Protecting bus RS485 and Detecting TRUE Tri-state

[kkdelabaca]

Hello,

I see that is very important protect the RS485 bus form Contact Discharges or Fast Transient Burst. Some devices have this protection inside the same chip but often are more expensive. What is the best element to protect the bus? I'm looking for one solution with best price.

Also, for one application I need to know when the bus is in High, Low and High Impedance. The next image show the typical circuit to transmit and recive (green circle). This device return '1' when A > (B+Treshold) and '0' in other cases.

**broken link removed**

Now, to create the difference between Low and High Impedance, I put one transceiver (red circle) with inverted inputs. This circuit can help to detect TRUE TRI-STATE? This connection can generate any problem?

BR,

 

[barry]

You should probably use fail-safe termination on your input, so that the bus is pulled to a known state when there is no driver (high impedance).

As far as protection, transorbs are pretty inexpensive. What do you consider expensive?

 

[HTA]

Hello,

if the RS485 bus transmit data differential. With one line at Low and the other at High level, or opposite. In the Tri-State it will float or have an undefined level. Thus you could use high value resistor to pull both to either a low or high level. This way you can detect the Tri-State stat if both line are at the same logic level. It is always recommended to have CMOS inputs not floating to avoid higher power consumption.

Enjoy your design work!

 

[kkdelabaca]

Tank you both!
Yes, I think that the solution of the transzorb will be OK.
One low cost transceiver will cost about 0,77$USD (SP485 from Sipex) and other drivers (Overvoltage Fault Protected) will cost about 3USD$. The price of one Transzorb is about 0.5USD$ (I will put 2 units).

About the polarization resistors values, I will put the typical values of 10K and the terminator of 120ohm.

Thanks!