DS2155 datasheet(149/262 Pages) DALLAS

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DS2156

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23.

LINE INTERFACE UNIT (LIU)

The LIU contains three sections: the receiver that handles clock and data recovery, the transmitter that

waveshapes and drives the T1 line, and the jitter attenuator. These three sections are controlled by the line

interface control registers (LIC1–LIC4), which are described in the following sections. The LIU has its

own T1/E1 mode-select bit and can operate independently of the framer function.

The DS2156 can switch between T1 or E1 networks without changing any external components on either

the transmit or receive side. Figure 23-3 shows a network connection using minimal components. In this

configuration, the DS2156 can connect to T1, J1, or E1 (75Ω or 120Ω) without any component change.

The receiver can adjust the 120Ω termination to 100Ω or 75Ω. The transmitter can adjust its output

impedance to provide high return-loss characteristics for 120Ω, 100Ω, and 75Ω lines. Other components

can be added to this configuration to meet safety and network protection requirements (Section 23.8).

23.1 LIU Operation

The analog AMI/HDB3 waveform off the E1 line or the AMI/B8ZS waveform off of the T1 line is

transformer-coupled into the RTIP and RRING pins of the DS2156. The user has the option to use

internal termination, software selectable for 75Ω/100Ω/120Ω applications, or external termination. The

LIU recovers clock and data from the analog signal and passes it through the jitter-attenuation mux

outputting the received line clock at RCLKO and bipolar or NRZ data at RPOSO and RNEGO. The

DS2156 contains an active filter that reconstructs the analog-received signal for the nonlinear losses that

occur in transmission. The receive circuitry also is configurable for various monitor applications. The

device has a usable receive sensitivity of 0dB to -43dB for E1 and 0dB to -36dB for T1, which allow the

device to operate on 0.63mm (22AWG) cables up to 2.5km (E1) and 6k feet (T1) in length. Data input at

TPOSI and TNEGI is sent through the jitter-attenuation mux to the waveshaping circuitry and line driver.

The DS2156 drives the E1 or T1 line from the TTIP and TRING pins through a coupling transformer.

The line driver can handle both CEPT 30/ISDN-PRI lines for E1 and long-haul (CSU) or short-haul

(DSX-1) lines for T1.

23.2 Receiver

The DS2156 contains a digital clock recovery system. The DS2156 couples to the receive E1 or T1

twisted pair (or coaxial cable in 75Ω E1 applications) through a 1:1 transformer. See Table 23-A for

transformer details. The DS2156 has the option of using software-selectable termination requiring only a

single fixed pair of termination resistors.

The DS2156’s LIU is designed to be fully software selectable for E1 and T1, requiring no change to any

external resistors for the receive side. The receive side allows the user to configure the DS2156 for 75Ω,

100Ω, or 120Ω receive termination by setting the RT1 (LIC4.1) and RT0 (LIC4.0) bits. When using the

internal termination feature, the resistors labeled R in Figure 23-3 should be 60Ω each. If external

termination is used, RT1 and RT0 should be set to 0 and the resistors labeled R in Figure 23-3 should be

37.5Ω, 50Ω, or 60Ω each, depending on the line impedance.

There are two ranges of user-selectable receive sensitivity for T1 and E1. The EGL bit of LIC1 (LIC1.4)

selects the full or limited sensitivity. The resultant E1 or T1 clock derived from MCLK is multiplied by

16 through an internal PLL and fed to the clock recovery system. The clock recovery system uses the

clock from the PLL circuit to form a 16-times over-sampler that is used to recover the clock and data.

This over-sampling technique offers outstanding performance to meet jitter tolerance specifications

shown in Figure 23-7.

DS2155 Datasheet(PDF) 149 Page - Dallas Semiconductor

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