A.2 Reference for the Serial Port A and B Connectors

1 

Not Connected 

None. 

2 

TXD 

Transmit Data 

Used by the data terminal equipment (DTE) to transmit data to the data circuit equipment (DCE). Except when control data is being sent, RTS, CTS, and DCD must be ON for this line to be active. 

3 

RXD 

Recive Data 

Used by the DCE in response to received data from the DTE. 

4 

RTS 

Ready to Send 

Used by the DTE to condition the DCE for data transmission. The transition to ON directs the DCE to go into transmit mode. The transition to OFF directs the DCE to complete the transmission. 

5 

CTS 

Clear to send 

Used by the DCE to indicate if it is ready to receive data from the DTE. When CTS, DSR, RTS and DTR are ON, the DCE is ready to transmit data received from the DTE across the communications channel. When only CTS is ON, the DCE is ready to accept dialing or control signals only. When CTS is OFF, the DTE should not transfer data across TXD. 

6 

DSR 

Data Set Ready 

Used by the DCE to indicate if it is ready to operate. When DSR is ON, the DCE is connected to the line and ready to exchange further control signals to start data transfer. 

7 

Gnd 

Signal Ground 

8 

DCD 

Data Carrier Detect 

Used by the DCE to indicate it is receiving a suitable signal from the communications channel. 

9 

Not connected 

None. 

10 

Not connected 

None. 

11 

Not connected 

None. 

12 

Not connected 

None. 

13 

Not connected 

None. 

14 

Not connected 

None. 

15 

TRXC 

Transmit Clock 

Used by the DCE to provide timing information to the DTE. The DTE provides data on TXD in which the transition of the bit corresponds to the rising edge of the clock. 

16 

Not Connected 

None. 

17 

RTXC 

Receive Clock 

Used by the DCE to provide timing information to the DTE. The falling edge of the clock corresponds to the center of the data bit received on RXD. 

18 

Not connected 

None. 

19 

Not connected 

None. 

20 

DTR 

Data Terminal Ready 

Used to control switching of the DCE to the communication channel.  

21 

Not connected 

None. 

22 

Not connected 

None. 

23 

Not connected 

None 

24 

TXC 

Terminal Clock 

Generated by the DTE to provide timing information to the DCE. Used only in synchronous mode and only when the driver requests a locally generated clock. Otherwise, TXC echoes the modem-generated clock. The falling edge of the clock corresponds to the center of the data bit transmitted on TXD. 

25 

Not connected 

None.