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ADDC (Automatic Data Direction Control) |
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When using RS-485 2-wire communication, one of the most important things to determine is when to switch the transmitter on and off. Because of the restrictions imposed by the RS-485 interface, only 1 node (on an RS-485 2-wire bus) can switch on it's transmitter at any given time. The node that wishes to send data must switch on it's transmitter, and then switch it off after the last data bit has been sent.
There are 2 ways to take care of the job of switching a transmitter on and off. One method is to use the RTS signal to control the transmitter manually by software. The other method is to use ADDC™ (Automatic Data Direction Control).
ADDC switches the transmitter on and off very precisely by hardware, effectively simplifying the complexity of timing control by software. |
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ASIC (Application Specific Integrated Circuit) |
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ASIC chips are specially designed chips that provide unique functions. ASIC chips can replace general-purpose commercial logic chips, and integrate several functions or logic control blocks into one single chip, lowering manufacturing cost and simplifying circuit board design. |
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Asynchronous communication |
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Asynchronous communication refers to digital communication (such as between computers) in which there is no timing requirement for transmission, and in which the start of each individual character is signaled by the transmitting device. |
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| B |
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Baud rate |
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Baud rate is related to transmission speed. As far as RS-232/422/485 interfaces are concerned, baud rate is measured in bps (bits per second). |
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| C |
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CompactPCI |
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CompactPCI is a very high performance industrial bus based on the standard PCI electrical specification in rugged 3U or 6U Eurocard packaging. Contrary to its desktop cousin, a CompactPCI board uses a high quality 2 mm metric pin and socket connector that meets IEC and Bellcore standards. CompactPCI boards are inserted from the front of the chassis, and the I/O can break out either to the front or through the rear. For more information, refer to http://www.picmg.com. |
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| D |
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DTE (Data Terminal Equipment) |
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DTE stands for Data Terminal Equipment, as defined by the RS-232 specification. Examples of DTE are computers, printers, and terminals. |
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DCE (Data Communication Equipment) |
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DCE stands for Data Communication Equipment, as defined by the RS-232 specification. The basic function of a DCE is to convert data from one interface, such as a digital signal, to another interface, such as an analog signal. One example of DCE is a modem. |
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| F |
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FIFO (First In First Out) |
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FIFO is a term that describes the behavior of some buffers. FIFO buffers send out characters in the order that they are received. FIFO buffers are used to reduce the frequency of interrupt processes for UART chips, such as the C16550C, used in serial communications. |
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Flow Control |
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Flow Control is necessary to regulate data flow between 2 devices with dramatically different data transmission speeds (such as a dot matrix serial printer and an RS-232 interface connection). Flow Control is needed to regulate data flow, to ensure that the two devices can communicate with minimal loss of data.
RS-232 communication uses one of two basic approaches to enforce Flow Control.
Software approach: XON/XOFF
XON (0x11) and XOFF (0x13) are defined as special control codes for use during communication. The 2 codes are transmitted along with data characters. The operating scheme is straightforward:
When either of the devices receives XOFF, it stops transmitting data until XON is received. The problem with this approach is that the data itself cannot contain these 2 codes.
Hardware approach: RTS/CTS
RTS (Request To Send) and CTS (Clear To Send) are separate signals, sent on separate wires, used for hardware Flow Control. RTS is an output signal that enables/disables data transmission for the other device. CTS is an input signal allowing the other device to enable/disable data transmission. The drawback to this approach is that 2 more wires are needed, but it also provides the capability to send binary data. |
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| I |
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Intelligent multiport board |
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An intelligent multiport board has on-board processing capability that allows it to cope with huge amounts of data, and in this way share the workload of the host processor. The on-board processor acts as a Front End I/O processor to handle the necessary data processing before it sends data to the host processor, and on-board memory provides a large buffer that eliminates the chance of losing data during data transmission. |
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| N |
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Non-intelligent multiport board |
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A non-intelligent multiport board is equipped with UART chips and the necessary peripheral ICs. All the data transmit/receive processing is done by the motherboard CPU. This creates a high workload for the CPU, and consequentlye can only be used for limited ports. However, non-intelligent multiport boards are an economical and robust solution for small scale applications. |
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| O |
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Optical Isolation |
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Communications devices connected by long cables may be damaged by the mismatch between ground voltage levels at the two ends of the wire. Optical isolation uses photo cells at both ends of the line to isolate the devices' sensitive components from this type of electrical damage. |
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| P |
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Parallel communication |
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Parallel communication refers to when data is tranmitted byte-by-byte. I.e., all bits of one or more bytes are transmitted simultaneously over separate wires. |
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| R |
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RS-232 |
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RS-232 is a serial communications standard that provides asynchronous communication capabilities, such as hardware flow control, software flow control, and parity check. It has been widely used for decades. Almost all gears, instruments with digital control interface, and communications devices are equipped with the RS-232 interface. The typical transmission speed of an RS-232 connection is 9600 bps over a maximum distance of 15 meters. |
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RS-422 |
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RS-422 is a serial communications standard that provides a much longer transmission distance, but fewer signal lines compared to RS-232. RS-422 uses differential transmission technology, and thus provides high-speed transmission of up to 10 Mbps. The maximum transmission distance is 1.2 km at a transmission speed of 9600 bps. |
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RS-485 |
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RS-485 is an enhanced version of RS-422. It uses a 2-wire bus topology, and is compatible with the RS-422 interface. By using an RS-485 2-wire bus, you can establish a very economical network. However, RS-485 only defines electrical signal specifications; users must define the software protocol themselves. |
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| S |
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Serial communication |
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Serial communication refers to when data is transmitted bit-by-bit, or sequentially, over a single wire. |
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Surge Protection |
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TOV (Transient Over Voltages), which come from lightning, electrostatic discharges, and other forms of high voltage, is one of the major causes of equipment damage. A surge protector absorbs TOV to effectively protect your equipment. |
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Synchronous communication |
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Synchronous communication refers to digital communication (such as between computers) in which a common timing signal is established that dictates when individual bits can be transmitted. In this case, individual characters are not delimited, allowing for very high rates of data transfer. |
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| T |
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TCP/IP (Transmission Control Protocol/Internet Protocol) |
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TCP/IP is a set of protocols developed to allow computers to share resources across a network. It was developed by a community of researchers centered around the ARPAnet. The most accurate name for the set of protocols we are describing is the "Internet protocol suite". TCP and IP are just two of the protocols in this suite. Because TCP and IP are the best known of the protocols, it has become common to use the term TCP/IP to refer to the whole family. Note that these computer services can all be provided within the framework of TCP/IP. |
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Termination Resistors |
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When an electrical signal travels through two different resistance junctions in a transmission line, the impedance mismatch will sometimes cause signal reflection. Signal reflection causes signal distortion, which in turn contributes to communication errors. The solution to this problem is to establish the same impedance at the line ends as in the line itself by terminating them with resistors. It is normally sufficient when the value of the termination resistor equals the characteristic impedance of the transmission line. The resistors should be added near the receiving side. |
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Throughput |
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Throughput refers to the performance of data transmission, and is measured by characters actually transmitted or received during a certain period of time. The throughput of a connection depends on CPU, memory, performance between the two devices, pattern of measurement, as well as the performance of the operating system. Throughput is usually measured in bps (bits per second). |
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| U |
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UART (Universal Asynchronous Receiver-Transmitter) |
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UART chips control the data transmission and reception of a computer's serial communications devices. The UART chip converts digital data between parallel data inside the PC and serial data from an RS-232/422/485 line driver. |
