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What voltage levels are required for transmitting data using modbus protocol?

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cool.man

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Hi,
I am using modbus protocol with rs485.for rs485 i am using sn75176 IC.
I want to know that what voltage levels are required for transmitting data over the wires.
Also is there any version of sn75176 available which can run at 3.3v?
 

cool.man

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Re: Modbus voltage level

Why this kind of circuit is needed after 75175 IC
 

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FvM

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Why this kind of circuit is needed after 75175 IC
You mean on the left side? A microprocessor, I guess. You'll need RX, TX and TX_Enable signals.

Referring to your original question, yes you can interface RS-485 with a 3.3V supplied driver. These drivers have the same pinout as 75176 and are available e.g. from TI. The voltage levels are specified by the RS-485 standard, there are no particular requirements for MODBUS.
 

cool.man

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FvM,
what i wanna ask is that in the attached picture a resistor network is connected to pin 6 and 7.
I wanna know why this configuration is done if we can run modbus at 5v or 3.3v
What are the advantages of this kind of circuit?
 

bigdogguru

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The Modbus specification only covers the implementation of a Application Layer (Layer 7 OSI Model) and a Datalink Layer (Layer 2 OSI Model), the actual Physical Layer (Layer 1 OSI Model) can take on several different forms. The preferred and most prevalent implementation of the Physical Layer (Layer 1 OSI Model) is the RS-485 (TIA/EIA-485), RS-232 (TIA/EIA-232-E) are also common, Modbus can even be implemented over TCP/IP on the Ethernet (IEEE 802.3) Physical Layer.

So your question,

What voltage levels are required for transmitting data using modbus protocol?
depends solely on the makeup of the Physical Layer the Modbus Datalink Layer is implemented over.

MODBUS over serial line specification and implementation guide V1.02

MODBUS MESSAGING ON TCP/IP IMPLEMENTATION GUIDE V1.0b

MODBUS APPLICATION PROTOCOL SPECIFICATION V1.1b

Modbus Organization

Modbus Related Technical Resources

Why this kind of circuit is needed after 75175 IC
I believe the circuit you're referring to provides fail-safe biasing, proper termination and transient protection.

RS-485/RS-422 Circuit Implementation Guide

Hope the info helps you with your implementation.
 

FvM

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The "RS-485" levels shown in the said paper are at best typical voltage levels, but not actually related to the RS-485 specification, that mainly focusses on required differential signalling levels and maximum common mode voltage. That't the problem when referring to unqualified internet resources.

Honestly, I don't understand which problem you are chasing behind?
 

cool.man

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Honestly, I don't understand which problem you are chasing behind?
Simply I wanna know that for protection what would be the theme at pin 6 and 7 of this IC.
 

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You have to distinguish between send and receive levels. Send levels are simply limited by the driver supply voltage, receive levels can be higher due to superimposed common mode voltages. 6.8V TVS diodes, as shown in the attachment of post #4 are a reasonable choice in my opinion.
 

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