THVD1510 Datasheet, PDF (19/39 Pages) Texas Instruments – 5-V RS-485 Transceivers With 18-kV IEC ESD Protection

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THVD1510, THVD1512
THVD1550, THVD1551, THVD1552
SLLSEV1 – SEPTEMBER 2017
Typical Application (continued)
10.2.1 Design Requirements
RS-485 is a robust electrical standard suitable for long-distance networking that may be used in a wide range of
applications with varying requirements, such as distance, data rate, and number of nodes.
10.2.1.1 Data Rate and Bus Length
There is an inverse relationship between data rate and cable length, which means the higher the data rate, the
shorter the cable length; and conversely, the lower the data rate, the longer the cable length. While most RS-485
systems use data rates between 10 kbps and 100 kbps, some applications require data rates up to 250 kbps at
distances of 4000 feet and longer. Longer distances are possible by allowing for small signal jitter of up to 5 or
10%.
10000
5%, 10%, and 20% Jitter
1000
100
Conservative
Characteristics
10
100
1k
10k 100k
1M
10M 100M
Data Rate (bps)
Figure 24. Cable Length vs Data Rate Characteristic
Even higher data rates are achievable (that is, 50 Mbps for the THVD1550, THVD1551 and THVD1552) in cases
where the interconnect is short enough (or has suitably low attenuation at signal frequencies) to not degrade the
data.
10.2.1.2 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 of varying phase 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)
10.2.1.3 Bus Loading
The RS-485 standard specifies that a compliant driver must be able to drive 32 unit loads (UL), where 1 unit load
represents a load impedance of approximately 12 kΩ. Because the THVD15xx family consists of 1/8 UL
transceivers, connecting up to 256 receivers to the bus is possible.
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Product Folder Links: THVD1510 THVD1512 THVD1550 THVD1551 THVD1552