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X.21 Loopback Plug Specifications

To retain FCC Class A qualification all cables must be fully shielded and the shielding must be connected to the metal connectors.

SG4P X.21 Cable
Description
Pin Number
Description

w
HD-62 'I-E' plug

w
DCD 3 A
1
8
RTS 2 A
RxData 3 A
2
9
TxData 2 A
RTS 3 A
3
6
DCD 2 A
TxData 3 A
4
7
RxData 2 A
Clock 3 A
26
31
Clock 2 A
DCD 1 A
12
19
RTS 0 A
RxData 1 A
13
20
TxData 0 A
RTS 1 A
14
17
DCD 0 A
TxData 1 A
15
18
RxData 0 A
Clock 1 A
37
42
Clock 0 A
GND
24
29
GND
RTS 3 B
22
51
DCD 2 B
TxData 3 B 3
23
52
RxData 2 B
RTS 1 B
33
61
DCD 0 B
TxData 1 B
34
62
RxData 0 B
Clock 3 B
45
50
Clock 2 B
DCD 3 B
46
27
RTS 2 B
RxData 3 B
47
28
TxData 2 B
Clock 1 B
55
60
Clock 0 B
DCD 1 B
56
38
RTS 0 B
RxData 1 B
57
39
TxData 0 B
GND
35
40
GND
Unused pins:
5, 10, 11, 16, 21, 25, 30, 32, 36, 41, 43, 44, 48, 49, 53, 54, 58, 59

Rx=Receive, Tx=Transmit, CTS=Clear to Send, RTS=Ready to Send,
DTR=Data Terminal Ready, DSR=Data Set Ready, DCD=Data Carrier Detect