Ready-To-Receive Using Rts; Transceiver Driver Enable Using Rts - NXP Semiconductors freescale KV4 Series Reference Manual

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46.9.5.1 Ready-to-receive using RTS

To help to stop overrun of the receiver data buffer, the RTS signal can be used by the

receiver to indicate to another UART that it is ready to receive data. The other UART can

send the data when its CTS signal is asserted. This handshaking conforms to the

TIA-232-E standard. A transceiver is necessary if the required voltage levels of the

communication link do not match the voltage levels of the UART's RTS and CTS signals.

The transmitter's CTS signal can be used for hardware flow control whether its RTS

signal is used for hardware flow control, transceiver driver enable, or not at all.

RS-485 is a multiple drop communication protocol in which the UART transceiver's

driver is 3-stated unless the UART is driving. The RTS signal can be used by the

transmitter to enable the driver of a transceiver. The polarity of RTS can be matched to

the polarity of the transceiver's driver enable signal. See the following figure.

Figure 46-90. Transceiver driver enable using RTS

In the figure, the receiver enable signal is asserted. Another option for this connection is

to connect RTS_B to both DE and RE_B. The transceiver's receiver is disabled while

driving. A pullup can pull RXD to a non-floating value during this time. This option can

be refined further by operating the UART in single wire mode, freeing the RXD pin for

Freescale Semiconductor, Inc.

Chapter 46 Universal Asynchronous Receiver/Transmitter (UART) / FlexSCI

Figure 46-89. Ready-to-receive

KV4x Reference Manual, Rev. 2, 02/2015

RS-485 TRANSCEIVER