Datasheet

P82B96_8 © NXP B.V. 2009. All rights reserved.
Product data sheet Rev. 08 — 10 November 2009 21 of 32
NXP Semiconductors
P82B96
Dual bidirectional bus buffer
Question: We have 2 meters of cable in a bus that joins the Tx/Rx sides of two P82B96
devices. When one Tx drives LOW the other P82B96 Tx/Rx is driven to −0.8 V for over
50 ns. What is the expected value and the theoretically allowed value of undershoot?
Answer: Because the cable joining the two P82B96s is a ‘transmission line’ that will
have a characteristic impedance around 100 Ω and it will be terminated by pull-up
resistors that are larger than that characteristic impedance there will always be negative
undershoot generated. The duration of the undershoot is a function of the cable length
and the input impedance of the connected IC. As shown in Figure 23, the transient
undershoot will be limited, by the diodes inside P82B96, to around −0.8 V and that will
not cause problems for P82B96. Those transients will not be passed inside the IC to the
Sx/Sy side of the IC.
Question: If we input 800 mV undershoot at Tx, Rx pins, what kind of problem is
expected?
Answer: When that undershoot is generated by another P82B96 and is simply the
result of the system wiring, then there will be no problems.
Question: Will we have any functional problem or reliability problem?
Answer: No.
Fig 23. Wiring transients limited by the diodes in P82B96
002aaf065
time (ns)
2
0
4
6
voltage
(V)
−2
horizontal scale = 62.5 ns/div
send
receive
5 V
Rx
Tx
Sx
P82B96
send
300 Ω
5 V
2 meter
cable
5 V
300 Ω
GND
receive
Sx
Rx
Tx
5 V