ABB Excount II User Manual
Also see for EXCOUNT-II: User manual
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Contents
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64 1HSA 801 080-15en EXCOUNT-II Users Manual7. Radio transmission protocol7.1 Packet communication protocolAll radio channels are subject to noise, intereference and fading. In manycases, radio channels are shared by several users of services. Packetcommunication protocols are widely used to achieve error-free communi-cations over imperfect and/or shared communication channels.Almost all short-range wireless data communications use some form ofpacket protocol to automatically assure information is received correctlyat the correct destination. A packet generally includes a training pream-ble, a start symbol, routing information (to/from, etc.) a packet ID, all orpart of a message and error detection bits. Other information may beincluded depending on the protocol.Figure 7.1.1 shows one of the packet formats used in EXCOUNT-II. Thestructure begins with a training preamble, which improves weak signaldetection at the receiver by ”training” the data slicer for best noise immu-nity, and providing signal transitions to train the clock recovery process.The training preamble usually consists of several bytes of a1-0-1-0-1-0 ... sequence.EXCOUNT-II RF Link Packet FormatPreamble StartSymbol To Byte FromBytePacketNumberSize/StatusByte* Message FCS HighByteFCS LowByteFigure 7.1.1The preamble is followed by a start symbol (often called a start vector),which is a distinct pattern of bits marking the start of the information sec-tion of the packet.The start symbol is followed by ”to” and ”from” address information. Inthe EXCOUNT-II protocol, the packet ID is followed by message size orstatus information.The message then follows . The following two bytes of the packet com-prise a 16 bit error checking code (frame check sequence), based on theX.25 packet standard (ISO 3309). The error checking code is recompu-ted at the destination to confirm error-free detection. The ISO 3309 framecheck sequence provides very high confidence of error detection forpackets up to 256 bytes in length.In summary, EXCOUNT-II protocols provides the following features:• 16-bit ISO 3309 error detection calculation to test message integrity• Automatic packet retransmission until acknowledgement is received; 8retries with semi-random back off plus ”acknowledge” and ”link failure”alarm messages. |
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