August 22, 2019
| By: Michael Jackson
Principal Writer, Maxim Integrated
I’ve been asked to design the RS-485 interface for our latest industrial controller. I’ve not been given exact specs yet, but I’m told it needs to be “as fast as possible” over the “longest possible distance.” I’m a graduate just out of college, so I’m not overly familiar with the interface. Any chance you could answer a few questions for me, so I can tie down what these things really mean? Anon
Hey Anonymous Graduate,
Nobody expects to see a sprinter enter the Olympic marathon. Nor would you expect to see a long-distance runner enter the 100 meters. One of the great features of RS-485 is that it has always had the ability to do both. It has the flexibility to switch from being a “sprinter” (delivering higher speeds over short distances) and a “long-distance runner” (lower speeds over longer distances). However, it has never had the ability to do both, until now… but before I say anything else, I’ll do my best to answer your questions.
RS-485 is an old standard. Where is it still being used?
True, RS-485 has been around for over 20 years. RS-485 is commonly used for signal transmission between equipment in industrial environments.
Why has it survived so long?
Because of its differential nature, it has high noise immunity, which is great with all those noisy electric/magnetic fields in a harsh factory or other industrial setting (Figure 1).
Figure 1. Industrial control system
How fast is it?
The original standard was defined to operate at speeds up to 10Mbps and at distances up to 1.2km, but not at the same time. The maximum data rate could only be achieved over relatively short distances (10m to 15m).
What’s the speed/distance trade-off?
A rule of thumb calculation is that the product of the data rate, in bits-per-second second, and distance, in meters, should not exceed 1 × 108.
But I’ve seen RS-485 transceivers that work much faster than 10Mbps?
True. Designers have pushed the standard way beyond the original specification and, while it’s not uncommon to see RS-485 transceivers working up to 40Mbps, this sort of speed can only be achieved over a few tens of meters.
So, what’s the problem?
As factories get bigger, cable runs get longer (100m or more). As we have seen for RS-485, longer distances have meant lower data rates, which nobody is prepared to tolerate any longer.
So, is this the end for RS-485?
Not at all. A new RS-485 transceiver IC, the MAX22500E (Figure 2), has quite literally changed the accepted status quo of what has been achievable with RS-485 by at least an order of magnitude. It can achieve an unprecedented data rate of 100Mbps over 10 meters of Cat5e cable (and potentially over even longer distances depending on the choice of cable selected).
Figure 2. Functional diagram of MAX22500E
While this level of speed is impressive by itself, the MAX22500E can comfortably operate at speeds up to 50Mbps over 100m of cable as you can see from the eye diagram in Figure 3.
Figure 3. MAX22500E operating at 50Mbps over 100m Cat5e
How can it do this?
It uses a clever technique called pre-emphasis that can be enabled or disabled (as required) depending on the cable length.
What else can it do?
This IC includes integrated protection to increase its immunity to ESD. This includes a -15V to +15V common-mode range, ±15kV ESD protection (human body model), ±7kV IEC 61000-4-2 air-gap ESD protection, ±6kV IEC 61000-4-2 contact discharge ESD protection, and short-circuit protection of the driver outputs. It operates over a 3V to 5.5V supply range to maintain compatibility with legacy systems. It also operates with a low-voltage logic supply down to 1.6V for easy interfacing to low-voltage microcontrollers without the need for voltage translation.
That’s great. Anything else?
Yes. A variation of the MAX22500E that comes without pre-emphasis is called the MAX22501E (for applications that do not have the same distance requirements). Another family member, the MAX22502E, provides full-duplex data transmission, if required.
Looks like RS-485 is going to be around for some time to come?
As they say, if it ain’t broke, don’t fix it!