TINY-2/MICRO-2 Technical Ref., 2. Ed 1989
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RS-232 Signal Table
The EIA RS-232C Standard
The Electronic Industries Association (EIA) RS-232C Standard defines the interfacing between data
terminal equipment and data communication equipment employing serial binary data interchange.
Electrical signal and mechanical aspects of the interface are well specified. The complete RS-232C
interface consists of 25 data lines. This would seem to be enough signals for a complex parallel
communication line, but many of the 25 lines are very specialized and few are undefined. Most
computer terminals only require from 3 to 5 of these lines to be operational.
Because RS-232C was originally intended for data communications equipment, a few lines must be
swapped between a computer and terminal if no modem of other data communications equipment
is used. These wires are swapped within the cable connecting the two devices. This line is called a
null modem cable.
Pin Description
1 Protective Ground
2 Transmitted Data
3 Received Data
4 Request to Send
5 Clear to Send
6 Data Set Ready
7 Signal Ground (Common Return)
8 Received Line Signal Detector
9 (Reserved for Data Set Testing)
10 (Reserved for Data Set Testing)
11 Unassigned (See section 3.2.3)
12 Sec. Rec'd. Line Sig. Detector
13 Sec. Clear to Send
14 Secondary Transmitted Data
15 Transmission Signal Element Timing (DCE Source)
16 Secondary Received Data
17 Receiver Signal Element Timing (DCE Source)
18 Unassigned
19 Secondary Request to Send
20 Data Terminal Ready
21 Signal Quality Detector
22 Ring Indicator
23 Data Signal Rate Selector (DTE/DCE Source)
24 Transmit Signal Element Timing (DTE Source)
25 Unassigned
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