Author Topic: RS-422 Tx/Rx mecanism  (Read 5996 times)


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RS-422 Tx/Rx mecanism
« on: January 28, 2013, 03:40:47 am »

I am trying to realise a RS485 in 4 wire mode which I feel is  nothing but RS-422 in balanced full duplex mode.
This is to support physical layer of a Industrial protocol on my board
On this board, The Tx_EN is controlled by a CPLD and its address is available to me . The Rx_EN is pulled to ground via a resistor.
These are the only details available from board's side.

Now i have some questions regarding the implementation of low level driver for my RS-422.

1. Since 422 is full duplex, is there any explicit need to enable the TX_EN pin memory mapped on CPLD ( Scenario : disabling Rx before to send as in 485 and so on..) - basically any special handling of the Tx/Rx EN pins

2. Requirement is that we need a point to point Rs422 link  between 2 devices ( both of them very near to each other)in this case is the Termination resistor requirred? ( Our board has a switch for modes -with /without termination) .

3. Any special handling required in Sw for the driver ( If both devices attempt to Tx or Rx to each other)

4. On Rx side, the data is received sometimes as just 2 bytes and sometimes upto 255 bytes , how to handle this feature?

5.  Example of 422 driver ( links, app.notes etc..)

Thanks in advance ,

Jan Axelson

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Re: RS-422 Tx/Rx mecanism
« Reply #1 on: January 28, 2013, 06:06:40 pm »
For full duplex, the transmitters on both ends can be always enabled.

Whether you need termination resistors depends on the length of the link and how fast the transitions are.

The software can be the same as for an RS-232 link.

The receiving software can poll or use interrupts.

.NET has a SerialPort class for serial communications.