Specifications
AE/LZB 119 1897 R1A SOFTWARE CONFIGURATION
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6.3 MODEM CONFIGURATIONS
6.3.1 General Information
Instead of a direct RS-422 control data link between the C3 Maestro console and the CIM in the CEC/IMC, some
applications may require modem-supported control data links. Normally, modems are employed only when the console-to-
CEC/IMC distance exceeds the capability of an RS-422 link (about 4,000 feet) or the cost of establishment and maintenance
of RS-422 link cabling between the console and the CEC/IMC exceeds the cost of establishment and maintenance of two
modems (a modem pair) and the required modem lines. In some cases, existing phone lines which may be utilized for modem
connections but not for RS-422 connections will influence the RS-422 vs. modems decision.
Either a dedicated line or a dial-up line may be employed between a modem pair. However, during normal console
operations (vs. during troubleshooting/diagnostic periods) a dedicated line—typically referred to as a “leased line” in the
communications industry—is almost always utilized since the C3 Maestro console requires a continuous control data link
between it and the CIM in the CEC/IMC during operation. Nearly all dispatch centers utilize modems with dedicated line
links if RS-422 links cannot handle the distances evolved.
Normally, a dedicated line between a modem pair utilizes 3002 data-grade 4-wire connections meeting the following
minimum specifications:
•
General Line Type 4-wire / 2 balanced pairs (one pair for TX, one pair for RX)
•
Impedance (each 2-wire pair) 600 ohms
•
Frequency Response
1000 Hz (reference) 0 dB
500 — 2400 Hz -1 to +3 dB
300 — 2700 Hz -2 to +6 dB
•
Maximum Frequency Error ±5 Hz
•
Maximum Net Loss 16 dB
•
Maximum Group Delay (800 — 2400 Hz) 2000 µS
•
Minimum Signal/Noise Ratio 24 dB
With a dedicated line, the modems can be set-up to automatically establish a connection. Therefore, no modem-related
maintenance is required if the connection is interrupted because of for example, a temporarily line breakage or a temporarily
power interruption at either modem.
In the case of the dial-up line like a standard telephone line, a connection must be established manually. Normally, dial-
up lines are used only for console troubleshooting/diagnostic purposes.