SMSC COM20020 manual Data Packets, Negative Acknowledgements

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Data Packets

A Data Packet consists of the actual data being sent to another node. It is sent by the following sequence:

∙An ALERT BURST

∙An SOH (Start Of Header--ASCII code 01H)

∙An SID (Source ID) character

∙Two (repeated) DID (Destination ID) characters

∙A single COUNT character which is the 2's complement of the number of data bytes to follow if a short packet is sent, or 00H followed by a COUNT character if a long packet is sent

∙N data bytes where COUNT = 256-N (or 512- N for a long packet)

∙Two CRC (Cyclic Redundancy Check)

characters. The CRC polynomial used is: X16 + X15 + X2 + 1.

ALERT BURST

SOH

SID

DID

DID

COUNT

data

data

CRC CRC

Acknowledgements

An Acknowledgement is used to acknowledge reception of a packet or as an affirmative response to FREE BUFFER ENQUIRIES and is sent by the following sequence:

∙An ALERT BURST

∙An ACK (ACKnowledgement--ASCII code 86H) character

ALERT	ACK
BURST

Negative Acknowledgements

A Negative Acknowledgement is used as a negative response to FREE BUFFER ENQUIRIES and is sent by the following sequence:

∙An ALERT BURST

∙A NAK (Negative Acknowledgement--ASCII code 15H) character

ALERT	NAK
BURST

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Contents General Description FeaturesTable of Contents PIN Configuration A1,A2/ALE Description of PIN FunctionsMiscellaneous DIP PIN Plcc PIN Name Symbol DescriptionTransmission Media Interface Instead, it must be connected to XTAL1 with COM20020 Operation Data Rates Network Reconfiguration Protocol Description Network ProtocolResponse Time Extended Timeout FunctionBroadcast Messages Invitations To Transmit Reconfiguration TimeIdle Time Line ProtocolNegative Acknowledgements Data PacketsAcknowledgements System Description MULTIPLEXED, 8051-LIKE BUS Interface with RS-485 Interface Figure C Traditional Hybrid Interface Backplane ConfigurationCOM20020 Network Using RS-485 Differential Transceivers Programmable Txen Polarity Differential Driver ConfigurationInternal Block Diagram Cable Type Impedance AT 5MHZ Functional DescriptionAttenuation NominalRead Register Summary Write Tentative ID Register Internal RegistersInterrupt Mask Register IMR Data RegisterStatus Register Diagnostic Status RegisterCommand Register Next ID RegisterConfiguration Register Setup Register BIT BIT Name Symbol Description Dupid BIT BIT Name Symbol DescriptionData Command Description Address Pointer Low Register Address Pointer High RegisterReset Configuration RegisterCKP3 CKP2 CKP1 Sequential Access Operation Software Interface Sequential Access MemoryAccess Speed Internal RAMTransmit Sequence Selecting RAM Page SizeRAM Buffer Packet Configuration Page Receive Sequence Transmit Command Chaining Command ChainingReceive Command Chaining Internal Reset Logic Reset DetailsInitialization Sequence Improved DiagnosticsAbnormal Results Normal ResultsOscillator Operational Description Maximum Guaranteed RatingsDC Electrical Characteristics Parameter Symbol MIN TYP MAX Unit CommentXTAL1, XTAL2 Input Capacitance Multiplexed BUS, 68XX-LIKE Control Signals Read Cycle Timing DiagramsMultiplexed BUS, 80XX-LIKE Control Signals Read Cycle Multiplexed BUS, 68XX-LIKE Control Signals Write Cycle Multiplexed BUS, 80XX-LIKE Control Signals Write Cycle NON-MULTIPLEXED BUS, 80XX-LIKE Control Signals Read Cycle NON-MULTIPLEXED BUS, 68XX-LIKE Control Signals Read Cycle NON-MULTIPLEXED BUS, 80XX-LIKE Control Signals Write Cycle NON-MULTIPLEXED BUS, 68XX-LIKE Control Signals Write Cycle These signals are to and from the hybrid Normal Mode Transmit or Receive TimingNTXEN NPULSE1 TTL Input Timing on XTAL1 PIN 28-PIN Plcc Package Dimensions 24-PIN DIP Package Dimensions SECTION/FIGURE/ENTRY Correction Revised DateStandard Microsystems Corp