The first parity bit is calculated from the first 12 bits of the code and the trailing parity bit from the last 12 bits. 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 system was designed for future expansion; the addressing system allowed for expansion to 255 nodes in a LAN (although only 32 could be used at that time), and by using "bridges" (which came to be known as "routers", although technically not the same) one could interconnect LANs into larger collections. However, when the same A-device is plugged into another USB system or a dedicated host becomes available, it can become a device. The latest version of the supplement also defines behavior for an Embedded Host which has targeted abilities and the same USB Standard-A port used by PCs. The Enhanced Parallel Port (EPP), originally defined by Zenith Electronics, is similar to IBM's byte mode in concept, but changes details of the handshaking to allow up to 2 MB/s. 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.

It does so by periodically measuring the capacitance on the USB port to determine whether there is another device attached, a dangling cable, or no cable. The device connected to the "A" end of the cable at start-up, known as the "A-device", acts as the default host, while the "B" end acts as the default peripheral, known as the "B-device". For example, a printer is connected as the A-device (host), but cannot function as the host for a particular camera, since it does not understand the camera's representation of print jobs. However, non-standard implementations exist, for example, printers that use DTR as flow control. For example, a smartphone may read from removable media as the host device, what is control cable but present itself as a USB Mass Storage Device when connected to a host computer. It can accept either a mini-A plug or a mini-B plug, while mini-A adapters allows connection to standard-A USB cables coming from peripherals. With OTG, a device can be either a host when acting as a link host, or a link peripheral. The device controlling the link is called the Host, while the other is called the Peripheral. The Wiegand interface uses three wires, one of which is a common ground and two of which are data transmission wires usually called DATA0 and DATA1, alternatively labeled "D0" and "D1" or "Data Low" and "Data High".

Control cables are used for signal transmission and as energy supply for smaller applications. It developed from various earlier printing telegraphs and resulted in improved transmission speeds. Inside the locomotive, a transformer steps the voltage down for use by the traction motors and auxiliary loads. 420 V DC, and a top-contact fourth rail is located centrally between the running rails at −210 V DC, which combine to provide a traction voltage of 630 V DC. The key advantage of the four-rail system is that neither running rail carries any current. Pre-flight safety checks of a fly-by-wire system are often performed using built-in test equipment (BITE). Most support a four-port hub system, but hubs using 16-port hub controllers are also available in the industry. By using HNP for reversing host/peripheral roles, the USB OTG device is capable of acquiring control of data-transfer scheduling. Host Negotiation Protocol (HNP): Allows the two devices to exchange their host/peripheral roles, provided both are OTG dual-role devices.

Attach Detection Protocol (ADP): Allows an OTG device, embedded host or USB device to determine attachment status in the absence of power on the USB bus, enabling both insertion-based behavior and the capability to display attachment status. 3.5 A, plus power for the hub itself. The new OTG protocols cannot pass through a standard USB hub since they are based on electrical signaling via a dedicated wire. The high signaling level of 5 VDC is used to accommodate long cable runs from card readers to the associated access control panel, typically located in a secure closet. Cable systems in the United States were becoming more of a necessity than a luxury both for individuals and communities as technology became a part of everyday life. The Federal Aviation Administration (FAA) of the United States has adopted the RTCA/DO-178C, titled "Software Considerations in Airborne Systems and Equipment Certification", as the certification standard for aviation software. 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.