CANOGA PERKINS 2240 Fiber Optic Modem - Manual (Page 79)

Chapter 4 Data Interfaces
High-Speed RS-422/Mil-Std 188-114C Interfaces
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4.10 High-Speed RS-422/Mil-Std 188-114C Interfaces
There are three High-Speed RS-422 interface models (TW, T22 and D22) and three High-Speed
Mil-Std 188-114C interface models (TW8, T88 and D88) available.
All can operate up to 20 Mbps (2240 limited to 2.048 Mbps). All support only clock and data
signals as shown in Table 4-19. Both the RS-422A and Mil-Std 188-114C are balanced
differential electrical signals.
The RS-422A operates between +1 and +4 volts whereas the Mil-Std 188-114C swings between
+/-3 volts. The termination impedances vary slightly as illustrated in Table 4-21. The two interface
types will communicate with each other but center tap ground jumpers E2 and E3 must be
removed from a Mil-Std 188-114C interface (refer to Table 4-22).
The basic differences between the models is the type of physical connectors used for the
interface. Table 4-21 lists the six interface models with the corresponding source and termination
impedances and physical connectors. Table 4-22 shows the jumper options available and the
factory default settings for the jumpers.
4.10.1 Model TW
The signaling used on this interface is RS-422A. Four TwinAx connectors
(BJ-77, 3-lug) are used for the physical connection (see Figure 4-16).
A switch is provided to select whether the fourth TwinAx (SCT/SCTE) is to be used as an output
(SCT) or as an input (SCTE). By setting the switch to the SCT position, the port becomes an
output providing the clock to the connected device. When set for SCTE, the port becomes an
input and will accept a clock from the connected device.
Table 4-20. TwinAx Supported Signals
Signal Full Name Direction
TxD Transmit Data To Modem
RxD Receive Data From Modem
SCR Serial Clock
Receive From Modem
SCT Serial Clock
Transmit From Modem
SCTE External Clock
Transmit To Modem
Table 4-21. Model Characteristics
Model
Electrical Interface
Type
Physical
Interface
Type
Physical
Interface
Figure/
Table
Driver
Impedance
Termination
Impedance
TW RS-422A 4 TwinAx Figure 4-16 <100 Ohms 100 Ohms ±10%
TW8 Mil-Std 118-114C 4 TwinAx Figure 4-16 <100 Ohms 78 Ohms ±10%