I am using Modbus RTU and i have 9487 holding reg. Can we read that much at a time?

Setting in configuration

 

The Modbus spec has a maximum limit of 125 registers in one transaction.

But that depends on server/slave implementation. There are server slaves that return as few as 10, 12, or 24 registers in one transaction. Sometimes that is not documented, one discovers the limit through empirical experimentation; increasing teh requested number until the server/slave faults.

 

For the software you're using as the Modbus master, the answer to your question is yes. But for your slave, it seems the answer may be no, as your slave seems to not support some registers in the range 2251 to 9487. Instead of replying with an exception response, your slave is not responding at all. Your slave does, however, allow reading the entire 125 register limit per request, as it is responding to requests to read 125 registers for registers below 2251.

In the Mdbus Configuration window you show, there is a setting for "Regs./Request" and it is set to 125 (which is the maximum limit for Modbus as David explained). In your packet captures, 125 decimal is 7D in hexadecimal (3rd to last byte of the TX packets), so your software is requesting 125 registers at a time and your slave is responding to some of the requests. Keep in mind this is going to take a long time to request so many registers. 9487 registers with 125 registers per request will require 76 requests. It seems your software's requests are 1 second apart, so this will take approximately 76 seconds to request all 9487 registers.

However, there is a problem being shown in your capture. Presumably (since not all packets are shown), the software is successfully reading registers 1 - 2250 (125 registers at a time). But when the software sends the requests to read registers 2251 - 2375, your slave doesn't respond (that's the multiple TX packets next to each other in your capture). It seems the software attempts 3 times to request these registers. After all 3 attempts time out, the software goes back to register 1 and sends a request to read registers 1 - 125, so it never gets past register 2250.

 

What is the best way to read all registers?

 

In order to read all registers from a Modbus slave, you first need to know what registers exist on the slave device. This can found in the slave device’s register list, which is typically included in the device’s user's manual or is a stand-alone document provided by the manufacturer.

What is the manufacturer and model of the slave device you're using?

 

Thanks for support.

 

Now the new issue with modbus is like my register address in SLAVE is 30103 and MASTER it is reflecting at 30105. For every register there is offset of 2.

 

First, take a look at this thread to understand the different conventions used for Modbus register numbering:
https://control.com/forums/threads/modbus-register-numbering.49844/

The 30,000 offset may simply indicate Input Register. So for example, 30103 may be Input Register 103. Try entering 103 in the "Start" column for "I. Regs." in the Mdbus Configuration window.