SN65HVD3082E：How to calculate pull up resistance in 485 circuit

jacky wang1

Part Number: SN65HVD3082E

Hi,

i want to know how to calculate pull up resistance on A piont，pull down resistance on B piont and resistance between A piont and B piont.

Thanks a lot!

over 6 years ago

0 Miguel Robertson over 6 years ago

TI__Guru 62920 points

Hi Jacky,

You can find out how to compute pull-up and pull-down resistance values for RS-485 bus biasing in this blog:

https://e2e.ti.com/blogs_/b/industrial_strength/archive/2016/12/06/rs-485-basics-two-ways-to-fail-safe-bias-your-network

If you have any questions on it please just let us know.

Regards,
Max

0 jacky wang1 over 6 years ago in reply to Miguel Robertson

Intellectual 340 points

Hi，

I understand A picture，but I do not understand B picture, a bit abstract，frame in B picture is receiver or driver or receiver and driver ？

I do not understand common-mode load 375，How to calc common-mode load is 375Ω？ Is this related to the different 485 type chip ?

if not 32 nodes，and only two 485 chip(such as A picture)，common-mode load still is 375Ω？

why the equivalent termination resistors in parallel is 60Ω ，How to calc 60Ω ？

unit load = 12V/1mA = 12 ,why unit load is 1 is not 12 ?(please see above picture),Maybe I don't really understand unit load？

Finally，would you please use schematic or Non-abstract picture(such as A picture) ，it can help me to understand easier.

Thanks a lot!

0 Miguel Robertson over 6 years ago in reply to jacky wang1

TI__Guru 62920 points

Hi Jacky,

The gray box in picture B doesn't show a single driver or receiver. Instead, it shows the equivalent loading of 32 "unit loads" placed in parallel on a bus.

The simple explanation of a unit load is that it is a unit that sinks at most 1 mA at 12-V applied voltage. From this standpoint, it can be thought of as a 12-kOhm resistance. If you had 32 12-kOhm resistances in parallel, it would give you a load of 375 Ohms (12 kOhm / 32 = 375 Ohms). You can read further details on the unit load concept in RS-485 in this blog:

https://e2e.ti.com/blogs_/b/analogwire/archive/2016/03/08/rs-485-basics-how-to-calculate-unit-loads-and-the-maximum-number-of-nodes-on-your-network

The equivalent termination resistance is 60 Ohms for similar reasons. That is, it is typical to place a 120-Ohm resistance at the two furthest ends of a bus. Two 120-Ohm resistances in parallel would give 60 Ohms between the A and B lines.

I hope this is all clear now - please let me know if not.

Regards,
Max

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SN65HVD3082E：How to calculate pull up resistance in 485 circuit