I have a device with a RS232 communication port (3 pin,tx,rx and gnd).

When I connect it to a PC RS232, they don't communicate together.

I use hyper terminal on the PC side and all setting are correct. Why it doesn't work? Do I need pull up or pull down resistors?


closed as unclear what you're asking by Leon Heller, PeterJ, Daniel Grillo, JIm Dearden, Matt Young Feb 11 '15 at 16:08

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    \$\begingroup\$ The problem is that the device doesn't have a RS-232 port, it has a TTL UART port. And you may have destroyed it by connecting them together. \$\endgroup\$ – Ignacio Vazquez-Abrams Feb 11 '15 at 6:35
  • \$\begingroup\$ no, connection port in device side is RS232, for example voltage in rx line is -6.2 \$\endgroup\$ – mohammad2050 Feb 11 '15 at 6:38
  • \$\begingroup\$ Is Tx of Controller connected to Rx of PC?? Which is your microcontroller?? \$\endgroup\$ – Swanand Feb 11 '15 at 6:46
  • \$\begingroup\$ no,It is not a microcontroller , it is output a device that is RS232 \$\endgroup\$ – mohammad2050 Feb 11 '15 at 6:49
  • \$\begingroup\$ @user3577549 Wait... What?? What device? And still Tx of "device" is connected to Rx of PC? \$\endgroup\$ – Swanand Feb 11 '15 at 6:50

If you are using the ISOLATOR output connections from the device then your signals are likely being converted to DC RS232 signals (and inverted) rather then being normal bipolar RS232 signals. Note that the ISOLATOR side also needs power and gnd inputs and likely a pull up resistor above the transistor collector.

So basically if that is the arrangement you have the voltages from/to your device are not compatible with the voltages from/to your PC.

If you connect the ISOLATOR collector pins to a pull up resistor at +5, and the emitters pins to gnd you can use a chip like a MAX232 to convert the collector signals back into the required voltages that match the requirement of the PC.

I was not able to access the tinyupload.

Did you add the power, gnd, and resistor parts as above? Are you using another chip like a MAX232 to reconvert the DC signals to be PC compatible?

If you still need to add the conversion circuit between the ISOLATOR side and the PC you may need to implement something similar to one of these:



You didn't mention if you needed to send data from the PC back to the device. If not then you may only need part of the circuit linked above.

  • \$\begingroup\$ thank in advance @Nedd I use of isolator that bought of store and I do not know the internal circuit , but i test between two pc and that work . in your opinion , Connection can not be created without isolator(Because the power supply is diffrent)!? if yes, do there is this problem for communicate device with the pc? you can use of this link instead tinyupload : wikisend.com/download/971802/AquaMaster Service edition 2.pdf \$\endgroup\$ – mohammad2050 Feb 11 '15 at 10:36
  • \$\begingroup\$ I think that is solved my problem with the circuit that you offer .. \$\endgroup\$ – mohammad2050 Feb 11 '15 at 10:45

Make sure the TX goes into RX and the RX goes into TX. Most most forget this.

Also, confirm if ground connection is solid.

  • \$\begingroup\$ i test this and other (for example RS232 ISOLATOR) but do not work ,,power supply is 9 volt in this device ,do this power supply will not create any problem ? There's your answer in the above comments \$\endgroup\$ – mohammad2050 Feb 11 '15 at 7:30

Software running through PC serial ports can require either hardware or software flow control and often that means "tying back" the pins for these, if the device you are plugging in does not do that.

Additionally, serial ports are infamous for protocol issues. Does the software require a set baud rate? Parity? Seven bits or eight? If the wiring is right but the protocols are wrong, it still won't work.

If you said what the device was, and what the software was, and confirmed that it was an actual serial port not a USB serial emulator, the problem might be easier to find. Serial ports are not complicated--if you have all the data for what both sides require.

  • \$\begingroup\$ There's your answers in the above comments \$\endgroup\$ – mohammad2050 Feb 11 '15 at 9:17

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