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Ethernet Cheapernet Physical Layer Made Easy with DP8391
Ethernet Cheapernet Physical Layer Made Easy with DP8391
With integration node electronics IEEE compatible local area networks silicon system design simplified This application note describes differences between Ethernet Cheapernet versions standard provides design guidelines implementing node electronics with National Semiconductor's DP8390 chip INTRODUCTION DP8390 chip designed provide physical media access control layer functions local area networks specified IEEE standard This standard based access method known carrier-sense multiple access with collision detection (CSMA this scheme network station wants transmit first ``lis-
National Semiconductor Application Note Alex Djenguerian June 1986
tens'' medium someone else transmitting station defers until medium clear before begins transmit However more stations could still begin transmitting same time give rise collision When this happens nodes detect this condition back random amount time before making another attempt IEEE standard supports different versions media 10BASE5 (commonly known Ethernet) 10BASE2 (Cheapernet) These used separately together hybrid form Both versions have similar electrical specifications implemented using same transceiver chip (DP8392) Cheapernet cost version user installable following table compares 10BASE2 (Cheapernet) Mbits baseband diameter (RG58A Single Shielded Flexible Connectors needed high flexibility RG58A cable
Parameter Data Rate Segment Length Network Span Nodes Segment Node Spacing Capacitance Node Cable
10BASE5 (Ethernet) Mbit baseband 2500 (cable marked) diameter Double Shielded Rugged N-Series Connectors diameter multiway cable with connectors length
Transceiver Drop Cable
Typical Connection Diagram Station
10BASE5 (Ethernet)
10BASE2 (Cheapernet)
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C1995 National Semiconductor Corporation
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RRD-B30M105 Printed
Although Cheapernet intended local several meter segments joined together with simple repeaters provide larger network span Similarly several Cheapernet segments tied into longer Ethernet ``backbone'' this hybrid configuration network com-
bines benefits Cheapernet flexibility cost with ruggedness much larger geographic range standard Ethernet Figure illustrates typical hybrid configuration
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FIGURE Hybrid Ethernet Cheapernet System TRANSMITTING RECEIVING PACKETS WITH DP8390 CHIPSET Node Block Diagram
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node electronics integrated into three chips DP8390 Network Interface Controller (NIC) DP8391 Serial Network Interface (SNI) DP8392 Coaxial Transceiver Interface (CTI) transmit packet host processor issues transmit command which normal-
transfers data local buffer memory then automatically handles transmission packet (from local buffer through on-board FIFO SNI) according CSMA protocol packet following format DATA
PREAMBLE DESTINATION SOURCE LENGTH 62-bits 2-bits 6-bytes 6-bytes
2-bytes 1500 bytes 4-bytes
PREAMBLE This section consists alternating bits packet travels through network some these bits would lost most network components allowed provide output some number bits after being presented with valid input START FRAME DELIMITER (SFD) This field consists consecutive signal that frame reception should begin DESTINATION SOURCE ADDRESSES Each these frames bytes long specifies address corresponding node LENGTH This byte field indicates number bytes data field DATA This field from 1500 bytes long Messages shorter than bytes require padding bring data field minimum length data field padded host determine number valid data bytes looking length field Messages longer than 1500 bytes must broken into multiple packets This field contains Cyclic Redundancy Code calculation performed Destination address through Data field error control shortest packet length thus adds bits long (excluding preamble SFD) Mbit this amounts which twice much maximum end-to-delay time that allowed IEEE protocol This ensures that collision arises network would recognized node locations combines data packet received from controller with clock signal encodes them into serial stream using standard Manchester encoding this coding scheme first half cell contains complementary data second half contains true data Thus transition always guaranteed middle cell
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FIGURE Manchester Coding encoded signal appears differential form SNI's output 10BASE5 (Ethernet) applications this signal sent transceiver Medium Attachment Unit (MAU) through twisted pair Tranceiver Drop cable (also known Attachment Unit Interface cable) This cable typically consists four individually shielded twisted wire pairs with overall shield covering these individually shielded pairs signal pairs which have differential characteristic impedance should terminated receiving ends cable meters length have maximum delay shields individual pairs should connected logic ground Data Terminal Equipment (DTE) outer shield chassis ground Figure shows picture cable corresponding assignments
Shell
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IEEE Name (Data (Data Shield) (Data (Data Shield) (Control (Control Shield) (Control (Control Shield) (Voltage Common) (Voltage Plus) (Voltage Shield) (Protective GND)
Pairs Transmit Pair Receive Pair Optional Pair Collision Pair Power Pair
DP8391 Name
Signal from
FIGURE Transceiver Cable Assignments
