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HP OpenVMS Systemsask the wizard |
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The Question is: What is the maximum cable length for a serial cable from a VT520 terminal to a serial port? The Answer is : The theoretical maximum distance is arguably limited by the speed of light in the chosen communications medium and by signal interference and by the speed of the data transfers required and is -- given the appropriate hardware -- effectively limited to interplanetary distances based solely on round-trip communications delays, though the general baud rate for such long-distance connections will be fairly low and the use of sliding-window-like or other protocols is recommended. (Tachyon and faster-than-light discussions deliberately omitted.) For information on the EIA-232 and EIA-423 requirements for traditional serial wiring runs, please consult the associated EIA standards, or please see the Boundless Technologies information, documentation, or other materials on the Boundless Technologies VT520 terminal. For network-based serial connections, please see the IEEE 802.3 network requirements. For VPN distances, planetary-scale connections are common. For orbital connections, a network of several helical antennas is recommended with specific sites based on the orbit. Theoretical values aside, the practical limit is generally determined locally and empirically, and this limit is heavily dependent on the particular host controller UARTs, the terminal UARTs, the cable loss, and the expected electromagnetic interference. (The performance and the expections of various UARTs and other similar signalling devices can and does vary (widely), of course.) The OpenVMS Wizard has personally seen operational 9600 baud terminal wiring runs over a kilometer in length, and has also seen other 9600 baud wire runs that will barely make three meters -- as an alternative to traditional EIA-232 or EIA-432 serial communcations wiring, network communications can potentially be supported over hundreds of kilometers or more, using the appropriate network connection. Your mileage will vary, of course. For assistance with hardware and hardware questions, please contact your hardware services organization, and please consult applicable hardware specifications and standards documents.
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