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Communication issues with RS-232 Signals and Shielded Cable

Prabhakarankft

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Hello Everyone!
I have designed two PCB for Textile machineries! Application is very simple Transmitter and receiver! Based on the data We will switch on Some Relays.

Transmitter will send some data through RS232(Shielded 5 core Cable ) to receiver. Receiver will receive the data and switch ON some relays! Transmitter is getting the power from Receiver PCB. In receiver We have one SMPS 15V-23.2A. Whenever AC230V power cable and core cables are placed very near, communication is missing over RS232 Cable!
It is happening very randomly. If we separate both the cables it is working fine without any issues. In practical We could not separate the Cables due to cable laying restrictions!

What should I do for Uninterrupted RS-232 Communication!

Shielded core cables are having 5 cores (Spec: 14/36 ). Pin details as follows. Shield of Core cable is not connected anywhere and total length of the Cable is 4.4 Meter
1. 15V Positive
2. Tx(RS232 Level)
3.Rx(RS232 Level)
4. Acknowledgement Pin @ 3.3V --MCU I/O Pin
5. GND

Relimate 5 pin Connector <----Core Cable 4.4 Metre----> DB9 Female Connector
 

danadakk

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You are getting capacitive noise pickup to that pin. Odd in that it has internal 5K ohm
termination R.. You in a lab with a lot of fluorescent lighting ?



Regards, Dana.
 

D.A.(Tony)Stewart

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When you get crosstalk errors from power line, it means there is a noisy machine drawing HF pulse currents on that AC cable. It needs a good AC line filter to suppress that but could be more expensive than the line filter for the data cable. You must PE ground only one end of the shield and you could retest next to the power cable and determine which end suppresses more.

Likely your SMPS supply is generating a lot of CM noise which could be shunted to PE earth with an RF cap.

For further suppression a standard LLC or Pi filter choke is used which is like a line filter , but low current higher inductance. with RF caps to ground and costs less. e.g. https://www.digikey.ca/en/products/detail/taiyo-yuden/TLF9UB203WK1/2573888 for RS-232 e.g. 100kbps
 
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