transmitted packet from well other signals (receive collision power) must electrically isolated from coax isolation means provided must withstand minute 10BASE2 2000 10BASE5 order detect collisions reliably electrical isolation done coax instead done side Attachment Unit Interface isolation three signal lines easily provided using three pulse transformers that come standard plastic from several manufacturers (Pulse Engineering Valor Electronics) inductance value these transformers vary from with larger inductance values slowing rise fall times smaller ones causing more voltage droop Manchester encoded data from reaches CTI's transmit input after passing through isolation transformer noise filter this input provides static noise margin b175 b300 These thresholds assure that differential Transmit data signals less than b175 narrower than always rejected while signals greater than b300 wider than always accepted b300 threshold provides sufficient margin since differential drivers transceiver drop cable provide minimum signal level after inductive droop maximum attenuation allowed drop cable signal frequencies Signals meeting squelch requirements waveshaped outputted coax medium This done follows transmitter's output driver switching current source that drives purely resistive load presented coax produce voltage swing approximately This
signal meet several critical electrical requirements RISE FALL TIMES rise fall times have Mbit This spec helps minimize electro-magnetic radiation reducing higher harmonic content signal contributes smaller reflection levels coax addition rise fall times required matched within minimize overall jitter system LEVEL component signal between tolerance here tight since collisions detected monitoring average level coax LEVEL component signal between level This specification guarantees minimum signal coax cable worst case condition signal shown would attenuated travels along coax maximum cable attenuation segment This applies both meters Ethernet cable meters Cheapernet cable With Mbit Manchester data this cable attenuation results approximately edge jitter either direction CTI's receiver compensate least portion this jitter meet combined jitter budget receiver also should overcompensate signal case short cable equalizer filter accomplishes this task Figure shows typical waveform seen cable corresponding differential output from CTI's receiver
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FIGURE Coax Transmit Waveform
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FIGURE Oscilloscope Waveforms
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Total Jitter without Noise Additional Jitter from Noise Coax Cable Additional Jitter from Noise Drop Cables Total System Jitter
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FIGURE Typical Signal Waveform SNI's Input addition equalizer squelch circuit coax input prevents noise cable from falsely triggering receiver absence valid signal Receive differential line from should isolated before reaches Manchester decoding This signal could have accumulated much jitter Figure illustrates jitter allocations different network components typical signal waveform SNI's input digital phase-locked loop decode Manchester data with random jitter which provides enough margin implementation converts Manchester received packet data clock pulses sends them controller Upon reception checks destination address valid verifies with generated board stores packet local buffer memory packet then moved host when this completed buffer area reclaimed storing packets collision occurs during this transfer process will detect sensing average level coax will send collision signal will translate this information controller form transmitting controllers will backoff different times retransmit later Also case illegally long packets (longer than jabber timer will disable coax driver that ``jabbering'' station will bring down entire network collision pair activated this case inform controller faulty condition After fault removed jabber timer holds before re-enabling coax driver COLLISION DETECTION SCHEMES There different collision detection schemes that implemented with receive transmit modes IEEE standard allows receive transmit transhybrid modes non-repeater nodes both Ethernet Cheapernet applications Repeaters required have receive mode implementation RECEIVE MODE Detects collision between stations network with certainty times TRANSMIT MODE Detects collisions with certainty only when station transmitting RECEIVE MODE receive mode scheme very simple truth table however tight threshold limits make design difficult threshold this case between maximum level station (b1300 minimum level stations (b1581 Several factors such termination resistor variation coax center conductor resistance driver current level variation signal skew input bias current non-transmitting nodes contribute this tight margin
Truth Table Various Collision Detection Schemes Mode Stations Transmitting Non-Transmitting Receive
Transmit
will detect collision detect collision
will detect collision
b1300 minimum level impulse response internal pass filter added incorporates pole Bessel filter combination with trimmed board bandgap reference provide this mode collision detection However would difficult receive mode extend cable length beyond limits standard also argued that necessary non-repeater nodes detect collisions between other stations TRANSMIT MODE this case collisions have detected with certainty only when station transmitting Thus collisions caused other nodes detected This feature relaxes upper limit threshold from b1581 b1782 result this longer cable segments used With resistor divider used Collision Detect Sense (CDS) lower threshold from receive transmit mode Typical resistor values 120X from from (This moves threshold about b100 IMPLEMENTING BASE5 (ETHERNET) WITH DP8392 provides (transceiver) functions except signal power isolation Signal isolation easily provided three pulse transformers that come single Dual-in-Line package These available from transformer vendors such Pulse Engineering (PE64103) Valor (LT1101) However power isolation converter required requires single volt supply This power derived from power pair drop cable which capable providing b6%) volt range supply current makes design converter quite easy Such converters being developed hybrid packages transformer manufacturers (Pulse Engineering PE64430 Reliability 2E12R9) They provide necessary voltage isolation output regulation also build simple converter with transistor self oscillating primary circuit some regulation secondary shown Figure
