⚫ Because the device driving a communication bus does not exist in the state of no communication,
a noise may lead unstable signal which causes communication failure such as a flaming error.
When the communication failure is occurred, connect one external pull up and pull down
resistance to the PC side to provide a FAIL-SAFE bias. (refer to Figure 42)
The FAILSAFE bias (Vfsb) must be greater than 200 mV to be in a guaranteed state.
The pull up resistance (Ra) and the pull down resistance (Rd) should be of equal value.
Figure 42 - FAILSAFE bias resistance
[Example computation]
The FAIL-SAFE bias can be expressed with the pull up resistance (Ra), the pull down
resistance (Rd), and the equivalent resistance of termination (Rb//Rc) on the simply voltage
divider as below.
Vfsb = Vcc x ( Rb//Rc / (Ra ＋ Rb//Rc ＋ Rd) )
When the impedance characteristic of a cable (Zo) is 120 Ω and the termination resistor Rb and
Rc match to Zo 120 Ω as well, the equivalent resistance of the termination resistor becomes 60
Ω (= 120 Ω // 120 Ω).
In case of Vcc = 5V and FAILSAFE bias Vfsb > 0.2V,
Ra＋Rd < (Rb//Rc x Vcc / Vfsb) - Rb//Rc = (60Ω x 5V / 0.2V) - 60Ω = 1440Ω
The pull up resistance (Ra) and the pull down resistance (Rd) are the same values, it is
calculated as follows.
Ra = Rd < 1440Ω/2 = 720 Ω
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