256-KBPS LINE DRIVER (V.35, RS-530, OR X.21)
2. Introduction
The 256-kbps Line Driver is a short-range modem for synchronous or asynchronous transmission—full- or half-duplex—over unconditioned lines. With a range of up to 11.8 miles (19 km), the line driver operates at user-selectable synchronous data rates from 19.2 to 256 kbps or asynchronous data rates from
19.2to 115.2 kbps. The line driver is available in standalone V.35 or RS-530 (ME444A/E-R3) or rackmount card V.35, RS-530, or X.21 (ME445C) versions.
The line driver uses conditional differential diphase modulation (EUROCOM Std. D1) to provide immunity from background noise, eliminate normal line distortion, and enable efficient transmission and reception of serial data over twisted-pair cable. The modem is coupled to the line through isolation transformers which, in conjunction with protective circuitry, safeguard against AC or DC overvoltages. With this protective circuitry, the line driver can operate even when DC is connected to the line.
Transmit timing is provided internally, derived externally from the PC or terminal, or regenerated from the receive signal. Receive timing is always regenerated from the receive signal.
The line driver features V.54 diagnostic capabilities for local analog loopback and local and remote digital loopback testing. In the digital loopback mode, the operator at either end of the line may test both modems and the line. The loopback is controlled either by pressing front-panel buttons or by manipulating signals on the V.35, RS-530, or X.21 (DTE) interface.
The line driver incorporates a built-in Bit Error Rate Tester (BERT). The internal BERT enables complete testing of both modems and the line without external test equipment. Pressing the line driver’s front-panel PATT button causes the unit to generate a pseudo-random test pattern (511-bit, according to ITU-TSS V.52) for testing end-to-end connectivity. The ERR LED flashes when a bit error occurs.
2.1 Physical Description
The line driver stands alone as a desktop unit, or can be mounted in a 19-inch rack with optional brackets (see Section 4.1). Also available is a rackmount card version that installs in the RackNest 2/14 (see Section 4.2). For descriptions of the line driver’s controls and indicators, see Section 5.1.
