Sometimes when you work on low-budget projects or 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 a MAX232 chip like this:
Max232 IC is a specialized circuit that makes standard voltages as required by RS232 standards. This IC provides the 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 is more expensive than other solutions.
One of such solution is Transistor based interface:
Transistors can be any general-purpose transistors. This circuit is elementary and works without problems. It is the cheapest solution as it requires a couple of transistors and four resistors. Two transistors do the 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 uses logic elements, and there are a couple of gates left on some IC. As almost every PC can work with positive-only signals, logic gates can provide necessary inversions. For instance, using CD4066B CMOS IC:
And of course NAND and NOR circuits:
And don’t forget to Power the logic IC’s with 5V.
uCHobby has an article describing how the two transistor RS232 converter works. You might find it interesting to reference.
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:
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!
Great all-in-one-place article!
I was about to use a Max232 to “translate” old vintage computer current-loop to RS-232. This is called a 20ma current loop digital interface. NOT to be confused with more contemporary 4-20ma current loop analog interface. (Did I get that right?).
Is there a simple sans-Max232 circuit for this (adapting 20ma current loop to RS-232)?
The MAX232 schematic is wrong (at least it didn’t work for me)!!!
I used this schematic
Max232 works for me. Could be other factors that cause failure.
Hi, I’m building an unlocker cable for my cellphone but I don’t have access to a MAX232 which is required. Can I use any of the above alternatives instead ??
Sure you can try. Most computer will work fine with any converter. But some may be sensitive to different RS232 signal level. Just give a try.
What is different between MAX232N and MAX232 ?
Your transistor based solution has a problem, it will echo what ever is received on Tx Line via Rx line coz of resistor R3.
Hi, Good project. I had a question: the zener diode you use is 1W or 1/2W?
0.5W is more than enough
I agree with sundar. I think R3 should not go to the Tx line but to ground!
Hi, i build the circuit with max232 but testing it by hyperterminal and connecting together ttl tx and rx on screen comes a lot of copy of same symbols like a home. Where can i be wrong?