R

A

B

R

RE

DE

D

D

LS

10000

1000

100

10

100

1k

10k

100k

1M

10M

100M

DATA RATE - bps

Conservative

Characteristics

5,10,20 % Jitter

SN888C

SLLSEI4 – SEPTEMBER 2013

www.ti.com

Cable Length Versus Data Rate

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, applications such as e-metering often operate at rates of

up to 250 kbps even at distances of 4000 ft and longer. Longer distances are possible by allowing for small

signal jitter of up to 5 or 10%.

CABLE LENGTH (FT)

vs

DATA RATE (BPS)

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 the short distance is because a stub presents a non-

terminated piece of bus line, which can introduce reflections if the distance is too long. 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

leading to a maximum physical stub length as shown in Equation 1.

LStub ≤ 0.1 × tr × v × c

where

•

tr is the 10 / 90 rise time of the driver

•

c is the speed of light (3 × 10

8 m/s or 9.8 × 108 ft/s)

•

v is the signal velocity of the cable (v = 78%) or trace (v = 45%) as a factor of c

(1)

Based on Equation 1, with a minimum rise time of 400 ns, Equation 2 shows the maximum cable-stub length of

the SN888C device.

LStub ≤ 0.1 × 400 × 10

-9 × 3 × 108 × 0.78 = 9,4 m (or 30.6 ft)

(2)

Figure 14. Stub Length

12

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