2 Traditional host and peripheral implementations

Figure 2-1 shows the traditional implementation of isolated host and peripheral ports in industrial equipment. This implementation uses a USB isolator that has fixed upstream- and downstream-facing definitions. The downstream-facing (host) port powers a 5-V supply to the VBUS and includes 15-kΩ pulldown resistors, as specified by the USB standard. The upstream-facing (peripheral) port does not provide power to the USB VBUS. Once this port connects to a host, it detects the presence of VBUS and pulls up either DP (for full speed and high-speed operation) or DM (for low-speed operation) with a 1.5-kΩ resistor. Here, DP refers to DPLUS/D+ or the positive terminal of the USB data differential pair, and DM refers to DMINUS/D-. Since a DRP must expect to either pull up DP and DM in peripheral mode (upstream facing), or offer a 15-kΩ pulldown resistor to ground and expect an external 1.5-kΩ pullup on DP and DM in host mode (downstream facing), it is clear that traditional isolated USB repeaters that have fixed upstream- and downstream-facing sides cannot support a DRP.