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• If you are shipping the USB Hub for repair, make sure you include everything that came in the original package. Before you ship, contact Black Box to get a Return Authorization (RA) number. 23 DUAL-AND QUAD-PORT USB HUBS Appendix B. Electrical Interfaces B.1 RS-232 Quite possibly the most widely used communication standard is RS-232. This implementation has been defined and revised several times and is often referred to as EIA/TIA-232. The IBM® PC AT® computer defined the RS-232 port on a DB9 connector, and subsequently the EIA/TIA approved this implementation as the EIA/TIA-574 standard. This standard is defined as the 9-Position Non-Synchronous Interface between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange. Both implementations are in widespread use and are referred to as RS-232 in this document. RS-232 is capable of operating at data rates up to 20 kbps at distances less than 50 ft. (15.2 m). The absolute maximum data rate may vary due to line conditions and cable lengths. RS-232 is a single-ended or unbalanced interface, meaning that a single electrical signal is compared to a common signal (ground) to determine binary logic states. The RS-232 and the EIA/TIA-574 specification define two types of interface circuits: Data Terminal Equipment (DTE) and Data Circuit- Terminating Equipment (DCE). The USB Hub is a DTE device. B.2 RS-422 The RS-422 specification defines the electrical characteristics of balanced-voltage digital interface circuits. RS-422 is a differential interface that defines voltage levels and driver/receiver electrical specifications. On a differential interface, logic levels are defined by the difference in voltage between a pair of outputs or inputs. In contrast, a single-ended interface, for example RS-232, defines the logic levels as the difference in voltage between a single signal and a common ground connection. Differential interfaces are typically more immune to noise or voltage spikes that may occur on the communication lines. Differential interfaces also have greater drive capabilities that allow for longer cable lengths. RS-422 is rated up to 10 Mbps and can have cabling 4000 feet (1219.2 m) long. RS-422 also defines driver and receiver electrical characteristics that will allow 1 driver and up to 32 receivers on the line at once. RS-422 signal levels range from 0 to +5 volts. RS-422 does not define a physical connector. 24 APPENDIX B: Electrical Interfaces B.3 RS-485 RS-485 is backward-compatible with RS-422; however, it is optimized for partyline or multidrop applications. The output of the RS-422/485 driver is capable of being Active (enabled) or Tristate (disabled). This capability allows multiple ports to be connected in a multidrop bus and selectively polled. RS-485 allows cable lengths up to 4000 feet (1219.2 m) and data rates up to 10 Mbps. The signal levels for RS-485 are the same as those defined by RS-422. RS-485 has electrical characteristics that allow for 32 drivers and 32 receivers to be connected to one line. This interface is ideal for multidrop or network environments. RS-485’s tristate (not dual-state) driver will allow the electrical presence of the driver to be removed from the line. Only one driver may be active at a time, and the other driver(s) must be tristated. RS-485 can be cabled in two ways: two-wire and four-wire mode. Two-wire mode does not allow for full-duplex communication, so data can be transferred in only one direction at a time. For half-duplex operation, the two transmit pins should be connected to the two receive pins (Tx+ to Rx+ and Tx to Rx). Four-wire mode allows full-duplex data transfers. RS-485 does not define a connector pinout, a physical connector, or a set of modem control signals. 25 DUAL-AND QUAD-PORT USB HUBS Appendix C: Asynchronous Communication In serial data communication, individual bits of a character are transmitted consecutively to a receiver that assembles the bits back into a character. Data rate, error checking, handshaking, and character framing (start/stop bits) are predefined and must correspond at both the transmitting and receiving ends. Asynchronous communication is the standard means of serial data communication for PC compatibles and PS/2® computers. The original PC was equipped with a communication or COM port that was designed around an 8250 Universal Asynchronous Receiver Transmitter (UART). This device allows asynchronous serial data to be transferred through a simple and straightforward programming interface. A start bit, followed by a pre-defined number of data bits (5, 6, 7, or 8), defines character boundaries for asynchronous communication. The end of the character is defined by the transmission of a pre-defined number of stop bits (usually 1, 1.5, or 2). Figure C-1. Bit diagram for asynchronous communication. An extra bit used for error detection is often appended before the stop bits. This special bit is called the parity bit. Parity is a...