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While the host-device arrangement works for some devices, many devices can act either as host or as device depending on what else shares the bus. This works out at £8 15s. per mile. Clearly, then, from a purely financial standpoint, a cable to India could be put out of existence by the land-line. An advantage of the Wiegand signalling format is that it allows very long cable runs, far longer than other interface standards of its day allowed. Many access control system manufacturers adopted Wiegand technology, but were unhappy with the limitations of only 8 bits for site codes (0-255) and 16 bits for card numbers (0-65535), so they designed their own formats with varying complexity of field numbers and lengths and parity checking. The first parity bit is calculated from the first 12 bits of the code and the trailing parity bit from the last 12 bits. This was a telegraph code developed for use on the French telegraph using a five-key keyboard (Baudot, 1874). Teleprinters generated the same code from a full alphanumeric keyboard.

The cause of these frequent fluctuations has been the more or less fierce struggle waged in the Atlantic, with some intermissions, since 1868, when the French entered the field in rivalry with the British. The politics of the Far East point in the same direction as regards British cable enterprise. It was in 1871 that British cable enterprise entered the Far East. The choice between host and peripheral roles is handled entirely by which end of the cable the device is connected to. For example, a smartphone may read from removable media as the host device, but present itself as a USB Mass Storage Device when connected to a host computer. USB On-The-Go (USB OTG or just OTG) is a specification first used in late 2001 that allows USB devices, such as tablets or smartphones, to also act as a host, allowing other USB devices, such as USB flash drives, digital cameras, mouse or keyboards, to be attached to them.

For instance, a computer printer is normally a device, but when a USB flash drive containing images is plugged into the printer's USB port with no computer present (or at least turned off), it would be useful for the printer to take on the role of host, allowing it to communicate with the flash drive directly and to print images from it. At the same time, a B-device will generate SRP (see below) and wait for the USB bus to become powered. If a logical 0 is transmitted by all transmitting node(s) at the same time, then a logical 0 is seen by all nodes. To transfer data between two devices, for example from a phone to a printer, the host first reads the data from one device, what is control cable then writes it to the other. The OTG or embedded host can leave the USB link unpowered until the peripheral (which can be an OTG or standard USB device) requires power. Session Request Protocol (SRP): Allows both communicating devices to control when the link's power session is active; in standard USB, only the host is capable of doing so.

However, when the same A-device is plugged into another USB system or a dedicated host becomes available, it can become a device. When a device is plugged into the USB bus, the host device sets up communications with the device and handles service provisioning (the host's software enables or does the needed data-handling such as file managing or other desired kind of data communication or function). Standard USB uses a host/device architecture; a host acts as the Host device for the entire bus, and a USB device acts as a Peripheral. After initial startup, setup for the bus operates as it does with the normal USB standard, with the A-device setting up the B-device and managing all communications. The wiring for the ID pin defines the initial role of each device. The Wiegand interface is a de facto wiring standard which arose from the popularity of Wiegand effect card readers in the 1980s. It is commonly used to connect a card swipe mechanism to the rest of an access control system. This back-up system served to reduce the risk of total flight-control-system failure ever happening because of a general-purpose flight software fault that had escaped notice in the other four computers.