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I'm designing a print for RS485 with the LTC2854 transceiver. I don't know what situation it will be placed in. So I don't know what voltage levels the other devices will communicate on. I will supply the transceiver with 3.3V and communicate on 0V and +3.3V.

Does this mean I can still receive the full range of voltages -7V - +12V of other devices on the bus? Or does this mean that it can only be implemented on 0V - +3.3V level systems?

In my understanding I should be able to detect difference between a zero and one, as long as the difference a-b of the transceiver is at least +/- 200mV. I won't be using biasing and the termination resistor can be turned on or off with LTC2854. Could you explain if my understanding is correct?

ehrickk
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1 Answers1

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Your understanding is correct.

When all devices on your bus comply with the RS485 standard you don't have to worry about voltages (on the RS485 side of the transceiver). Your device will be able to talk to any other RS485-compatible device.

To elaborate a bit more: note that any transceiver acts as a kind of translator, it converts the electrical layer on one side to the other. For instance, in your case, you have 3.3V logic on one side and RS485 on the other. The transceiver takes care of converting the single-side 3.3V logic to the differential levels required by the RS485 bus.

What you would not be able to do is use the same LTC2854 to interface a TTL 5V logic device to RS485. To do that you need a different transceiver, for instance, a LTM2881-5.

There are some nuances to the general R485 universal compatibility stated above, like bus biasing or impedance (related to the terminating resistors) that can be sometimes (mostly when you have to cover long distances or you are in a noisy industrial environment) a headache but you should always be able to solve those given enough time, patience and ingenuity.

Marcos G.
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