
DS26303: 3.3V, T1/E1/J1, Short-Haul, Octal Line Interface Unit 
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6.3.3 Dual-Rail Mode 
Dual-rail mode consists of TPOS, TNEG, and TCLK pins on the system side. NRZ data is sampled on the falling 
edge of TCLK as shown in 
Figure 10-12
. The zero substitution B8ZS or HDB3 is not allowed. The TPOS/TNEG 
data is encoded in AMI format on the TTIP and TRING pins. The data that appears on the TPOS pin is output on 
TTIP and data on the TRING is output on TRING after pulse shaping. The single-rail-select register (
SRMS) 
is 
used for selection of dual-rail or single-rail mode. The data that arrives at the TPOS and TNEG can be overwritten 
in the maintenance mode by setting the BERT Control Register (
BTCR
). 
6.3.4 Single-Rail Mode 
Single-rail mode consists of TPOS, TNEG, and TCLK pins on the system side. NRZ data is sampled on the falling 
edge of TCLK as shown in 
Figure 10-12
. The zero substitution B8ZS or HDB3 is allowed. The TPOS data is 
encoded in AMI format on the TTIP and TRING pins after pulse shaping. The single-rail-mode select (
SRMS)
 is 
used for selection of dual-rail or single-rail mode. The data that arrives at the TPOS can be overwritten in the 
maintenance mode by setting in BERT control register (
BTCR
). 
6.3.5 Zero Suppression—B8ZS or HDB3 
B8ZS coding is available when the device is in T1 mode selected by the TS2, TS1, and TS0 bits in the 
TS
 register. 
Setting the LCS bit in the 
LCS
 register enables B8ZS. Note that if the individual LIU is configured in E1 mode, then 
HDB3 code substitution can be selected. Bipolar violations can be inserted via the TNEG/BPVI pin or transmit 
maintenance register settings only if B8ZS or HDB3 coding is turned off. B8ZS substitution is defined in ANSI 
T1.102 and HDB3 in ITUT G.703 standards. 
6.3.6 Transmit Power-Down 
The transmitter is powered down if the relevant bits in the 
TPDE 
register are set. 
6.3.7 Transmit All Ones 
When transmit all ones is invoked, continuous 1s are transmitted using MCLK as the timing reference. Data input at 
TPOS and TNEG is ignored. Transmit all ones can be sent by setting bits in the 
TAOE
 register. Transmit all ones 
are enabled if bits in register 
ATAOS
 are set and the corresponding receiver goes into an LOS state in the status 
register 
LOSS.
6.3.8 Drive Failure Monitor 
The Driver Fail Monitor is connected to the TTIP and TRING pins. It will detect a Short Circuit on the Secondary 
side of the Transmit Transformer. The drive current will be limited to 50 ma if a short circuit is detected. The 
DFMS
status registers and the corresponding Interrupt and Enable Registers can be used to monitor the driver failure. 
6.4 Receiver 
The DS26303’s eight receivers are all identical. A 2:1 transformer steps down the input from the line. The DS26303 
is designed to be fully software-selectable for E1 and T1/J1 without the need to change any external resistors for 
the receive side. The output of the internal termination circuitry is fed into a peak detector. 
The peak detector and data slicer process the received signal. The output of the data slicer goes to clock and data 
recovery. A 2.048/1.544 PLL is internally multiplied by 8 by another internal PLL and fed to the clock recovery 
system derives E1 or T1 clock. The clock-recovery system uses the clock from the PLL circuit to form an 8-times 
oversampler, which is used to recover the clock and data. This oversampling technique offers outstanding 
performance to meet jitter tolerance specifications. Depending on selection options, B8ZS/HDB3/AMI decoding is 
performed. These decoded data is provided to the system side in either single-rail or dual-rail mode. The selection 
of single rail or dual rail is done by settings in the 
SRMS 
register. 
6.4.1 Peak Detector and Slicer 
The slicer determines the polarity and presence of the received data. The output of the slicer is sent to the clock 
and data recovery circuitry for extraction of data and clock. The slicer has a built-in peak detector for determination 
of the slicing threshold.