Comtech EF Data SNM-1001 - Manual (Page 43)

Theory of Operation SNM-1001 Network Control Modem
4–12 Rev. 1
4.4 Digital Interfaces
The modem interface module is a daughter card that plugs onto the demodulator board.
The module provides the interface for terrestrial data and overhead signals, and the fault
reporting output of the modem.
EIA-422/449 and EIA-232-C interfaces are available for input and output of terrestrial
data when the modem is in the Continuous mode. Both baseband and interface loopbacks
are provided. The terrestrial data rate is 19.2 kbit/s.
4.4.1 EIA-422/449 Interface
4.4.1.1 Functional Description
The EIA-422/449 digital interface provides the level translation, buffering, and
termination between the internal modem signals and the EIA-422/449 interface on the
rear panel.
Electrical characteristics of the EIA-422/449 interface signals are defined in EIA STD
EIA-422. Details of the mechanical interface are found in EIA STD EIA
- 449.
A functional diagram of the interface is shown in Figure 4-4.
The EIA-422/449 interface provides a Send Timing (ST) clock signal at the modem data
rate.
•
In the Internal Clock mode, the data to be transmitted, Send Data (SD), must be
synchronized to ST.
•
In the External Clock mode, the clock is accepted on the Terminal Timing (TT)
input to clock-in the data to be transmitted.
•
In either Internal or External Clock mode, the phase relationship between the
clock and data is not important as long as it meets the EIA-422/449 jitter
specifications.