1、 1 of 14 REV: 052203 INTRODUCTION Figure 1 illustrates a general interface for T1/E1 transceiver chips. The circuit is an unprotected network interface and shows a general idea about how to distribute resistance around the transformers. When designing the network interface for specific applications,
2、 more or fewer components may be necessary. One example is an over voltage protection network. For this application, it is necessary to add voltage suppression- and current-limiting devices. This type of design is discussed in more detail later in this application note. The following paragraph is a
3、brief overview of the line interface for the Dallas Semiconductor T1 and E1 parts. The transmitter output drivers present a low impedance to inbound surges and must be able to drive sufficient current into the primary winding of the transmit transformer in order to produce the required output pulse.
4、 The transmitter outputs are designed to fit an output pulse into a template using a step-up transformer under a matched load with 0g87 of inline resistance. The step-up transformer is based on the supply voltage and the matched load depends on the line impedance100g87 for T1 and 75g87 or 120g87 for
5、 E1. The receiver inputs present a high impedance to inbound surges and requires very little input current to operate. The receiver inputs are designed to recover a signal using a 1:1 transformer with 0g87 of series resistance under a matched load. For these reasons, the transmitter and receiver pin
6、s require different protection techniques. Figure 1. General Network Interface Circuit Application Note 324 T1/E1 Network Interface Designwww.maxim- Application Note 324: T1/E1 Network Interface Design 2 of 14 RECEIVE CIRCUIT The receive circuit is the most straight forward. Generally a 1:1 transfor
7、mer is used to interface to the receiver inputs. The primary consideration in the receive circuit is the accurate termination of the transmission line. A T1 signal is carried on 100g87 balanced twisted pair while an E1 signal is carried on either 75g87 unbalanced coax or 120g87 balanced twisted pair
8、. The components involved in the termination network are the RR, and RPRXresistors. The ideal termination circuit would be if the RPRXresistors were 0g87 and the resistance of RRequaled half of the characteristic line impedance. Since the RPRXresistors may be necessary as part of a protection networ
9、k, they form a voltage divider and RRmust be adjusted. As the resistance of the RPRXresistors increases, the resistance of RRdecreases. If the RPRXresistors become too large, then the receiver may be unable to recover weak signals. The following equation describes the termination: Zterm = RPRX+ RPRX
10、+ 2 RR / N2 Substitute: Zterm= 100g87 (T1) or 75g87, 120g87 (E1) and N = 1 Then solve for RPRXand RRThe RPresistors in the circuit are for current limiting purposes and do not significantly affect the termination due to the high impedance of the receiver inputs. Capacitor C1, along with resistors RR
11、, form a high frequency cutoff filter for improved noise immunity. INTERNAL RECEIVE TERMINATION To ease the design of receive termination for both T1 and E1 circuits, Dallas Semiconductor has designed the ability to select the termination using software into the DS2148 and DS2155. By designing the r
12、eceive circuit for 120g87 termination, the internal line interface unit can selectively add resistance to the line to achieve termination settings of 75g87, 100g87 or 120g87. When using internal termination two changes must be made to the external components. First, the Rr resistors must be 60g87 (t
13、his sets up a line termination of 120g87) and the Rp resistors must be omitted from the circuit. Omitting the Rp resistors is necessary because the 470g87 resistors would interfere with the additional resistance that the internal circuitry adds. Second, the RPRXresistors must be omitted for the inte
14、rnal termination to match the impedance properly. If these resistors are present, the received signal will be degraded by the resulting impedance mismatch in line termination. If the receive circuitry needs to be protected, voltage suppression must be used. This is discussed later in the application
15、 note. TRANSMIT CIRCUIT Since the signal pulses for T1 and E1 are different and the source voltage for the driver can be either 3.3V or 5.0V, the transmit circuit description is more complicated than the receive circuit. When designing the transmit circuit, several considerations must be taken into
16、account. Some applications require that the source impedance be closely matched to the characteristic impedance of the network. Along with this there may be a need to provide for protection of the circuit against power line cross (UL) and transient (FCC) conditions. The following is a description of
17、 the transmitter interface for both T1 and E1 networks. The circuit in Figure 1 is a basic network circuit and does not provide protection. Circuits designed with diodes, fuses, and voltage suppressors are more universal with an improved level of protection. For this reason, these descriptions are f
18、or reference only. Protected network interfaces are discussed later in more detail. T1 DEVICE TRANSMIT CIRCUIT The transmitter outputs of Dallas Semiconductor T1 parts are designed to generate the correct pulse amplitude at the network interface for varying line lengths. Since the different line len
