Can We Find Common Ground on Common Mode Range?
January 21, 2021
| By: Robert Gee
Executive Business Manager, Core Products Group, Maxim Integrated
| and Alec Makdessian
Executive Director, Core Products Group, Maxim Integrated
Common mode voltage—it can be a scourge for serial communication systems such as RS-485 or even CAN communications in noisy industrial systems. High common mode voltage can be due to an electrically noisy environment or nearby equipment, such as an AC motor, transformers, or power cables. Or, it could stem from the voltage differences in ground planes; for example, two nodes or systems that operate from different transformers or ground levels. These voltage differences occur because cabling can be hundreds of feet long, especially in RS-485. For example, a 1000ft long 24 AWG cable (25.67 Ohms per 1000ft) carrying 450mA of current can cause a difference in ground potential of 11.6V. It's easy to see that in a large factory or building, such voltage differences can be quite common. So whether the high common mode voltage is due to noise or ground level voltage differences, it can hamper reliable communications. In an application area like an automated factory, such errors can mean lost uptime and productivity.
It's a pain to have to figure out why control, feedback, and other types of messages aren't getting through between equipment that relies on open lines of communication. In some designs, you may need to install a conduit to completely shield it from the external noise source. Obviously, this is costly. You can also use a twisted shielded pair cable and carefully lay it out to keep distance from electrical noise sources, significantly lengthening the cable run—another expensive and time-consuming approach. Imagine if you had 20 motors to avoid along your path!
In serial communications systems, the RS-485 protocol defines common mode range (CMR) for the receiver side (-7V to +12V). This is the voltage range that allows messages to be received. But the RS-485 standard doesn't have such a definition for the driver side. This means that messages can't be sent if the common voltage level in the system goes above the transceiver's threshold.
What if we had a larger ground potential difference than the 11.6V in the example above? Most standard RS-485 transceivers would fail to communicate. The larger the CMR the transceiver can accommodate, the more reliable the system becomes. Thus, it behooves the designer to select components with the widest CMR
Aside from high common mode voltage, there are other communications challenges to overcome for RS-485 communications systems:
- Damage to the transceiver's data lines due to accidental wiring to local power supplies
- Electrical overstress damage to data lines during production, field installation, or maintenance work
- The space and increased costs required by external protection circuitry
- A system design that is transitioning to lower supply rail voltages (from 5V to 3.3V, for example)
RS-485 Transceiver with Industry's Highest CMR
A new RS-485 transceiver is now available to address key communication system design challenges. The MAX33072E half-duplex RS-485 transceiver provides the industry's highest CMR (+/-40V) on both the driver and receiver sides, enabling two-way communication in the presence of large ground potential between nodes. The device helps overcome other communications challenges by providing:
- Protection from accidental shorts to local power supplies with +/-65V fault protection
- Safeguarding against inadvertent electrostatic discharge with +/-40kV electrostatic discharge (human body model) protection
- Integration of protection circuit to save PCB area, cost, and design time
Wide supply range (3V to 5.5V) to support legacy and newer designs
What about CAN?
Even though cable length in CAN systems is much shorter than RS-485 systems, the CAN bus can suffer the same communication difficulties due to a mismatch in potential between transmitter and receiver. Many CAN transceivers have historically provided a split termination node where the common-mode voltage of the bus is forced to Vcc/2 by biasing this split termination node.
In newer devices, such as the newly introduced MAX33053E and MAX33054E family, the split termination pin is no longer needed as these devices have "built-in" an input common-mode range greater than +/-25V, exceeding the ISO11898 specification of -2V to +7V. In addition, these devices provide a high level of +/- 65V of fault protection and +/-25KV of ESD protection, making these devices ideal for harsh industrial environments
Both RS-485 and CAN remain a popular standard for signal transmission between equipment in industrial environments. They can deliver high speeds over short distances or lower speeds over longer distances. Electrically noisy environments can stress these communication systems in applications like programmable logic controllers (PLCs), building automation equipment, motor drivers, process automation equipment and the like. For these types of electrically noisy applications, choosing the most appropriate transceiver like the MAX33072E (for RS485) and the MAX33053E/54E can minimize communications errors or even device failures, keeping equipment running for increased uptime.