User guide

ASCOM USER GUIDE
High Speed Base Band Modems
53
8.2 X.21 BIS (V.24/V.28)
8.2.1 25 Way D-type X.21 bis (V.24/V.28) Connector pin allocation.
8.2.2 Module Link Settings
There are no links on this module
8.2.3 External Timing
If External timing is required ie the modem is to be used as a DTE then a change over cable
will be required as shown in the diagram below:
2 103 2 103
3 104 3 104
4 105 4 105
7 102 7 102
8 109 8 109
24 113 17 115
Master
High Speed
Modem
Set to
External
Bit Timing
Timing
Source
Pin Circuit Description Type SITS89/43
7 102 Common Return Common
2 103 Transmitted Data Load
3 104 Received Data Generator
4 105 Request to Send Load
5 106 Ready for Sending Generator
6 107 Data Set Ready Generator
20 108.1 Connect Data Set to Line Load
8 109 Data Channel Received Line Signal Detector Generator
15 114 Transmitter Signal Element Timing (DCE) Generator
17 115 Receiver Signal Element Timing (DCE) Generator
21 140 Remote Loopback for Point-to-Point circuits Load
18 141 Local Loopback Load
25 142 Test Indicator Generator
24 113 External Transmitter Signal Element Timing Load
8
52 ASCOM USER GUIDE
High Speed Base Band Modems
J2 – X.21 SEXT (External Signal Element Timing) Circuit Termination – This link connects a 120
ohm load resistor across the SEXT circuit in position 1-3. This helps maintain signal rise times
and minimises reflections at rates greater than 9.6kbps over long lines. At lower rates and
over short lines the termination resistor may be omitted, (link set position 1-2).
8.1.3 External Timing
If External timing is required ie the modem is to be used as a DTE then a change over cable
will be required as shown in the diagram below
2 T-A 2 T- A
9 T-B 9 T-B
4 R-A 4 R-A
11 R-B 11 R-B
1 SE-A 13 S-A
15 SE-B 6 S-B
5 I-A 5 I-A
12 I-B 12 I-B
3 C-A 3 C-A
10 C-B 10 C-B
8 GND 8 GND
Master
High Speed
Modem
Set to
External
Bit Timing
Timing
Source