RS485 node powered down condition

Hi Umesha,

All of the RS485 devices have a parameter called "unit load". This parameter actually describes what you are looking for.

The RS485 Standard dictates that the unit load is the worst case scenario for leakage current into the bus pins when the voltage is swept from -7 to +12 volts when the device is both powered and unpowered. Therefore, the device will not load the bus any more than indicated by the unit load for unpowered or powered situation.

Please let me know if you have any other questions.

Thank,

John

Hi John 

Thanks for the soloution

I have one more question on the common mode voltage.

Any reason that all the isolated RS485 transceiver will not be offered with high fault protect common mode voltages (i.e Greater than -7 to +12V). and also what are the pros and cons if we isolate RS485 at logic level(TX,RX,EN) and at transceiver level.

Thanks for your support

Umesha HM

Hi Umesha,

I understand where your question is coming from. In the isolated RS485 datasheets they still have the line of -7 to +12 as the recommended voltage at either bus I/O terminal.

The reason why this isn't as much of a limitation as you are thinking is because the GND2 reference will float to whatever potential is on the bus. Therefore, the GND1 and GND2 reference voltages can be very different, let's say 50 volts, and the voltage difference between the A&B pins and the GND2 reference can still be within the -7 to +12 volts that the datasheet indicates. That’s the benefit of the isolation, the GND2 can float to whatever potential the bus is at.

In terms of isolating at the logic levels or the transceiver level, are you asking the difference between using a standalone transceiver and standalone isolator as opposed to an integrated solution? My thoughts are...

The pros of the integrated solution:

• There is one guaranteed propagation delay, as opposed to two worst case propagation delays with a multi-chip solution
• Everything is integrated into one device saving board space and manufacturing costs

Cons:

• Since there are only a few isolated RS485 XCVRs that we offer, the freedom of isolation loevel and speed is limited in an integrated solution

Thanks,

John