Datasheet

Product data sheet Rev. 2 — 6 May 2011 22 of 32

NXP Semiconductors

PCA9601

Dual bidirectional bus buffer

Question: We have 2 meters of cable in a bus that joins the TX/RX sides of two

PCA9601 devices. When one TX drives LOW the other PCA9601 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 PCA9601s 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 25

, the transient

undershoot will be limited, by the diodes inside PCA9601, to around −0.8 V and that will

not cause problems for PCA9601. 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 PCA9601 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 25. Wiring transients limited by the diodes in PCA9601

002aaf079

time (ns)

voltage

(V)

−2

horizontal scale = 62.5 ns/div

send

receive

5 V

PCA9601

send

300 Ω

5 V

2 meter

cable

5 V

300 Ω

GND

receive

5 V