Switching RS-485 with analog switches

Thread Starter

Avshi

Joined Jan 16, 2015
48
Is it a good idea to switch an rs-485 signal with THIS analog switch or any analog switch? If no, can it be done at all in a reasonably inexpensive fashion?
Thanks,
Avi
 

Marley

Joined Apr 4, 2016
502
The supply voltage range is not high enough on you suggested analog switch. Expect RS485 signals to be between 0 and 5V even though a differential of only ±200mV is required for signalling.
Why do you need to do this?
RS485 can be used in a bus topology using multiple transmitters and receivers. However only one transmitter will be transmitting at any time.
In fact RS485 only specifies the "Physical Layer" (the line voltages, etc), it does not specify any "Communications Protocol" that would be required to do this.
 

Thread Starter

Avshi

Joined Jan 16, 2015
48
I'm using a transmitter that is powered by a 3.3V supply (here). My intention is to connect a bunch of sensors via wires and I don't want to limit the cable length coming from the board. The bus topology isn't right for my use. The stub length on the bus is supposed to be quite small (about 6 inches) as per the application note for rs485 from st (here). Even if I were to just communicate with one sensor at a time, the connection to other sensors would cause ringing. I'll be communication to sensors through an ethernet hub.
 

Marley

Joined Apr 4, 2016
502
I see. You want a point-to point connection for each sensor.
Instead of using analog switches, why not use a number of the ST4485EB ICs (one for each sensor) and select them using the /RE and DE pins?
 

Thread Starter

Avshi

Joined Jan 16, 2015
48
Simply because the IC is large. I don't have that much space. I'm on a 10cmx10cm board and having 12 of these ICs would be impossible I think. Maybe I should look for a smaller one, but I don't think it's likely that a smaller one exists at this datarate.
 

Thread Starter

Avshi

Joined Jan 16, 2015
48
I looked at the data sheet for the switch a little more closely and Vin for the switches must be 0V to 4.3V so that's a bust. I'll just going to use p and n mosfets
 
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