RS232/RS485 Converter Circuit Receiving Problem

communicationreceiverrs232rs485transmitter

I'm trying to make an RS232/RS485 converter circuit. I used a circuit from http://airborn.com.au/layout/232cnv/232to485.html site and i changed some components on that circuit. Then I established my converter circuit:

schematic

I used copper pour as ground and i connected lacking ground connections with additional wires after the board was prepared. To test my circuit, I used another RS232/RS485 converter circuit which I know that is working without a problem. I connected A and B outputs of my circuit's RS485 connector to the A and B outputs of other circuit's connector. Then i connected these two converter circuits to computer with RS232 USB cable. I tested the communication between these circuits using CuteCOM serial communication terminal. I used two terminals for my circuit and other. (Baud Rate:9600)

I observed that data is transmitting from my converter circuit to other but not receiving from other circuit to mine.

I also tested the other converter circuit with another RS232/RS485 converter circuit which I know that is working without a problem, and i realised that there is not any problem during transmitting and recieving process, so data can be sended and recieved.

These are the list of the changes that i made on original scheme in for my converter scheme;

  1. Maxim's MAX232CWE was used instead of ICL232.
  2. Texas Instruments' SN65HVD3082E was used instead of 75176.
  3. As voltage regulator, Exar's SPX3819 was used instead of LM2936Z5.
  4. As transistor, BC256A (pnp) was used instead of BC557 (pnp).
  5. 22 uF capacitors were used instead of 2.2 uF, for the ones connected to MAX232CWE.
  6. On original scheme, a 6 pin connector was used as RS485 connector and two pins were used for A,B connections, one pin was connected for ground, one pin was connected to the 9th pin (RI – Ring Indicator) of the RS232 connector with a jumper. Unlike these, in my circuit, 2 pin connector was used as RS485 connector and only A,B connecitons were made. (RI and ground connections were not used.)
  7. On original circuit, A and B pins of SN65HVD3082E RS485 Transreciever were terminated with a 120 Ohm resistor and a jumper. I did not do this on my circuit.

I'm investigating about the problem which can be related with changes that i made on original circuit or another reason. I will be very happy if you share your idea about the problem of just transmitting and not recieving data on my converter circuit.

Best Answer

in my circuit, 2 pin connector was used as RS485 connector and only A,B connecitons were made. (RI and ground connections were not used.)

RS485 proper wiring requires a common earth: -

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You cannot expect disparate systems having their own ground connections to adequately handle the common mode noise that is normally present between separate grounds.

In addition the maxim document linked states: -

Although RS-485 can be successfully transmitted using multiple types of media, it should be used with wiring commonly called "twisted pair." What Is Twisted Pair, and Why Is It Used?

As its name implies, a twisted pair is simply a pair of wires of equal length and twisted together. Using an RS-485-compliant transmitter with twisted-pair wire reduces two major sources of problems for designers of high-speed long-distance networks: radiated EMI and received EMI.

I always use screened twisted pair with matching terminators.

You have not even attempted to use the 120R (across A and B) terminator.

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