In Modbus Comm.,RS485, 2-Wire, Half Duplex,

How much of Data (No. of READ,WRITE, WORDS and BITS) we can transfer.
I mean Maximum Quantity

 

The amount of data transferred is for practical purposes unlimited. The maximum number of registers at a single Modbus station is 65535 of each type, and conceivably you could read them all. It would take some time, but it is possible.

--
Bob

 

The Modbus RTU message frame is a maximum of 256 bytes.

The overhead for a response message to a function code 3 query is
function code = 1 byte
byte count = 1 byte
CRC = 2 bytes

That leaves 252 bytes for data, but function code 3 uses 16 bit, 2 byte registers, which allows for 126 registers.

 

The most common function codes for reading Bits and Words are FC01, FC02, FC03 and FC04. The responses to these function codes all include a Byte Count field that is a single byte in size. So the maximum number of data bytes that can be returned in a single read is 255. That limits you to a maximum of 2040 Input Status or 2040 Coils per message and 254 bytes (127 Registers) of Holding Registers or Input Registers. These numbers are per MESSAGE. You can have up to 65535 Registers or bits in a single Modbus device.

The most common function codes for writing multiple bits and registers are FC15 and FC16. These function codes have a Byte Count field in the query so the same limitations apply in the write messages.

Jerry Miille

 

A single Modbus RTU message can be a maximum of 256 bytes.

The reply message format is...
1st byte: Slave ID
2nd byte: function code
3rd byte: number of data bytes to follow
then the data bytes
last 2 bytes: CRC

The overhead is actually 5 bytes, 3 at the start of the message and 2 for the CRC at the end.

This leaves 251 bytes for data. Since the data is in 2 byte registers, this allows for 125 registers (in a 255 byte reply message).

For coils, since each coil is one bit and 8 coils can be sent in each byte. 251 bytes will allow for a maximum of 251 x 8 = 2008 coils in each reply.

But How much data can we transfer? If we send multiple requests for different blocks of data, we can transfer all 65536 registers (0-65535) for function code 03 and another 65536 read-only registers for function code 04, as well as 65536 coils each for function codes 01 and 02.

65536 registers = 65536 words
= 131,072 bytes
= 1,048,576 bits

65536 coils = 65536 bits
= 8196 bytes
= 4096 words

 

The Byte Count field in the message is the number of data bytes present. You can take this number and add 5 to get the total message length.

Jerry Miille

 

Yes, Jerry's overhead count is correct. I forgot the slave ID byte. David

 

Do also remember that the answer may be highly 'vendor specific'.

For example, a power meter with only six registers of data (32-bit status, Kw and KwH) will only allow polling 6 words of data regardless of the Modbus.org documentation.

Other small devices with limited RAM may also requiring reading registers in smaller chucks - like 10 or 32 registers at a time just because they don't choose to allocate enough buffer space in RAM to handle larger messages.

 

Thanks!!

 

I see that everyone forgot about the functions number 20/21 (14/15 Hex) which allows access to more registers (6XXXXX ).

Regards
Andrzej
www.modbus.pl