19、gths affect the pulse shape, the parts have programmable output levels. Every part has a transmitter LBO table in the data sheet, which shows the settings to choose based on the transformer turn ratio and the line length. A default T1 pulse for a known line length is generated under the following co
20、nditions: 5.0V supply, RPTX= RT= 0g87, and the Tx transformer has a turn ratio of 1:1.15. For circuits requiring protection the value of the resistors may be increased. However, as series resistance is increased, so is signal attenuation and a transformer with a larger turn ratio must be selected to
21、 compensate for the loss. A nominal 0dB T1 pulse is 3V under a 100g87 load or 3V at 30mA on the network interface. An unprotected circuit using a 1:1.15 transformer with series resistance of 0g87, will have to produce a 3V g180 1/1.15 = 2.6V pulse at the Application Note 324: T1/E1 Network Interface
22、 Design 3 of 14 output pins of the device. The current drive into the primary winding of the tansformer will be 30mA g180 1.15 = 34mA. If resistors RPTXor RTare added to protect the device from surges, a 1:1.36 transformer will be necessary. While the current pulse in the secondary loop of the 1:1.3
23、6 transformer will remain the same, the current pulse in the primary of the transformer will be 30mA g180 1.36 = 40mA. Because the output voltage pulse is still 2.6V, the net impedance (RL) seen by the transmitter will be 2.6V / 40mA = 65g87 and is described by: RL= ZLOAD / N2+ RPTX/ N2+ RPTX/ N2+ 2
24、 RTSubstitute: RL= 65g87, ZLOAD= 100g87, and N = 1.36 65g87 = 100g87 / 1.362+ (RPTX+ RPTX) / 1.362+ 2 RT Simplify: 10.9g87 = (RPTX+ RPTX) / 1.85 + 2 RTIf the RPTXresistors are 0g87 then RT= 5.5g87. Since 4.7g87 is the closest standard value that is less than the theoretical limit, it can be used in
25、the circuit. If RTis 0g87 then the RPTXresistors can be as much as 10g87 each and will also provide current limit protection for the transformer. In this circuit, the RPTXresistors can be combined into a single component on the network side of the transformer. Any resistance that is on the line side
26、 must be divided equally in the TIP and RING output circuits so that the line is balanced. Devices operating from a 3.3V supply require a 1:2 step up transformer to produce a sufficient voltage pulse on the transformer secondary. In order to produce the required 3V pulse under a 100g87 load, a curre
27、nt pulse of 30mA g180 2 = 60mA is required from the transmitter output drivers. Adding series resistance to this network would require a turn ratio greater than 1:2 and thus even larger currents from the transmitter. For this reason, it is recommended that 3.3V circuits be designed with 0g87 of seri
28、es resistance and use other components for over voltage protection. Schottky diodes placed in a bridge configuration connected to TTIP and TRING will turn on sooner than the silicon diodes in the transmit output drivers and conduct energy away from the CMOS device. The use of Schottky diodes and fus
29、es as protection methods for both 3.3V and 5.0V designs is discussed later. E1 DEVICE TRANSMIT CIRCUIT The transmitter outputs of Dallas Semiconductor E1 parts are designed to generate the correct pulse at the network interface under varying termination conditions. The parts have programmable output
30、 levels which along with the transmit transformer are used to compensate for resistive components between TTIP and TRING and the network interface. This ensures that signals arrive at the network interface with a peak voltage of 3.0V for 120g87 applications or 2.37V for 75g87 applications. Unlike in
31、 T1, E1 applications can have additional resistance in the transmit path that matches the source impedance to the characteristic line impedance. This extra resistance results in a further reduction in signal reflections and line noise being coupled into the transmitter outputs. Return Loss is a meas
32、ure of how well the source and line impedance are matched. A higher return loss results in more attenuation of any noise or reflected signals and is calculated by: Return Loss (dB) = 20 log10|ZSOURCE+ ZLOAD|/|ZSOURCE- ZLOAD| Where: ZLOAD = 120g87 or 75g87 and ZSOURCE= RPTX+ RPTX+ (2 RT+ 5) g180 N2Th
33、e constant of 5 in the ZSOURCEequation above is the internal impedance of the transmitter. The return loss for an unprotected network interface without a high return loss condition is shown below. In the example resistors the supply voltage is 5.0V, R1& R2& Rt = 0g87, the Tx transformer has a turn r
34、atio of 1:1.15, and the line impedance is 75g87. Return Loss (dB) = 20 log10|ZSOURCE+ ZLOAD| / |ZSOURCE- ZLOAD| Substitute: Zload = 75g87, N = 1.15, and RPTX& RT= 0g87 Return Loss = 20 log10|5 g180 1.152+ 75| / |5 g180 1.152- 75| Return Loss = 20 log10 1.21 Return Loss = 1.5 dB Application Note 324:
35、 T1/E1 Network Interface Design 4 of 14 In this example, 83% of the noise or reflected signal can be coupled into the transmitter outputs. To improve the return loss, the value of Rtcan be increased. Changing RTto a value of 27g87 increases the return loss to 34dB. This means less than 2% of the inb
36、ound signal will be reflected. When changing the value of the RPTXor RTresistors it is important to note that there will be changes in the output pulse amplitude. When designing with Dallas Semiconductor E1 parts, consult the LBO table in the data sheet for proper transformer and resistor selection.
