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From: Winfield Hill
Subject: Re: single discrete RS232 driver
Date: 17 Dec 2002 11:17:05 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
Miguel AB Silva wrote...
> In the project I'm working in I use a MAX238 to drive 4 RS232 lines and
> recieve another 4 RS232 lines. My problem is: I need another line to be
> drived from TTL to RS232 (from a module to a PC computer). IMHO it would be
> enough to drive this line directly to the RS232 transceiver in the PC (its
> a control signal with very low speed) as i tested it with the PC´s in the
> company. Can I assume that this will work with any PC ? Still my boss wants
> the signal to be interfaced respecting the RS232 interface, i. e., my
> circuit should output > 3v for one level and < -3v for the complementary
> level. Can anyone give an hint on how to solve this ? Maybe based on the
> positive and negative voltages available on certain pins of the MAX ?
Your boss is correct, it's dangerous to send 0 / +5V signals and depend
on all RS-232 receivers seeing your signal correctly.
OK, step carefully, and with respect, you're entering the Mickey-Mouse
logic zone! After acclimatizing to the special climate of this zone,
you can easily make a +5 to -10V output swing using a single p-channel
MOSFET or PNP transistor. I'll leave the gate drive to you. Connect
the FET's drain to a resistor pulled down to the part's -10-volt pin.
If you want +/-5V swings, add a second identical resistor to ground.
When you're finished, tell your boss you're taking part in a great
engineering tradition. :>)
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