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The basic settings of the ISDN Primary Rate interfaces (PRI) can be made here. The visibility of some settings depends on other settings.

  • NT Mode: The interface is operated in TE (Terminal Equipment) mode as standard. It behaves like a normal ISDN terminal and synchronises itself to the network clock (clock slave). A checked check box operates the interface in NT (Network Termination) mode. It behaves like an ISDN network termination (NTBA) and provides the clock pulse (clock master).
  • Clock Mode :
  • Derived from NT mode (Master/Slave configuration depends on setting of NT Mode checkbox : NT Mode => Master, otherwise Slave).
  • Slave (the device obtains its clock pulse from its peer device, a clock master).
  • Master (the device provides the clock pulse).
  • Swap tx/rx: Switches the interface of the transmit line with that of the receive line.
  • Do not use for synchronisation: Slave Mode interfaces are by default used to obtain the clock for the internal PCM bus. With this checkmark the interface can be excluded as clock source. This can be checked to control where the device gets its clock from. (For a direct cable from one PRI interface to another PRI interface of the same box, this checkmark has to be set on the slave interface for such a connection, because synchronization does not work).
  • µ-law: This check box must be checked if the device is located in a country that uses the ISDN µ-law standard. This includes the North America and Asian region, for example.
  • T1 : Switches the interface from the E1 European standard (2.048MBit/s or 30 ISDN B channels) to the T1 American standard (1.544 MBit/s or 24 ISDN B channels), which is used in the US, Canada and Japan.
  • CAS : Use CAS signaling instead of ISDN signaling. With the CAS method (Channel Associated Signalling), the signalling data is transferred in 4 bits (ABCD) instead of messages. On E1 lines CAS is signaled on channel 16 (D channel). On T1 lines the CAS information is send on the individual B-channel in the LSB every 6th frame. T1 in CAS mode has 24 voice channel ( instead of 23 voice channels with ISDN ).
 If you enable the CAS method, the content of chapter
 "Configuration/PRI1-n/Protocol" changes.
  • No CRC4 : A checked check box disables the Cyclical Redundancy Check. Select No CRC4 only in the rare cases the links supports no CRC.
  • Activate 'power-off' relay : Closes the relay of the power-off loop. This performs a physical connection of the RJ45 jack with the jack of the next interface. Checking this check box also puts the two corrosponding PRI interfaces into high impedance state, so that they appear as disconnected from their RJ45 jacks.
  • Loopback : A checked check box enables the loopback function. This is only necessary for test purposes. The loopback function simulates a connected device on the relevant interface.
  • Tx attenutation for T1 mode : Allows the transmit signal level for T1 mode to be decreased for short lines. '0db' selects the TIA968 option A output pulse, this is the "normal" pulse with full amplitude. '7.5db' and '15db' selects the attenuated TIA 968 Option B and C output pulses and is used if a legacy repeater is connected with a short cable. Only visible if T1 is checked. For details ask the operator or read the TIA968 or "O'Reilly T1. A Survival Guide" chapter6.
  • Send flags on FDL : A checked check box transmits FDL messages (Facility Data Link). Only visible if T1 is checked. The T1 link managment protocol AT&T TR 54016 ( also called Telemetry Asynchronous Block Serial Protocol or TABS ) needs flags on the FDL to work reliable.

Pinout of the PRI connector

RX and TX can be swapped for NT operation with software configuration so that no crossover cable is needed. Transmit and receive signal are for 120Ohm symetric interfaces. See also ISO/IEC 10173 and USOC RJ48-C.

TE Mode or ( NT Mode with 'Swap tx/rx' checked )

Pin Function
1 RX+
2 RX-
4 TX+
5 TX-


NT Mode or ( TE Mode with 'Swap tx/rx' checked )

Pin Function
1 TX+
2 TX-
4 RX+
5 RX-


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