To add isolation to the RS-232 and RS-485 connection to a PC, the isolated RS-232 and isolated RS-485 portion of this reference design uses capacitive galvanic isolation, which has an inherent life span advantage over an opto-isolator. In particular, industrial devices are usually pressed into service for much longer periods of time than consumer electronics; therefore, maintenance of effective isolation over a period of 15 years or longer is important.

The variant of the ISO7731B device used in the RS-232 and RS-485 circuitry of this reference design provides galvanic isolation up to 5 kV_RMS for one minute per UL. This digital isolator has three isolated channels where two channels are forward channels and the other is a reverse channel. In this design, two isolation channels are used for the TX and RX. If RS-485 is selected for communication, the third isolation channel is used for the control signal used to enable the receiver or driver. If RS-232 is selected, the third isolation channel is not needed. If RS-232 is desired in a customer’s system instead of RS-485, only two isolation channels are needed, so a two-channel ISO7721B device could be used to reduce cost instead of using the three-channel ISO7731B device (keep in mind that these two devices are not pin-to-pin compatible). Each isolation channel has a logic input and output buffer separated by a double capacitive silicon dioxide (SiO2 ) insulation barrier. This chip supports a signaling rate of 100 Mbps. The chips can operate from a 2.5 V, 3.3-V, and 5-V supply and logic levels.