# Loopback on LTC2863 RS-422 transceiver not working [closed]

I am using this transceiver chip to communicate between 2 devices. Now, as a loop back test I am connecting the Tx positive to Rx positive and likewise for the negative. Ideally, I should get an echo when I send some data via the hyper-terminal. But I get no values.

I then, as a test, reversed the connections. In other words, Tx positive to Rx negative and such. I started getting more sensible repeatable data.

Am I understanding this component wrong ?

update:

Updated to change transceiver part number from LTC2862 to LTC2863, after receiving later information.

update:

I am using the above system where in I am connecting A* to Y* and B* to Z*. Should this loop back work ?

## closed as unclear what you're asking by brhans, Bence Kaulics, Daniel Grillo, Voltage Spike, Dave Tweed♦Aug 24 '16 at 19:40

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• A diagram of your connections is needed, because I think you are not doing a "normal" loopback - that LTC2862 transceiver (I fixed the broken link in your original posting) only has one pair of differential transceiver pins - "A" and "B". – SamGibson Aug 24 '16 at 5:26
• I am using the LTC2863 which has only 1 diff tranciever. – Board-Man Aug 24 '16 at 6:12
• There seems to be a mistake in your reply: "I am using the LTC2863 which has only 1 diff tranciever." The LTC286_3_ has 2 diff transceivers. But earlier you said that you used the LTC286_2_ which only has 1 diff transceiver, and because it has only 1 diff transceiver, it cannot transmit and receive at the same time and so cannot do what I interpret as a "normal" loopback. Therefore I am confused by your description. That is why I asked for a connection diagram :-) Of course I cannot force you to provide a diagram, but I can't even start to understand the question, until you provide one. – SamGibson Aug 24 '16 at 9:19

I think I can "piece together" your problem now.

I am using the above system

In your update, you seem to refer to the LTC2863 pinout. Therefore much of the confusion was that you originally mentioned in a (broken) link, that you were using the LTC2862 not the LTC2863. :-(

Assuming you really did mean LTC2863, then:

I am using the LTC2863 which has only 1 diff tranciever.

Not exactly. The LTC2863 has 1 differential transmitter (driver) output and 1 differential receiver input. It has no combined transceiver connection. (The LTC2862 has a single transceiver connection pair.) The fact that the LTC2863 has separate transmitter and receiver connections does allow it to have a loopback attached to it.

I am connecting A* to Y* and B* to Z*. Should this loop back work ?

Yes, that should work correctly. (FYI, you don't need the "*" characters - that is just shown on some of their diagrams, because those pins have 15 kV HBM ESD protection.)

Here is the explanation about what those pins are:

You would be connecting pin Y to pin A (non-inverting driver to non-inverting receiver) and pin Z to pin B (inverting driver to inverting receiver). Thereby correctly connecting the inverting signals together and the non-inverting signals together.

Although it is a slightly unusual example circuit, this diagram from the datasheet confirms that those are the correct connections (driver pin Y connected to receiver pin A and driver pin Z connected to receiver pin B):

• Unfortunately this does not work in my case. But when I connect y to b and z to a it shows good results. And it's weird !!! – Board-Man Aug 24 '16 at 16:08
• The only way to progress with investigating this apparent anomaly, would be to look at 'scope traces. Do you have an oscilloscope? If so, I'll tell you what traces to collect. But even with the scope traces, if the PCB / wiring has been wrongly connected, that could make it difficult to find the cause, without PCB photos and full schematics etc. – SamGibson Aug 24 '16 at 16:32