SN65HVD1780: Communication error and abnormal response from the Slave devices

Jeet Sodagar

Part Number: SN65HVD1780

Tool/software:

Greetings,

I am using SN65HVD1780 IC for RS485 communication between my master and slave devices. Cable length is less than 50 meters and baudrate is 9600. I have used 120 Ohm termination resistor at the both end of the network.

Issue is that when I use USB to RS485 converter and read every single node by converter it works perfectly; but when I connect slaves with my master device few random slave devices does not respond to query.

I am using daisy loop network and 4.7K resistor for the pull-up (A+) and pull-down(B-).

6 months ago

0 Clemens Ladisch 6 months ago

Guru 318700 points

How many pull-up/-down resistors are there?

Can you show an oscilloscope trace of the A/B signals of a failing query?

0 Jeet Sodagar 6 months ago in reply to Clemens Ladisch

Prodigy 55 points

There are 16 Slave devices and every device has one pull-up(A+) and pull-down resistor(B-). I would like to trace A/B signal myself but as it is installed on site and environment is explosive. Due to this it is not feasible to measure A/B signal.

0 Clemens Ladisch 6 months ago in reply to Jeet Sodagar

Guru 318700 points

Only one set of pull-ups/-downs is needed.

Please show the schematic of the master and of one slave.

0 Jeet Sodagar 6 months ago in reply to Clemens Ladisch

Prodigy 55 points

Schematic of slave device:

Schematic of master device:

Sorry, if I have made some basic mistakes as this is my first time working with this protocol.

0 Clemens Ladisch 6 months ago in reply to Jeet Sodagar

Guru 318700 points

Remove R20 and R21.

And there must be only exactly two termination resistors, at the physical ends of the bus.

0 Jeet Sodagar 6 months ago in reply to Clemens Ladisch

Prodigy 55 points

Yes, I have removed other termination resistors and there are only two resistors in the field. I will let you know the results after testing it in the field. Is there any issue with Rfs as I have seen online that Rfs value should be calculated as per UL, Vab and Vs (Note).

Also, Is there any chance that damaged RS485 IC can cause the issue? as there was one damaged slave module; which we replaced.

0 Clemens Ladisch 6 months ago in reply to Jeet Sodagar

Guru 318700 points

The SN65HVD1780 has fail-safe inputs and does not need fail-safe resistors. If you add resistors, they just increase the noise margin.

A slave that is not currently transmitting does not affect the bus.

0 BOBBY 6 months ago in reply to Jeet Sodagar

TI__Guru* 76861 points

Hi Jeet,

Just to be clear, when Clemens said remove R20 and R21, he meant to short them with a 0 ohm resistor.

Jeet Sodagar said:
Also, Is there any chance that damaged RS485 IC can cause the issue? as there was one damaged slave module; which we replaced.

We'd probably need to see the scopeshots to be sure but fail safe resistors (even tons in parallel) should not damage an RS485 driver since they have a short circuit limit which would prevent excessive current from damaging them when they drive the bus.

Usually damage is due to over voltage due to transients.

-Bobby

0 Jeet Sodagar 6 months ago in reply to BOBBY

Prodigy 55 points

Noted. I will try to get scopeshots if possible and share it.

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SN65HVD1780: Communication error and abnormal response from the Slave devices