THVD1500 Datasheet, PDF (16/29 Pages) Texas Instruments – 300 kbps RS-485 Transceivers With 8-kV IEC ESD Protection

THVD1500
SLLSEY4 – JULY 2017
www.ti.com
Typical Application (continued)
9.2.1.2 Stub Length
When connecting a node to the bus, the distance between the transceiver inputs and the cable trunk, known as
the stub, should be as short as possible. Stubs present a non-terminated piece of bus line which can introduce
reflections as the length of the stub increases. As a general guideline, the electrical length, or round-trip delay, of
a stub should be less than one-tenth of the rise time of the driver, thus giving a maximum physical stub length as
shown in Equation 1.
L_(STUB) ≤ 0.1 × t_r × v × c
where
• t_r is the 10/90 rise time of the driver
• c is the speed of light (3 × 10⁸ m/s)
• v is the signal velocity of the cable or trace as a factor of c
(1)
9.2.1.3 Bus Loading
The RS-485 standard specifies that a compliant driver must be able to driver 32 unit loads (UL), where 1 unit
load represents a load impedance of approximately 12 kΩ. Because the THVD1500 consists of 1/8 UL
transceivers, connecting up to 256 receivers to the bus is possible.
9.2.1.4 Receiver Failsafe
The differential receivers of the THVD1500 are failsafe to invalid bus states caused by the following:
• Open bus conditions, such as a disconnected connector
• Shorted bus conditions, such as cable damage shorting the twisted-pair together
• Idle bus conditions that occur when no driver on the bus is actively driving
In any of these cases, the differential receiver will output a failsafe logic high state so that the output of the
receiver is not indeterminate.
Receiver failsafe is accomplished by offsetting the receiver thresholds such that the input indeterminate range
does not include zero volts differential. In order to comply with the RS-422 and RS-485 standards, the receiver
output must output a high when the differential input V_ID is more positive than 200 mV, and must output a low
when V_ID is more negative than –200 mV. The receiver parameters which determine the failsafe performance are
V_IT+, V_IT–, and V_HYS (the separation between V_IT+ and V_IT–). As shown in the Electrical Characteristics table,
differential signals more negative than –200 mV will always cause a low receiver output, and differential signals
more positive than 200 mV will always cause a high receiver output.
When the differential input signal is close to zero, it is still above the V_IT+ threshold, and the receiver output will
be high. Only when the differential input is more than V_HYS below V_IT+ will the receiver output transition to a low
state. Therefore, the noise immunity of the receiver inputs during a bus fault conditions includes the receiver
hysteresis value, V_HYS, as well as the value of V_IT+.
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