Alternatives of MAX232 in low budget projects

Sometimes when you work on low budget projects of don’t want to go shopping for max232 or similar IC you can save your time and money by using other TTL to RS232 converter. We usually used to use MAX232 chip like this:

RS232_adapter.PNG

Max232 IC is a specialized circuit which makes standard voltages as required by RS232 standards. This IC provides best noise rejection and very reliable against discharges and short circuits. If your project is more advanced and has to reliable you must use specialized RS232 to TTL converter IC’s. As I said Specialized are more expensive than other solutions.

One of such solution is Transistor based interface:

interface_schematic.gif

Transistors can be any general purpose transistors. This circuit is very easy and works without problems. It is cheapest solution as it require couple of transistors and four resistors. Two transistors does a trick to get a negative voltage as required by some PCs. When the PC does not transmit data, its TX pin is stuck to a negative voltage. The negative voltage is picked up from the TX pin and brought back - through resistor R3 - to the PC’s RD pin.

RS232 alternatively can be interfaced using Logic gates. This is handy when your application is using logic elements and there are couple gates left on some IC. As almost every PC can vork with positive-only signals logic gates can provide necessary inversions. For instance using CD4066B CMOS IC:

interface_4066.gif

And of course NAND and NOR circuits:

interface_4001.gif

interface_4011.gif

And don’t forget to Power the logic IC’s with 5V.

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3 Responses to “Alternatives of MAX232 in low budget projects”

  1. Electronics-Lab.com Blog » Blog Archive » Alternatives circuits of MAX232 Says:

    [...] circuits of MAX232 - [Link] Filed in [...]

  2. David Fowler Says:

    uCHobby has an article describing how the two transistor RS232 converter works. You might find it interesting to reference.

    http://www.uchobby.com/index.php/2007/06/11/ttl-to-rs232-adaptor-explained/

  3. Dan Philips Says:

    Hi,

    Good to know about this article. However if you need the full 5-pin interface and are working with long RS232 cables at high speed it may still be useful to use a MAX232 IC.

    You can get the following quite cheap:
    http://www.tronisoft.com/4201.php

    The thing like is the PSU is built in so you can power the circuit and you Microchip PIC projects in one go. Great for breadboarding!

    Dan

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