Several areas board layout require special care most critical these coax connection Ethernet requires that capacitance less than coax with another allocated mechanism Receive Input (RXI) Transmit Output (TXO) lines should kept absolute minimum mounting very close center Also external diode (see Figure designer must minimize stray capacitance particularly anode side diode this metal lines especially ground planes should kept possible from lines order meet stringent capacitive loading requirements coax imperative that directly soldered board without socket special lead frame package allows direct conduction heat from through these leads board thus reducing operating temperature significantly good heat conduction pins should connected large metal traces planes separate voltage sense (CDS) provided accurate detection collision levels coax receive mode where threshold margin tight this should independently attached coax shield minimize errors ground drops resistor divider network this used transmit mode operation described earlier differential transmit pair from should terminated with differential resistive load splitting termination resistor into equal values capacitively grounding center node common mode impedance reduced about which helps attenuate common mode transients drive differential line with sufficient voltage swings CTI's collision receive drivers need external 500X resistors using external resistors power dissipation chip reduced enhancing long term reliability only precision component required resistor This resistor sets many important parameters chip such coax driving levels output rise fall times collision oscillator frequency jabber timing receiver squelch timing should connected between pins (RRb) DP8392 features heartbeat function which externally disabled using This function activates collision output short time cells) every transmission used ensure controller that collision circuitry intact properly functioning enables Heartbeat when grounded disables when connected
IEEE standard requires static discharge path provided between shield coax cable ground resistor standard also requires have susceptibility levels electromagnetic interference capacitor will provide
sufficient discharge path from coaxial cable shield ground individual shields should also capacitively coupled Voltage Common typical Ethernet connection diagram using receive mode with Heartbeat enabled shown Figure
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FIGURE Simple Cost Converter
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FIGURE Ethernet Implementation with
CHEAPERNET APPLICATION WITH DP8391 DP8392 assignment both designed minimize crossover printed circuit traces Some components needed Ethernet like interface needed Cheapernet instance Cheapernet's relaxed load capacitance compared with Ethernet) obviates need external capacitance isolation diode Also since transceiver drop cable used Cheapernet there's need termination resistors Moreover without loading differential outputs pulldown resistors both CTI's collision receive drivers SNI's transmit driver larger save power These resistors instead 500X 500X instead 270X crystal connection requires special care IEEE standard requires absolute accuracy transmitted signal frequency external capacitor between pins normally needed required frequency range Section data sheet describes choose value this capacitor
also provides loopback capability fault diagnosis this mode Manchester encoded data internally diverted decoder input sent back controller Thus both encoding decoding circuits tested transmit differential output driver differential input receiver circuits disabled during loopback This mode enabled active high input different modes half step full step selected SNI's transmit output standards require half step mode operation where output goes differential zero during idle eliminate large idle currents through pulse transformers other hand differential output remains fixed state during idle full step mode thus used with transceivers which work either mode different modes selected with input
Figure shows typical Cheapernet connection diagram using DP8391 DP8392
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FIGURE Cheapernet Connection Diagram power isolation similar here Ethernet application except input usually instead Hybrid converters also being developed this application Pulse Engineering PE64381) Figure shows discrete implementation with input output
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FIGURE Converter b9V)
Ethernet Cheapernet Physical Layer Made Easy with DP8391
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LIFE SUPPORT POLICY NATIONAL'S PRODUCTS AUTHORIZED CRITICAL COMPONENTS LIFE SUPPORT DEVICES SYSTEMS WITHOUT EXPRESS WRITTEN APPROVAL PRESIDENT NATIONAL SEMICONDUCTOR CORPORATION used herein Life support devices systems devices systems which intended surgical implant into body support sustain life whose failure perform when properly used accordance with instructions provided labeling reasonably expected result significant injury user critical component component life support device system whose failure perform reasonably expected cause failure life support device system affect safety effectiveness
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National does assume responsibility circuitry described circuit patent licenses implied National reserves right time without notice change said circuitry specifications
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