A unit load is a measure of the load that a receiver, driver, or transceiver presents to the bus. A 1-unit load RS-485 IC has about a 12 kΩ input resistance, while a 1-unit load RS-422 device has a 4 kΩ input resistance. There can be 32 unit loads on an RS-485 bus, and ten unit loads on an RS-422 bus. Fractional unit load devices present a load to the bus that is a fraction of that on a 1-unit load device. Typical RS-485 fractional devices are one quarter (1/4) or one eighth (1/8) unit loads with input resistances of 48 kΩ and 96 kΩ respectively. Using fractional unit load devices allows four or eight times more devices on the bus without violating the RS-485 loading limitation. Therefore a device advertising 256 devices allowed on the bus is an 1/8-unit load IC. Note that a fractional unit load device can almost always be substituted for a 1-unit load device. Some ICs have a low power shutdown function that reduces the supply current to low µA or even sub-µA values. This typically occurs when both the transmitter and receiver have been disabled for 600 ns or more. ICs with a hot plug function have circuitry that keeps the driver and receiver outputs tri-stated while Vcc powers up or down, regardless of the state of the enable control lines. Some customers and competitors call this “glitch-free” power up. This feature ensures that drivers connected to a bus do not crash the bus during power cycling, and also prevents the receiver from generating erroneous start bits. IEC61000-4-2 is the European ESD standard that is much more critical than the human body model (HBM) that is commonly referenced in the United States. IEC61000 levels are typically specified for only the bus pins, as these are the pins that are exposed to contact via their presence in a connector. The most robust interface devices are specified for at least ±15kV to the air gap method, and for at least ±8kV to the contact method. Overvoltage protected transceivers (also called OVP or fault protected) have transmitter outputs and receiver inputs that are designed to withstand voltages much greater than the required -7V to +12V. Renesas has OVP devices that withstand voltages up to ±60V. This feature is especially useful when the DC power supply wire must be routed in the same conduit as the data cable, and an OVP device often eliminates the need for TVS protection devices. Renesas OVP RS-485 devices also operate over much wider common mode ranges than standard transceivers with ranges specified from ±15V to ±25V. All of the previously mentioned features are available on Renesas transceivers. One feature we do not offer is galvanic isolation, where the logic side is isolated from the RS-485 side of the IC. Isolation prevents high voltages on the RS-485 bus from blowing through to the logic side and damaging an expensive microcontroller. NVE is a provider of isolated RS-485 products that uses Renesas RS-485 die in their products. If the designer needs an isolated RS-485 transceiver, they are recommended to visit NVE.com.