How FTP works on MODBUS over UART


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How does FTP work on MODBUS over UART?

Basically, I want to reduce transaction timing (query and response).
Modbus supports only 256 bytes in one transaction and if I want to transfer 40 KBytes of data then I need more transactions of 256 bytes.
So transferring this bulk data I want to use FTP in MODBUS.

Please help me in this regard.
Why don't you use ASCII as transmission mode which has a double size 2x252 chars instead of 256? MODBUS ASCII FTP implementation is more suitable to reduce transmission time.
Modbus is a protocol, and FTP is a different protocol. FTP over Modbus doesn't make sense. If you want to use FTP, then just use FTP. I don't however know of an implementation of FTP that runs over RS-232 though. There are other file protocols however that are designed to work over serial links.

Modbus is optimised for frequent repeated transfers of small messages - which is what 99.9% of applications need to do in industrial control. FTP is optimised for occasional transfers of large files. They're different because they need to do two different and contradictory things.

If you are trying to read data from a device that only speaks Modbus, you need to work with how the Modbus protocol works. If you want to describe your application more generally though, someone might be able to make another suggestion. It's pretty hard to come up with any ideas when we don't know what equipment you are working with, and what restrictions you have.

If we look at your numbers though, you are transferring 40kB of data over a serial link. If the baud rate is 9600 baud, that works out to about 40 seconds just to get the bits through the wire at 100% efficiency. The most effective measure you could take within the limitations you have stated would be to run at a higher baud rate, if that is possible.