Black Box Explains…Network Time Protocol (NTP).
Network Time Protocol (NTP) dates to the early 1980s and is one of the oldest protocols used on the Internet today. NTP was developed to synchronize a network through the... more/see it nowuse of NTP servers and latency compensation to ensure that every device on a network shows the correct time.
NTP is based on a hierarchical system of clocks and NTP servers. At the top of the hierarchy are clock sources such as atomic or GPS clocks. These clocks are referred to as Stratum 0. NTP servers connected directly to a clock source are referred to as Stratum 1, servers connected to Stratum 1 servers are called Stratum 2, and so on.
Many Stratum 1 NTP servers—often called just time servers—are available for reference on the Internet. The National Institute of Standards and Technology (NIST) Internet Time Service provides many of these servers. Generally, servers closer to the clock—Stratum 1 and Stratum 2—are considered to be more accurate than servers further from the clock. NTP provides Coordinated Universal Time (UTC), which is based on International Atomic Time (TAI), and has support for leap seconds.
To set the correct time, a client device polls several NTP servers, throws out any times that appear to be anomalous, and averages the remaining times to come up with a best estimate of the correct time. An NTP transaction consists of four timestamps:
A client timestamp when a request is sent to the server.
A server timestamp when the request is received by the server.
A server timestamp when the reply is sent to the client.
A client timestamp when the reply is received by the client.
The client device uses these timestamps to compensate for latency and calculate the correct time. Accuracy can usually be maintained within a few milliseconds, even over the Internet.
A compatible standard, Simple Network Time Protocol (SNTP), polls only one NTP server to obtain the time.
Most of today’s computer operating systems include support for NTP or SNTP. Most managers of large networks set up a Stratum 2 NTP server and have computers on the network get their time from that server using SNTP. Having one NTP server for the network rather than having each device going to outside servers for the time ensures accurate and consistent time across the network.
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Black Box Explains...RS-232.
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RS-232, also known as RS-232C and TIA/EIA-232-E, is a group of electrical, functional, and mechanical specifications for serial interfaces between computers, terminals, and peripherals. The RS-232 standard was developed by... more/see it nowthe Electrical Industries Association (EIA), and defines requirements for connecting data communications equipment (DCE)—modems, converters, etc.—and data terminal equipment (DTE)—computers, controllers, etc.) devices. RS-232 transmits data at speeds up to 115 Kbps and over distances up to 50 feet (15.2 m).
The standard, which is functionally equivalent to ITU V.24/V.28, specifies the workings of the interface, circuitry, and connector pinning. Both sync and async binary data transmission fall under RS-232. Although RS-232 is sometimes still used to transmit data from PCs to peripheral devices, the most common uses today are for network console ports and for industrial devices.
Even though RS-232 is a “standard,” you can’t necessarily expect seamless communication between two RS-232 devices. Why? Because different devices have different circuitry or pinning, and different wires may be designated to perform different functions.
The typical RS-232 connector is DB25, but some PCs and other data communication devices have DB9 connectors and many newer devices have RJ-45 RS-232 ports. To connect 9-pin PC ports or RJ-45 to devices with 25-pin connectors, you will require a simple adapter cable. collapse
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