33
8575
8575
N/B Maintenance
N/B Maintenance
The TSB41AB1 provides a 1.86-V nominal bias voltage at the TPBIAS terminal for port termination. This bias
voltage, when seen through a cable by a remote receiver, indicates the presence of an active connection. This
bias voltage source must be stabilized by an external filter capacitor of 1µF. TPBIAS is typically V
DD
0.2 V
when the port is not connected to another node.
The line drivers in the TSB41AB1 operate in a high-impedance current mode, and are designed to work with
external 112Ω. line-termination resistor networks in order to match the 110Ω. cable impedance. One network
is provided at each end of a twisted-pair cable. Each network is composed of a pair of series-connected 56Ω.
resistors. The midpoint of the pair of resistors that is directly connected to the twisted-pair-A terminals is
connected to its corresponding TPBIAS voltage terminal. The midpoint of the pair of resistors that is directly
connected to the twisted-pair-B terminals is coupled to ground through a parallel R-C network with
recommended values of 5kΩ and 220pF. The values of the external line termination resistors are designed to
meet IEEE Std 1394-1995 when connected in parallel with the internal receiver circuits. An external resistor
connected between the R0 and R1 terminals sets the driver output current, along with other internal operating
currents. This current-setting resistor has a value of 6.34kΩ± 1.0%.
When the power supply of the TSB41AB1 is off while the twisted-pair cables are connected, the TSB41AB1
transmitter and receiver circuitry presents a high impedance to the cable and does not load the TPBIAS voltage
at the other end of the cable. Fail-safe circuitry blocks any leakage path from the port back to the device power
plane.
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