One network protocol that has withstood the test of time in harsh, noisy environments is RS-485, and recent advances in signal conditioning and processing will ensure that it continues to do so

BY MICHAEL JACKSON, Analog IC Design Engineer, and SEAN LONG, Director,
Applications, Industrial & Healthcare Business Unit
Maxim Integrated
www.maximintegrated.com

As modern industrial environments become ever larger
and more complex, production engineers are challenged in the design and layout
of automated industrial process control systems, particularly regarding communications
network reliability, range, and data rates. However, one network protocol that
has withstood the test of time in harsh, noisy environments is RS-485, and
recent advances in signal conditioning and processing will ensure that it
continues to do so.

This article will briefly review the original RS-485
specification and present its relative strengths and weaknesses. It will then
show how its capabilities are being extended to meet the demands of modern
industrial network protocols.

The evolution
of RS-485
For over 20 years, common network protocols such as
PROFIBUS have used the RS-485 standard for transmission of electrical signals
across physical media in harsh, electrically noisy, industrial environments (Fig. 1).

Fig. 1:
Industrial environments are increasingly complex yet remain harsh and
electrically noisy, challenging designers to get more out of their network
interfaces.

However, as these network protocols have developed and
evolved, the original RS-485 specification is being pushed to its limits (and
beyond).

The original ANSI/TIA/EIA-485-A-1998 standard, now
commonly known as RS-485, was approved in March 1998. RS-485 is a
bidirectional, half-duplex standard featuring multiple “bussed” drivers and
receivers in which each driver can relinquish the bus. RS-485 is an
electrical-only, physical-layer (PHY) standard that relies upon differential
signaling for noise immunity, typically on a balanced transmission cable, such
as unshielded twisted pair (UTP).

With a –7-V to 12-V bus common-mode range, receiver
input sensitivity is ±200 mV, which means that to recognize a “mark” (logical
1) or “space” (logical 0), a receiver must see signal levels above 200 mV or
below –200 mV. Maximum receiver input impedance is 12 kΩ, and the driver output
voltage is ±1.5 V minimum, ±5 V maximum. Fig. 2 lists the key
specifications in the original RS-485 standard.

0518_Feature_Networking_Fig-2

Fig. 2: The original RS-485 specification had a maximum data rate of 10
Mbits/s, achievable over short distances of 10 m to 15 m. Modern
implementations achieve up to 40 Mbits/s over a few meters.

RS-485 advantages and disadvantages
RS-485 is commonly used for signal transmission
between equipment in industrial environments because of its differential
nature, which ensures high noise immunity in the harsh and noisy
electric/magnetic fields typical of a factory setting. However, while the
original standard was defined to operate at speeds up to 10 Mbits/s and at
distances up to 1.2 km, these could not be simultaneously achieved. The maximum
data rate is achievable over relatively short distances (10 m to 15 m). A rule
of thumb to estimate the trade-off between the data rate and distance is that
the product of the data rate, in bits per second, and
distance, in meters, should not exceed 1 × 108.

The requirement of industrial network protocols to
operate at increasing data rates means that designers of modern RS-485
transceivers have pushed the original specification well beyond its original
limit. It is not uncommon for modern transceivers to work at speeds that are
multiples of the original specification — for example, 30 Mbits/s to 40 Mbits/s.
These data rates are only achievable over relatively short distances (on the
order of a few meters). However, the scale and complexity of modern process
control systems requires signals to travel at higher speeds over medium
distances (up to 100 m). To achieve this, the implementation of RS-485 needed
to be re-examined.

Higher speeds over longer distances
Engineers noted that inter-symbol interference places the main limit on the maximum distance for RS-485 communications
and that pre-emphasis can very effectively reduce this interference. Pre-emphasis
is applied at the source of a transmitted signal, before the electrical
channel, and improves the signal quality at the destination. The technique is
common. In IC-layout programs, for instance, pre-emphasis adjusts the line
widths to compensate for etch-rate variations that occur around corners. In
disk-drive controllers, pre-emphasis compensates for poor frequency response
near the center of the disk. Another type of pre-emphasis reduces tape hiss by
boosting low-level signals in the middle- and high-frequency audio bands when
recording and then reversing this process during playback. 

Based on this finding, engineers developed RS-485 transceivers that
incorporate internal pre-emphasis to further extend the capabilities of modern
devices beyond their already advanced performance and far beyond the original
specification. For example, a modern device is shown to be able to transmit
over 100 Mbits/s over 10 m (Fig. 3).

0518_Feature_Networking_Fig-3

Fig. 3: A
modern RS-485 transceiver can achieve 100 Mbits/s over 10 m using Cat5e UTP
cable with a good eye opening. (Scale of 2.5 ns/div)

As illustrated by the differential voltage levels of
approximately ±2 V (measuring well above the minimum ±200 mV specified in the
standard) in the eye diagram illustrated (Fig. 3), this device
achieves a data rate of 100 Mbits/s over 10 m of Cat5e cable (and potentially
over even longer distances depending on the choice of cable selected).

It’s also possible to get relatively high data
transmission rates over longer factory network cable runs: up to 50 Mbits/s
over 100 m of cable. Although not quite as distinct, the opening of the eye at
this speed (approximately ±0.7 V) is still greater than the minimum voltage
levels defined by the original RS-485 standard (Fig. 4).

0518_Feature_Networking_Fig-4

Fig. 4: Over
longer factory cable runs of up to 100 m, RS-485 can achieve 50 Mbits/s, again
using Cat5e, though the eye opening may not be as distinct. (Scale at 5ns/div)

Using pre-emphasis, the performance of the transceiver
— in this case, the Maxim Integrated MAX22500E — can be improved further over
these longer distances (Fig. 5).

0518_Feature_Networking_Fig-5

Fig. 5:Using
pre-emphasis, the eye opening improves dramatically when operating at 50
Mbits/s over 100 m, going to ±1 V from ±0.7 V. (Scale at 5ns/div)

The speed and distance are the same as for Fig. 4, but enabling the pre-emphasis feature increases the
voltage levels so that the opening of the eye in the diagram is visibly more
distinct.

This ability to enable pre-emphasis for data
transmission over longer distances provides equipment designers with an extra
degree of flexibility in achieving their desired level of speed-versus-distance
performance using the RS-485 interface.

Other features to expect from modern RS-485 devices
such as the MAX22500E include a –15-V to 15-V common-mode range and integrated ESD
protection to ±15-kV ESD protection (Human Body Model), ±7-kV IEC 61000-4-2
Air-Gap ESD protection, ±6-kV IEC 61000-4-2 contact discharge ESD protection,
and short-circuit protection of the driver outputs.

Conclusion
Although RS-485 is robust and reliable, there is
a significant trade-off to be made between the speed and distances attainable.
Modern transceivers use pre-emphasis and other techniques to provide reliable
transmission of data up to 100 Mbits/s over 10 m of cable and up to 50 Mbits/s
over 100 m in either full- or half-duplex mode. This means that designers of
industrial automation and control equipment can confidently plan for the
continued use of RS-485 as a communications interface in complex industrial
environments for the foreseeable future.