Maximum current consumption for RS-485 chip?

[hello188]

Hi. I am designing a Modbus and Profibus interface.

How much current do I have to allot for an RS-485 chip?

On data sheets, maximum current on B-A line says about 55mA ish.

So, is it safe bet to allot about 65 mA??

My circuit works fine, but i don't know what will happen if it goes out in the world.

thank you

[Ttelmah]

Depends on the chip you are using, and the data rate you need.

There should be a 'chip quiescent' value in the data sheet (which tells you what the core electronics draws when doing nothing), and a 'worst case' - short circuit output current. Sum of these gives approximately the worst case.

The 'standard' MAX485, can draw 250mA into a short. However 'low power' chips, limit the slew rates available, and at the same time some limits the output current that can be delivered, to keep the peak down.

Have a look at the Maxim document:

<http://www.maximintegrated.com/app-notes/index.mvp/id/1090>

General consumption is then dependant on data rate and the chip chosen.

You then need to consider 'what happens' with a short. As well as being the worst case on current drawn, it can also lead to thermal problems with the driver, if continuous. Hence some designs use a current monitor to the chip, and will switch it off if a short is detected. There are some chips which can do this automatically. The other alternative, is if you can design the circuit to make a short 'impossible' (RC network in the drive).

65mA, sounds too low, if a short circuit is possible.

Best Wishes

[SherpaDoug]

In part it depends on what is allowed to happen if the 485 lines are shorted. Should the rest of the product continue to function? Do you still need to be able to land the plane even if you can't report status? In which case you may want to limit the current the 485 can draw. Or is existence pointless without communication. There is no need to monitor tire pressure if you can't tell anyone about it. In which case a backup plan has no function.

Depending on the message format you may get away with something very simple. If you occasionally send brief messages and spend most of the time receiving, you could just use a current limiting resistor that will supply enough current to run in receive mode. Then add a capacitor that holds enough energy for a transmit message near the 485 chip. If the line shorts the resistor will prevent the power bus from being pulled down. The capacitor will power the transmitter and recharge between transmits.
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[temtronic]

...another area to consider is the current drawn by the bus pullup,bias,pulldown resisitors.
depending on the RS-485 chips used is might be uamps to millamps...

hth
jay

[asmboy]

my suggestion, or how i always approach this?

1- estimate/design
2-BUILD!!
3-TEST ( as brutally as possible)

repeat as needed.

[hello188]

Thank you guys!!, I now get it.