Recently, a designer asked, "Do you have an app note that explains why 3.3V and 5V 485 chips can operate on the same system?"
Here is a short explanation of why you can mix and match RS-485 devices that use different supplies.
The RS-485 standard (TIA-485-A) specifies the voltages and currents needed on the differential bus lines to ensure any compliant transceivers can communicate with each other. The drivers must be capable of sourcing a 1.5V signal (either A > B +1.5V, or B > A +1.5V) into a standard load, and the receivers must be able to discriminate a differential signal as small as 200 mV (+ or -). All of the original RS-485 transceivers used 5V supplies, which then allows for 3.5V of “headroom” for the driver circuits. Newer designs for transceivers which operate with 3.3V supplies require more efficient driver stages, in order to develop 1.5V differential output signals. On the receiver side, the allowable common-mode voltage range is from -7V to +12V according to the standard, which comes from adding 7V on either side of the original 0V and 5V power rails. In order to discriminate a 200 mV differential signal on top of a 12V offset, the differential receivers first attenuate the input signals in order to bring the range within the linear region of the input stages. For example, with 5V rails, a 4:1 attenuation through resistor dividers would “shrink” the 12V common-mode offset to 3V, which should be within the linear range of the input stage. At the same time, the differential signal is also attenuated, so internally the receiver’s comparator stage would need a sensitivity of 50 mV in order to detect an external differential signal of 200 mV. For devices with 3.3V supply, the attenuation must be slightly higher, for example a 6:1 attenuation would reduce a 12V common-mode offset to 2V; this will also require more sensitivity (about 33 mV) at the receiver comparator. These differences between devices with 5V supply and 3.3V supply account for the typically higher price for 3.3V transceivers compared to 5V transceivers; the output stages require larger transistors and the receiver stages require more accurate and sensitive circuit designs. Texas Instruments used a variety of advanced process technologies and innovative circuit designs to develop RS-485 transceivers that fully comply with the RS-485 standard, whether the supply voltage is 3.3V or 5V.
