CPU : 315-2 PN/DP       6ES7 315-2EH14-0AB0   Firmware V3.2

CP: 343 - 1 Lean 6GK7 343-1CX10-0XE0 Firmware V3.0

FC5 AG_SEND Version 4.2

FC6 AG_RECV  Version 4.7

I read manual

S7300:

- With the latest versions of the Ethernet CPs, only program blocks AG_SEND and AG_RECV are used; data with a length of up to 8192 bytes can be transferred.
- With S7-300 CPs (up to 6GK7 343-1EX10-0XE0 with firmware version V2.2), use FC60 on TCP connections instead of FC6. For the CP 343-1 (EX10), you can use FC5/FC6 up to block version V3.0.

When you call an FC, you transfer the address and length of the data area in the CPU. Remember, that the maximum length of the data area depends on the block type and block version being used.

AG_SEND and AG_RECVUp to version V3.0 of these blocks, a maximum of 240 bytes can be sent or received. The current block versions allow a data area of up to 8192 bytes for an S7-300. With an S7-400, the FCs AG_LSEND / AG_LRECV must still be used for larger data areas.
AG_LSEND / AG_LRECVUsing the CPs of the S7400 and with earlier versions of the S7300, larger data areas can only be transferred with the FCs AG_LSEND or AG_LRECV. Please check the length of the data area in the product information of the CP.

AG_SSEND / AG_SRECVWith CPs of the S7-400 that support PROFINET communication in conjunction with CPUs as of version 5.1, data can be transferred at higher transmission speeds with the FCs AG_SSEND or AG_SRECV (does not apply to the CP 443-1 Advanced 6GK7?43-1EX41-0XE0).You can check which CP types are supported by CPUs as of version 5.1 in the manual of your CP (Section "Requirements for use").

The following table shows the limit values of the various connection types.

FC ISO transport ISO-on-TCP TCP UDP
AG_LSEND (S7-400)AG_SEND (S7-300) 8192 bytes 8192 bytes 8192 bytes 2048 bytes
AG_SEND (S7-400) 240 bytes 240 bytes 240 bytes 240 bytes
AG_LRECV (S7-400)AG_RECV (S7-300) 8192 bytes 8192 bytes 8192 bytes 2048 bytes
AG_RECV (S7-400) 240 bytes 240 bytes 240 bytes 240 bytes
AG_SSEND (S7-400)AG_SRECV (S7-400) 1452 bytes 1452 bytes 1452 bytes 1452 bytes

it says i can send 1~8192 byte and receive 1~8192byte

but I can send only 240 byte.

I use TCP connection . PLC <-> PC

Hello

Sorry Im not an expert in this topic, I have read also about 8K, but perhaps its also a matter of PDU Size, because this is at S7300 240Bytes (effective)

Perphaps this one helps:

https://support.industry.siemens.com/cs/ww/en/view/8707570

Bye Murof