Rockwell-automation 1770-KF2 Data Highway or Highway Plus Interface Mo Instrukcja Użytkownika Pobierz pdf (Strona 213)

Detailed Flowcharts

Appendix D

D9

Figure D.10

Sharing

the Transmit Side of the UART

TXALLOC

UART Usage

TXFREE

XMIT RCVE

Not-in-Use

Signal

11664

Since the transmit side of the UART is shared by the transmit and receive processes, a

mutual exclusion mechanism is used to gain sole access during transmission of each

indivisible code sequence. The UART is like a small bridge; and the mutual exclusion

mechanism is like two flagmen, one at each end of a onelane bridge. If two heavy

trucks (one called XMIT and one called RCVE) come to the bridge at the same time,

the flagman on one side lets one truck through and makes the other wait. When the

first truck has crossed, the first flagman signals the second flagman, who then allows

the second truck to cross. In a similar manner, TXALLOC and TXFREE work together

to ensure the XMIT and RCVE do not try to use the UART at the same time.

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Rockwell-automation 1770-KF2 Data Highway or Highway Plus Interface Mo Instrukcja Użytkownika Strona 213