Serin2 already has handshake implemented! Why not use it instead of trying to re-invent the wheel?

From the manual:

SERIN2 DataPin{\FlowPin},Mode,{ParityLabel,}{Timeout, Label,}[Item...]

Receive one or more Items on Pin in standard asynchronous format. SERIN2 is similar to the BS2 SERIN command. DataPin is automatically made an input. The optional FlowPin is automatically made an output. DataPin and FlowPin may be a constant, 0 - 15, or a variable that contains a number 0 - 15 (e.g. B0) or a pin name (e.g. PORTA.0).
The optional flow control pin, FlowPin, may be included to help keep data from overrunning the receiver. If it is used, FlowPin is automatically set to the enabled state to allow transmission of each character. This enabled state is determined by the polarity of the data specified by Mode.

Ioannis