Datasheet
R
A B
R RE DE D
D
L
S
10000
1000
100
10
CABLE LENGTH - ft
100 1k 10k 100k 1M 10M 100M
DATA RATE - bps
Conservative
Characteristics
5,10,20 % Jitter
SN65HVD82
SLLSED6 –OCTOBER 2012
www.ti.com
The theoretical maximum bus length is assumed with 4000 ft or 1200 m, and represents the length of an AWG
24 cable whose cable resistance approaches the value of the termination resistance, thus reducing the bus
signal by half or 6 dB.
The theoretical maximum number of bus nodes is determined by the ratio of the RS-485 specified maximum of
32 unit loads (UL) and the actual unit load of the applied transceiver. For example, the SN65HVD82 is a 1/8 UL
transceiver. Dividing 32 UL by 1/8 UL yields 256 transceivers that can be connected to one bus.
CABLE-LENGTH VERSUS DATA RATE
There is an inverse relationship between data rate and cable length. That is, the higher the data rate the shorter
the cable and conversely the lower the data rate the longer the cable. While most RS-485 systems utilize data
rates between 10 kbps and 100 kbps, applications such as e-metering often operate at rates of up to 250 kbps
even at distances of 4000 ft and above. This is possible by allowing for small signal jitter of up to 5 or 10%.
Figure 15. Cable Length vs Data Rate Characteristic
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. The reason for this is that a stub presents a non-terminated piece of
bus line which can introduce reflections if too long. As a rule of thumb the electrical length or round-trip delay of a
stub should be less than one tenth of the driver’s rise time, thus leading to a maximum physical stub length of:
L
Stub
≤ 0.1 × t
r
× v × c, with t
r
as the driver’s 10/90 rise time, c as the speed of light (3 × 10
8
m/s or 9.8 × 10
8
ft/s),
and v as the signal velocity of the cable (v = 78%) or trace (v = 45%) as a factor of c.
Thus, for the SN65HVD82 with a minimum rise time of 400 ns the maximum cable stub length yields L
Stub
≤ 0.1 ×
400 × 10
-9
× 3 10
8
× 0.78 = 9.4 m or 30.6 ft.
Figure 16. Stub Length
14 Submit Documentation Feedback Copyright © 2012, Texas Instruments Incorporated
Product Folder Links: SN65HVD82