Flow control in RS485

[terry]

Hi,

I see many serial-converters have a db9/25 socket and a 4 wires 485
socket (TX+,TX-,RX+,RX-). I want to know know the RS485 part
implements the CTS/RTS flow control.

For example, if the local computer wants to stop the remote side to
send due to buffer full, how the converter can do that just by the 485
wries TX+,TX-,RX+,RX-?

Thanks!

 

[Nicholas O. Lindan]

On Fri, 03 Dec 2004 17:58:59 -0800, terry wrote:

Hi,

I see many serial-converters have a db9/25 socket and a 4 wires 485
socket (TX+,TX-,RX+,RX-). I want to know know the RS485 part
implements the CTS/RTS flow control.

It doesn't.

If the protocol is designed correctly you don't need flow control. Viz CAN bus,
I2C et. al..

There are RS232<>RS485 boxes that carry through all the '232 logical
signals, shifting them to '485 levels. Sort of a 50 wire RS485 ...

--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer: Electronics; Informatics; Photonics.
Remove spaces etc. to reply: n o lindan at net com dot com
psst.. want to buy an f-stop timer? nolindan.com/da/fstop/

 

[Rene Tschaggelar]

terry wrote:

Hi,

I see many serial-converters have a db9/25 socket and a 4 wires 485
socket (TX+,TX-,RX+,RX-). I want to know know the RS485 part
implements the CTS/RTS flow control.

For example, if the local computer wants to stop the remote side to
send due to buffer full, how the converter can do that just by the 485
wries TX+,TX-,RX+,RX-?

The directional control is usually built into the protocol,
meaning all nodes track all messages and know when a message
starts and stops. Then some busses have fixed message length,
while others have a length field in the message.

Note that a PC prefers RS422 over RS485. RS422 has
the master (PC) continously on while the peripheral
nodes share the other pair towards the PC.
In this case the PC doesn't have to switch the direction.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net