Hardware Reference
In-Depth Information
the received data and returning it to its source. When the DTE initiates such a test, trans-
mitted data is passed through the local modem, the telephone line, the remote modem,
and back, to exercise the channel and confirm its integrity. The remote modem signals the
local modem to assert test mode on pin 25 when the remote loopback test is underway.
Pin 25—Test mode (TM).
This signal is relevant only when the DCE device is a modem.
When asserted (logic 0), it indicates that the modem is in an LL or RL condition. Other
internal self-test conditions may also cause the TM signal to be asserted, depending on
the modem and the network to which it is attached.
The TIA-232E uses a 25-pin D-type connector, as shown in Figure 9.1a. Since only a small
subset of the 25 signals is actually used, a 9-pin connector (DB9) is used in most PCs. The signal
assignment of DB9 is shown in Figure 9.1b. The DB9 is not part of the TIA-232E standard.
Signal
Direction
Signal
Direction
Signal Name
Signal Name
1
2
3
4
5
6
7
8
9
10
11
12
13
Protective ground
Transmitted data
Received data
Request to send
Clear to send
Data set ready
Signal ground
Carrier detect
Reserved
Reserved
Both
to DCE
to DTE
to DCE
to DTE
to DTE
Both
to DTE
to DCE
to DTE
Secondary transmitted data
Transmit clock
Secondary received data
Receiver clock
Unassigned
Secondary request to send
Data terminal ready
Signal quality detect
Ring indicator
Data rate select
Transmit clock
Unassigned
14
15
16
17
18
19
20
21
22
23
24
25
to DTE
to DTE
to DCE
to DCE
to DTE
to DTE
Both
Unassigned
to DCE
Secondary carrier detect
to DTE
Secondary clear to send
to DTE
Figure 9.1a
■
EIA-232E DB25 connector and pin assignment
Ground
5
Ring indicator
9
4
DTE ready
8
Clear to send
3
Transmitted data
Request to send
7
2
Received data
DCE ready
6
Received line signal detect
1
Figure 9.1b
■
EIA-232 DB9 connector and signal assignment
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