AN3123

UART modem operation

Doc ID 16862 Rev 2

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UART modem operation

The UART can support both data terminal equipment (DTE) and data communication

equipment (DCE) modes of operation. Table 13 gives a description of the signals in each

mode.

Two dedicated "modem control" wires are used in hardware flow control to send the "stop"

and "start" signals. When the computer is ready to receive data it asserts RTS by putting a

positive voltage on the RTS pin. When the computer is not able to receive any more bytes, it

negates RTS by asserting a negative voltage on the pin. The RTS pin is connected by the

serial cable to another pin on the modem. This other pin's only function is to receive this

signal.

This "other" pin will be the modem's RTS pin. For a modem, a straight-thru cable is used.

For the opposite direction of flow a similar scheme is used. The CTS pin is used to send the

flow control signal to the CTS pin on the PC.

Thus modems and non-modems have the roles of their RTS and CTS pins interchanged.

RS-232 hardware handshaking has ben specified in terms of communication between Data

Terminal Equipment (DTE) and Data Communications Equipment (DCE).

Most RS-232 connections use 9-pin DSUB connectors. A DTE uses a male connector and a

DCE uses a female connector.

Table 13.

Modem input/output signals in DTE and DCE modes

Signal

Description

DTE

DCE

nUARTCTS

Clear to Send

Request to Send

nUARTDSR

Data Set Ready

Data Terminal Ready

nUARTDCD

Data Carrier Detect

-

nUARTRI

Ring Indicator

-

nUARTRTS

Request to Send

Clear to Send

nUARTDTR

Data Terminal ready

Data Set Ready

nUARTOUT1

-

Data Carrier Detect

nUARTOUT2

-

Ring Indicator