SN65HVD01: Regardless of the data speed, I would like to know the extent of the distance limit in the specification of this IC.

won seungyeon

Prodigy 70 points

Part Number: SN65HVD01

Spec

: SN65HVD01 (power Supply 3.3v)

: Cable awg 24 (twist + shield)

: data rate 200kbps under

: Stub line 1 (1:1 communication)

1) What is the maximum cable length that can be RS 485 communicated in the above situation?

2 I think RS485 communication will not be possible due to voltage drop. Is it right?

This link says the voltage difference should be more than 1.5v.

The IC uses 3.3 volts. How many meters can Ic communicate?

https://www.ti.com/lit/an/slla272d/slla272d.pdf?ts=1653979769684&ref_url=https%253A%252F%252Fwww.google.com%252F

Signal Levels
RS-485 standard conform drivers provide a differential output of a minimum 1.5 V across a 54-Ω load, whereas standard conform receivers detect a differential input down to 200 mV. The two values provide sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors. This robustness is the main reason why RS-485 is well suited for long-distance networking in noisy environment.

over 2 years ago

0 Clemens Ladisch over 2 years ago

Guru 257040 points

An AWG 24 cable with a length of 1.2 km has about the same resistance as the termination, which reduces the bus signal by half. This is usually assumed to be the limit, but there is no fixed value. Longer cables reduce the noise margin even further and greatly increase the risk of errors. The actual amount of errors depends on how noisy the environment is.

+1 Parker Dodson over 2 years ago

TI__Mastermind 45397 points

Hi Won,

Clemens is correct - 1.2km is generally considered the "max" of RS-485, and by spec it typically states the max rate at ~100kbps at 1.2km. It is typically considered the limit for RS-485 in general. This is based off the minimum differential output across 54 Ohms being at least 1.5V (this voltage is measured at driver - cable attenuation will cause this voltage to be smaller when it is received on the other side of the cable hence smaller logic thresholds than 1.5V)

With that being said however for your application:

1. Since there are only two devices (1 to 1 communication - please let me know if I misunderstand) and its at 200kbps you should be able to max out the cable length or get very close to it. As a general rule of thumb in an impedance matched system the speed vs. distance curve can be understood as:

Where RS-485/RS-422 is shown in red.

2. Yes at enough length the issue becomes attenuation of the data-signal. Longer cables add more capacitance which requires lower data-speeds as well as more resistance which also will cause attenuation of the data-signal. With your current setup though you should be able to achieve pretty close to the limit of cable length (if the system is impedance matched - which it should be with proper termination)

Please let me know if you have any other questions!

Best,

Parker Dodson