RS-232,RS-422/485 Troubleshooting Trick and Tips

RS-232,RS-422/485 Troubleshooting  Trick and Tips

Let's Start with RS-232

Protocol

RS-232 is a Full duplex protocol means user can send and receive data at same time. Its a 2 way transmission like mobile Phone but It can travel distance of 50Ft /17mt.

How to Identify Transmit line with the help of Multimeter in RS-232

RS-232 sends signal in form data packets and these data packets contains 0's and 1's like any digital signal.These 0's and 1's has certain voltage level,In this case mostly its around +12v for 0 and -12v for 1.


Now a interesting fact- Whenever RS-232 line is ideal it will sit on logic 1(-12v), And the baud rate or speed  rov use most of the times line is ideal.So we will use this property to identify transmit line.


Place black lead of multimeter to shield and red lead to any of the other two wires, The wire which will show -12V on multimeter is transmit line.

Now -12v is in books in reality anything over -4v is good enough but make sure voltage should not be less than -3v on receiver otherwise data will be corrupted.

Getting –ve voltage does not mean that you are receiving or sending any data, It only tells you that computer is ON to verify that you are actually sending or receiving data, take out O’scope or LED.

Place LED between shield and transmit line if LED blinks that means you are transmitting data but if LED is solid bright it means computer is on but not transmitting. Do the same for Receive line.

In O’scope you can actually see data packets and pulse width by adjusting time scale division on o’scope and now a days in modern o’scope everything is adjusted automatically.


If you are dealing with any kind of signal,Always use Shielded twisted wire pair and never use twisted pair which is made by yourself by twisting 3 wires together as it will not serve the purpose.

RS-422/485

RS-422/485 both of them have same properties except the fact RS-422 is a Full duplex protocol and RS-485 is half duplex,I have already explained full and half duplex in previous post when i discussed protocols,

so RS-422/485 signals are never measured with respect to ground because is a differential signal between transmit(Tx) and receive(Rx) line so we need to measure between tx and rx.

Now what you expect to see is between 2v-4v it doesn't matter that voltage is negative or positive as signal is like a battery depending on where you have placed multimeter leads.

The signal is good above 0.2v as below this voltage level signal will get corrupted.

One more important thing when you are connecting RS-422/485 that never connect ground on both sides of circuits otherwise signal will start flowing through ground,However companies recommend to connect ground wire on one side of circuit but i usually don't connect it all on either side and it works without any problem.