Hardware Reference
In-Depth Information
The sequence of events that occurs during data transmission using the TIA-232E is easier
to understand by studying examples. Two examples are used to explain the procedure.
In the first example, two DTEs are connected with a point-to-point link using a modem.
The modem requires only the following circuits to operate:
•
Signal ground (AB)
•
Transmitted data (BA)
•
Received data (BB)
•
Request to send (CA)
•
Clear to send (CB)
•
Data set ready (CC)
•
Carrier detect (CF)
Before the DTE can transmit data, the DSR circuit must be asserted to indicate that the
modem is ready to operate. This signal should be asserted before the DTE attempts to make a
request to send data. The DSR pin can simply be connected to the power supply of the DCE to
indicate that it is switched on and ready to operate. When a DTE is ready to send data, it asserts
the RTS signal. The modem responds, when ready, with the CTS signal asserted, indicating that
data may be transmitted over circuit BA. If the arrangement is half-duplex, then the assertion
of the RTS signal also inhibits the receive mode. The DTE sends data to the local modem bit
serially. The local modem modulates the data into the carrier signal and transmits the resultant
signal over the dedicated communication lines. Before sending out modulated data, the local
modem sends out a carrier signal to the remote modem so that it is ready to receive the data.
The remote modem detects the carrier and asserts the CD signal. The assertion of the CD sig-
nal tells the remote DTE that the local modem is transmitting. The remote modem receives the
modulated signal, demodulates it to recover the data, and sends it to the remote DTE over the
RxD pin. The circuit connections are illustrated in Figure 9.2.
The next example involves two computers exchanging data through a public telephone line.
One of the computers (initiator) must dial the phone (automatically or manually) to establish
Computer
(DTE)
Modem
(DCE)
Modem
(DCE)
Computer
(DTE)
TxD
RxD
TxD
TxD
TxD
RxD
RxD
RxD
Direct link
CD
CD
CD
CD
CTS
CTS
CTS
CTS
RTS
RTS
RTS
RTS
DSR
DSR
DSR
DSR
GND
GND
GND
GND
Figure 9.2
■
Point-to-point asynchronous connection
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