. Why? If I would know why,
>I could do the necessary modifications. I've found another document, in the URL:
>from a person that has used the chip MAX232 to connect the SF-4300. What
>is the problem with this chip?

Experiments with my SF-4300B showed that it's IO por was only capable of
supplying current (called sourcing). It did not seem cabable of drawing
current out of the attached interface (called sinking).

The MAX232 has an internal resistor between the input and +5V as shown in
the data sheet. This means that the casio has to sink the current to work.
Circuits 3&4 rely on the casio sourcing current which is no problem.

You could put a load resistor on the input to the MAX232 to get it to work
but this will increase the current drain on the casio when the interface is
in use which is not desirable. An alternative is to insert a non-inverting
op-amp between the two units but this consumes space.

>Second, the figure for Interface 2 says that MAX232 has 16 pin and 233
>has 20 pin. But the text says that 232 has 14 pin and 233 has 18 pin. If
>I buy a 232 or 233, how do I connect them?

The circuit diagram is correct for the 233. The wording should read 20 pin
and not 18 pin.

The MAX232 is a completely different device which requires external
capacitors. You could contact maxim for a data sheet.
Alternatively I could email you the Adobe Acrobat version of the datasheet
(covers both devices). It is quite large at 1.16MB though.

>Third, the pinout of my 7805 regulator chip is not like the one in your
>image. (Please see attached image)

>What is the relation between your picture and my component?

The 7805 is a 1A device while the 78L05 is a 100mA device.
I doubt very much if the 7805 can be used as it will increase the power
cunsumption of the circuit to a level which the serial port cannot supply.

>Four, in diagrams 1 and 2 the anodes of the diodes 1N4148 are connected
>together and to the input of the 5V stabilizer, and the cathodes are
>connected to pin 20 and 4 of the 25-pin RS-232 connector.

This is correct. It is done like this so that power is supplied to the
circuit if either of these RS232 signals is +ve. This way the circuit is
compatible with more of the possible software configurations.

>BUT, in the
>diagram of the interface 3, a diode is inverted and the two diodes are
>not connected together. Is that correct?

Yes this is correct. This circuit requires a +ve and a -ve supply from the
serial port. Therefore the diodes are arranged so that only the correct
voltages are supplied to the corresponding pins.
Gareth Blades (home)
Agradezco al señor Blades sus amables respuestas.