NEC UPD98413 User Manual page 24

CHAPTER 2 PIN FUNCTION
TSX I Transmit start of transfer.
LVTTL
Signal for controlling the port address specification using
TDAT[7:0] at the start of transfer. When TSX is high and
TENB_B is high, the port used to start transfer can be
selected with the port address input to TDAT[7:0].
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PRELIMINARY
NEC confidential and Proprietary