ISO1412: How to communicate to 10 devices using RS485 500 meters away

Adam Torma

Part Number: ISO1412

Tool/software:

e2e,

I want to communicate 10 devices using RS485 (ISO1412BDW) but here I need to go up to 400 meters in distance. Naturally there will be a drop in the signal line voltage, how can I keep this signal line voltage at 5V level?

In this respect, we are thinking of using an RS485 repeater to amplify the signal, is there a reference design you can recommend for this?

Can you support me on this issue?

Adam

4 months ago

0 Andrew J 4 months ago

TI__Mastermind 23645 points

Hi Adam,

Thank you for reaching out. RS-485 is a robust protocol that is designed communicate long distances (a maximum length of about 1km of cable). The drop in signal level is accounted for due to RS-485 being a differential protocol. An RS-485 receiver will accept a valid signal level as long as the differential voltage is greater than +/-200mV.

Section 9 "Data Rate Versus Bus Length" of our The RS-485 Design Guide (Rev. D) shows that data rate will need to be decreased to ~4Mbps in order to have proper performance at 400m cable length.

Best,
Andrew

0 Isee 4 months ago in reply to Andrew J

TI__Expert 5990 points

Thank you! Do we have an exemplary design for a repeater circuit?

Regards

Andreas

0 Clemens Ladisch 4 months ago in reply to Isee

Guru 294210 points

You do not need a reapeater. RS-485 drivers output not 5 V, but a differential signal of at least ±2 V. You can use very long cables before the voltage drop over the wire reduces the differential voltage at the receiver to less than ±0.2 V (see note C above).

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ISO1412: How to communicate to 10 devices using RS485 500 meters away