Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 213)

Detailed Flowcharts
Appendix D
DĆ9
Figure D.10
Sharing the Transmit Side of the UART
TXALLOC
UART Usage
TXFREE
XMIT RCVE
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 oneĆlane 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.