37、 Each setting is based on the operational voltage, the transformer turn ratio, and RT. The LBO tables from the DS21554 and DS21354 data sheets are shown in Table 1 and Table 2, respectively. Table 1. LBO Select in LICR for 5V Devices L2 L1 L0 APPLICATION TRANSMIT TRANSFORMER RETURN LOSS1RT20 0 0 75g
38、87 normal 1:1.15 stepup 0 0 0 1 120g87 normal 1:1.15 stepup 0 0 1 0 75g87 normal with protection resistors 1:1.15 stepup 8.2 0 1 1 120g87 normal with protection resistors 1:1.15 stepup 8.2 1 0 0 75g87 with high return loss 1:1.15 stepup 21dB 27 1 1 0 75g87 with high return loss 1:1.36 stepup 21dB 18
39、 1 0 0 120g87 with high return loss 1:1.36 stepup 21dB 27 Table 2. LBO Select in LICR for 3.3V Devices L2 L1 L0 APPLICATION TRANSMIT TRANSFORMER RETURN LOSS1RT20 0 0 75g87 normal 1:2 stepup 0 0 0 1 120g87 normal 1:2 stepup 0 0 1 0 75g87 normal with protection resistors 1:2 stepup 2.5 0 1 1 120g87 no
40、rmal with protection resistors 1:2 stepup 2.5 1 0 0 75g87 with high return loss 1:2 stepup 21dB 6.2 1 0 1 120g87 with high return loss 1:2 stepup 21dB 11.6 Note 1: Empty cells indicate that the return loss is less than 21dB. Note 2: The value of RTshown assumes that both RPTXresistors = 0g87. INTERN
41、AL TRANSMIT TERMINATION To ease the design of transmit termination for E1 circuits and include T1 transmit termination, Dallas Semiconductor has designed the ability to select the termination using software into the DS2155. By designing the transmit circuit with 0g87 of inline resistance, the transm
42、it interface unit can selectively add inline resistance to match the transmitter impedance to a 75g87, 100g87 or 120g87 line. When using internal termination two changes must be made to the external components. First, the RTresistors must be 0g87. Second, the RPTXprotection resistors must be omitted
43、 for the internal termination to match the impedance properly. If these resistors are present, the transmit signal will be degraded by the resulting impedance mismatch in line termination. If the transmit circuitry needs to be protected, voltage suppression should be used. This is discussed later in
44、 the application note. INTRODUCTION TO PROTECTION CIRCUITS Dallas Semiconductor single-chip transceivers (SCTs) and line interface units (LIUs) are used in applications connecting directly to the outgoing telephone lines which can expose the devices to hazardous overvoltage conditions. For such appl
45、ications, protection networks must be used to direct high voltages or currents away from the sensitive low-voltage CMOS devices. Protection networks are divided into two catagories, primary and secondary voltage protection. Primary voltage protection is usually provided by gas discharge tubes or car
46、bon block located at the point where the line enters the premises. Since the primary voltage protection only limits the voltage surges to 1000VPEAKand power line cross to 600V (RMS), secondary voltage protection is necessary. The secondary voltage protection provides additional voltage and current l
47、imiting so the network interface device is not damaged. Longitudinal (common mode) surge types are from Tip to Ground or Ring to Ground while Metallic (differential) surge types are between Tip and Ring. Longitudinal surges are formed on the Tip and Ring conductors by lightning currents that enter t
48、he conductive shield of the cable. Metallic surges are a byproduct of longitudinal Application Note 324: T1/E1 Network Interface Design 5 of 14 surges and are formed between the Tip and Ring conductors by imbalances in the the operation of the primary protectors or equipment on the line. The followi
49、ng secondary voltage protection examples provide immunity from metallic and longitudinal surges as well as for power-line cross. These circuit designs are targeted for compliance with one or more standard including but not limited to: Underwriters Laboratories UL 60950 (formerly UL 1950) Telecommunications Industry Association TIA/EIA-IS-968 (formerly FCC Part 68) Telcordia (formerly Bellcore) GR 1089-Core International Telecommunication Union ITU-T K.20, K.21 VOLTAGE SUPPRESSION PROTECTION CIRCUITS To meet the increasing demands placed on the telecommunications industry, traditional c
