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Bridge gap between USB and traditional interfaces

Posted: 01 Oct 2015 ?? ?Print Version ?Bookmark and Share

Keywords:serial communication? USB? RS-232? interface? Digital isolators?

As a standard for serial communication to peripheral devices, USB is ubiquitous today. However, industrial devices still rely in many cases on the older standard of RS-232 for debug or master-slave connections. In order to add robustness through differential communication, RS-422 or RS-485 are also widely used, and RS-485 remains the physical layer standard used by various wired multi-node industrial network or Fieldbus protocols. As a result, there can be a mismatch between the available interfaces on tools used for debugging or monitoring, such as laptop computers that only have USB for serial communications, and industrial equipment, which may only expose RS-232, RS-422 or RS-485 interfaces.

In addition to the hardware interfaces required for any USB to UART translation, galvanic isolation is a functional and potentially safety requirement for any interface circuit between test or monitoring equipment, and industrial equipment that is potentially in active use, in an electrically harsh environment. Combinations of interface, iCoupler and isoPower isolation technology from Analog Devices, integrated together in a variety of ICs, allow easy solutions for bridging the gap between USB and traditional industrial interfaces.

Why use galvanic isolation?
When interfacing with industrial equipment, it is of paramount importance to avoid interfering with the operation of those systems. Galvanic isolation prevents any flow of current while allowing data transfer, protecting both the unit under test and the debug unit. When connecting debug consoles (for example, laptop computers), several hazards are apparent, as shown in figure 1.

???If the debug equipment is grounded, differences in ground potentials can result in loop currents
???If the debug equipment is not grounded, static can discharge into the units under test
???Hazardous voltages can occur on Fieldbus networks due to switching noise from motors and relays
???Some basic diagnostic equipment (e.g. a laptop) may not have the same rigorous levels of interface protection as industrial equipment.

Figure 1: Potential electrical hazards.

In a USB to UART interface, there are two possibilities, either the USB interface can be isolated, or the UART interfaces (RS-232/RS-485/RS-422) can be isolated. In theory, both could be isolated in order to protect the interface module itself from either equipment, or to provide safety isolation from two basic isolation interfaces in series.

Isolating using digital isolators
Digital isolators using iCoupler technology offer robustness for galvanic isolation, with lower power requirements than opto-couplers and enhanced reliability. Other IC functionality (e.g. transceivers) can be integrated into single-chip solutions requiring little extra PCB area compared to non-isolated solutions. This is a particular advantage for isolating RS-232 or RS-485 interfaces where an IC is already required to transmit and receive at the appropriate voltages.

For USB, two signals, D+ and D-, need to be isolated transparently in order not to interfere with USB handshaking or require custom software drivers. Using a digital isolator for USB such as ADuM3160 achieves this in a wide-body SOIC footprint. Power is available on one side of the isolator, across the USB cable, and so a similar power supply should be provided on the isolated side as well. Using isoPower, a similar chip-scale transformer technology for power isolation, such a power supply can also be realised in a wide-body SOIC footprint, using the ADuM5000. If additional levels of

power are required, an external transformer and rectifier can be used, requiring a transformer driver and a secondary regulator C which are combined in the ADuM3070.

Figure 2: ADM3252E block diagram.

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