Data can be safely written, that
is, tx_enain asserted, when tx_rdyout is asserted. After tx_rdyout is negated, additional writes
using tx_enain can be safely performed
provided tx_ovfout is never asserted.
When tx_rdyout is asserted again,
additional data can be written. If at any time the back-pressure
mechanism is violated, the tx_ovfout is
asserted to indicate the violation. Up to four write cycles might
be safely performed after tx_rdyout is
negated, but no more until tx_rdyout is
asserted again.