What is different read input register and read holding register in modbus application

[ino_measure]

Hello,

Sorry to make question like this (maybe it so easy with Modbus professional). But I think I can put this type of question here.

Please teach me What is different between "read input register" and "read holding register" in modbus application?

Is there any different type of data output, storage address data in PLC (Register in modbus definition) with two these kind commands?

By the way, let's me make one more question: when I check in VISA test Panel (MAX) by write*IDN?\n (6 byte), I Read the same content *IDN?\n? What is trouble with this situation. Why the content of Read in VISA is same loopback test? I think when I send *IDN?\n? command, the equipment will response with information of that equipment. Is this right?

My configuration between PLC and PC as follow:

PLC --> PCMCIA 114 (Card) -->TSX SCP CM4030 (Wire)--->TSX SCA 50 (T-Junction box)--->AD400EI (RS 232/RS485 converter) as bellow picture

Does any one sugest me using what free program to test modbus command, to test configuration like this? 

Thank you in advance.

[JordanG · Accepted solution]

Good morning,

Regarding your last question, you can download our Modbus Library for LabVIEW. It's a free set of VI functions that provide Modbus communication from any standard Ethernet or serial port. I have attached a link to download this library.

NI Modbus Library for LabVIEW

https://forums.ni.com/t5/Reference-Design-Content/LabVIEW-Modbus-API/ta-p/3524019

I have also attached a little more documentation for you that contains the answers to the other questions you asked. The Read Input Registers document contains information regarding read input registers and the Into to MODBUS document outlines holding registers. You can quickly search these documents with Ctrl + F. I have also included an article regarding serial communication. Steps to perform a loopback test are also included in this document. Post back here if you still have questions!

Introduction to MODBUS

https://www.ni.com/en/shop/labview/introduction-to-modbus-using-labview.html

Serial Communication Starting Point

https://www.ni.com/en/shop/labview/understanding-the-spi-bus-with-ni-labview.html

Jordan

[ino_measure]

Hi Jordan,

I appreciate your kindly sending me some document and guiding me how to study. My problem already was solved after take a look your document and try to ask schneider's expert. the main reason: the communication of system (between PC and PLC) has trouble.

After this event, I studied more from other engineer using LabVIEW and PLC's User (Special schneider' PLC).

Thank you so much, hope get your help in near future.

Regards