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robismyname
Joined: 17 Jan 2008
Posts: 28
Location: Central Florida
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 02 May 2008 5:43 rs232 transceiver made with 1 transistor |
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| Does any one have a link or suggestions on how to make a rs232 transeiver with only one transistor(PNP 222A)? I have found one link that shows how to make one with two transistors (http://www.coolcircuit.com/circuit/rs232_driver/)
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FvM
Joined: 22 Jan 2008
Posts: 1768
Helped: 327
Location: Bochum, Germany
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| 02 May 2008 9:00 Re: rs232 transceiver made with 1 transistor |
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| You have to invert the signal for both directions, so you need two.
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banjo
Joined: 24 Dec 2005
Posts: 431
Helped: 79
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| 02 May 2008 14:06 rs232 transceiver made with 1 transistor |
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FvM is correct when dealing with dedicated UARTs. However, if you are using a microcontroller and are willing to do a bit-banged serial communication in software without using the dedicated UART in the micrcontroller, then no transistors are required.
This is possible because you can write the software to do the signal inversion for you. Then all that is required are some series resistors on the data lines to limit the current. Some designs also use clamping diodes to keep the pins from exceeding VCC and gnd. However, really cheap designs make use of the protection diodes built into the microcontrollers.
To use this approach, the baud rate needs to be fairly low, the project needs to be very sensitive to cost, and the microcontroller needs to have available processing bandwidth to support the software UART.
For hobby circuits that you will only make a few copies of, I would suggest sticking with the MAX232 or similar IC. The is the most robust. Even the two transistor version requires stealing voltage from the serial port. It works on many computers, but has been known to fail on some laptops.
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vamanan
Joined: 09 Aug 2005
Posts: 105
Helped: 5
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| 08 May 2008 12:29 rs232 transceiver made with 1 transistor |
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| What about the response time of the circuit ?. If there is a mismatch at high baud rate then there will be serious problem. Other thing is that the RS232 voltage level is also 5 V in this circuit. Is it OK ? or did I read it wrong ?. If this is OK why dont we club the TTL signal to the port straight away ? I also feel better use MAX232 as banjo rightly said.
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AdvaRes
Joined: 14 Feb 2008
Posts: 456
Helped: 20
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| 08 May 2008 12:41 Re: rs232 transceiver made with 1 transistor |
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| robismyname wrote: |
| Does any one have a link or suggestions on how to make a rs232 transeiver with only one transistor(PNP 222A)? I have found one link that shows how to make one with two transistors (http://www.coolcircuit.com/circuit/rs232_driver/) |
Use the transistor to do a simple pull-up. You will need only one wire to connect the receiver and the transceiver.
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