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Mark,

You can do this with a Max202 or similar, 555 Timer and Max485.

You use the Max 202 to provide level translation from the PC to TTL level.

The signal lines go from Max202 directly to Max485 IC.

You have a 555 timer set up as retriggerable that is triggered by low
going pulse on the TXD line. The output of the 555 drives the TX enable of
the Max485.
The time out period for the 555 is set up to grater than 1 byte period of your
slowest data rate.

For power, you can use RTS, DTR signals to "suck" a little power from the PC port.
(The signals can be gated using diodes and fill a large cap for storage during
transmission packets).

You may like to use the Cmos (7555) timer for lower current operation.

If you have not yet been there, try browsing

http://www.bb-elec.com/convert_serial_port/rs485.asp


Kind Regards
David Huisman (CEO)
Orbit Communications Pty Ltd
Wireless Solutions that Work
Ph: +61-2-4393-3627
Fax: +61-2-4393-3685
Web: www.orbitcoms.com

----- Original Message -----
From: Mark Jordan
To:            PICLIST@MITVMA.MIT.EDU
Sent:         Wed, 24 Mar 2004 22:32:40 -0300
Subject: [EE:] RS232 to RS485?

        Hi,

        I need to build a RS232/RS485 converter powered by the PC
serial port only.
        Have googled for schematics but cannot find nothing useful,
just commercial products.
        Can't it be done using just transistors or CMOS ICs for
lower power consumption?

        Thanks for any information.
        Mark Jordan

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