There is no flow control provided by the HSM. It is the responsibility of the application to ensure that the input buffer in the HSM, which is 32K bytes long, is not exceeded.
No single command contains more than 2048 bytes (1024 for SNA-SDLC), including any STX and ETX characters. Where an Async connected HSM operates in half duplex the response to a command must be received before a new command request is sent.