RS-232C INTERFACING RS-232à ió thå mosô recenô versioî oæ thå EIÁ  (Electronicó Industrù   Association©  standarä  foò  lo÷  speeä  seriaì   datá communication®  Iô  defineó  á numbeò oæ  parameteró  concerninç voltagå levels¬ loadinç characteristicó anä timinç relationships® Typicaì  practicå requireó mountinç femalå connectoró (DB-25S© oî thå  chassió oæ thå communicationó equipment¬  anä malå  (DB-25P© connectoró oî thå cablå connectinç twï sucè devices. Therå  arå twï maiî classeó oæ RS-232à devices¬  DTÅ  (Datá Terminaì  Equipment©  anä  DCÅ (Datá  Communicationó  Equipment)® Terminaló arå typicaì DTE's¬ anä DCE'ó maù bå printeró oò modems® Typically¬  onå onlù interfaceó á DTÅ tï á DCE¬ aó opposeä tï onå DTÅ tï anotheò DTE¬ oò onå DCÅ tï anotheò DCE® Therå arå wayó oæ interfacinç twï likå deviceó bù buildinç á non-standarä cable® Iæ onå oæ thå deviceó involveä ió á modem¬  thå 'hand-shaking§  lineó musô bå supported® Theså lines maù bå useä tï allo÷ thå receivinç devicå  tï  causå  thå  transmittinç devicå tï  pauså  whilå  thå receivinç devicå catcheó up. RS-232à ió intendeä foò relativelù shorô (5° feeô oò  less)¬ relativelù  lo÷  speä (19,20° bits/seconä oò lesó (BAUD)©  seriaì communication®  Botè  asynchronouó anä synchronouó seriaì  encodinç arå supported¬  anä Digitaì signaló arå useä aó opposeä tï  Analoç signals®  RS-232à wilì supporô simplex¬  half-duplex¬  oò full-duplex channels. Á  Simpleø channeì meanó thaô communicatioî ió onå  way®  Aî examplå  woulä  bå  á receivå  onlù  Printer®  Iî  á  half-dupleø channel¬  datá  maù traveì iî eitheò direction¬  buô aô anù  giveî timå datá wilì onlù bå travellinç iî onå direction® Thå linå musô bå  turneä arounä beforå datá caî traveì iî thå otheò  direction® Iî  á  full-dupleø  channel¬  datá maù traveì iî  botè  directionó simultaneously. Therå arå twentù fivå circuitó whicè makå uð thå  interface® Iî thå simplesô case¬  aî interfacå maù bå implementeä witè aó fe÷ aó  twï  circuits®  Á circuiô ió defineä tï bå á continuouó  wirå froí onå devicå tï another. DEFINITION OF THE MOST COMMON CIRCUITS 1. CG Chassis Ground This circuit is Frame Ground and insures that the two devices are at the same potential, to prevent electrical shock to the operator. This circuit is optional and is not used as the reference for any other circuit. Š 2. TD Transmit Data This circuit is the path whereby serial data is sent from the DTE to DCE. This circuit must be present if data is to travel in that direction. 3. RD Receive Data This circuit is the path whereby serial data is sent from the DCE to DTE. This circuit must be present if data is to travel in that direction. 4. RTS Request To Send This circuit is the signal that indicates that the DTE wishes to send data to the DCE. Note that the DTE must always be ready to accept data. Once the DTE has data to send it will set RTS to ON (logic 0), and wait for an on condition on CTS from the DCE. After sending the data the DTE will set RTS to OFF (logic 1). 5. CTS Clear To Send This circuit is the signal that indicates that the DCE is ready to accept data from the DTE. In normal operation it will be in the OFF state. When the DTE asserts RTS, the DCE will do whatever is necessary to allow the data to be sent. At this time the DCE would set CTS to the ON state, which would allow the DTE to send the data. Note that the DTE must have an incoming CTS before it can transmit. 6. DSR Data Set Ready This circuit informs the DTE that the DCE is alive and well. It is normally set to the ON state by the DCE on power-up and left there. The DTE must have an incoming DSR in order to function normally. 7. SG Signal Ground This circuit is the neutral to which all other voltages are referenced. It must be present at all times. (It is the common line for all DC voltages.) 8. DCD Data Carrier Detect This circuit is the signal whereby the DCE informs the DTE that it has an incoming carrier. It may be used by the DTE to determine if the channel is idle, so that the DTE can request it with RTS. Note that some DTE's must have DCD before they will operate. 15. TC Transmit Clock This circuit provides the clock for the transmitter section of a synchronous DTE. It may or may not be running at the same rate as the receiver clock, and must be present on synchronous interfaces. 17. RC Receiver Clock This circuit provides the clock for the receiver section of a synchronous DTE. It may or may not be running at the same rate as the transmitter clock, Š and must be present on synchronous interfaces. Note that both RC and TC are sourced by the DCE. 20. DTR Data Terminal Ready This circuit provides the signal that informs the DCE that the DTE is alive and well. It is normally set to the ON state by the DTE at power-up and left there. Note that a typical DCE must have an incoming DTR before it will function normally. Thå  followinç  tablå  listó alì  thå  informatioî  foò  thå variouó circuitó whicè makå uð thå RS-232à interface. PIN NAME EIA CCITT DTE DCE FUNCTION 1 CG AA 101 == == Chassis Ground 2 TD BA 103 >> Transmit Data 3 RD BB 104 << Receive Data 4 RTS CA 105 >> Request To Send 5 CTS CB 106 << Clear To Send 6 DSR CC 107 << Data Set Ready 7 SG AB 102 == == Signal Ground 8 DCD CF 109 << Data Carrier Detect 9* << Pos Test Voltage 10* << Neg Test Voltage 11 Not Used (usually) 12+ SCDC SCF 122 << Secondary DCD 13+ SCTS SCB 121 << Secondary CTS 14+ STD SBA 118 >> Secondary TD 15# TC DB 114 << Transmit Clock 16+ SRD SBB 119 << Secondary RD 17# RC DD 115 << Receive Clock 18 Not used (usually) 19+ SRTS SCA 120 >> Secondary RTS 20 DTR CD 108.2 >> Data Terminal Ready 21* SQ CG 110 << Signal Quality 22 RI CE 125 << Ring Indicator 23* CH 111 >> Data Rate Selector CI 112 << Data Rate Selector 24* XTC DA 113 >> Ext. TC 25* >> Busy * Denotes rarely used + Denotes only if secondary channel implemented # Denotes only on synchronous interfaces >> Denotes from DTE to DCE << Denotes from DCE to DTE == Denotes Ground line CCITÔ  ½  Internationaì  Telegrapè  anä  Telephonå   Consultativå Committee TYPES OF DATA COMMUNICATIONS Therå  arå  twï  wayó oæ sendinç datá  serially®  Theù  arå ŠAsynchronouó  anä  Synchronous®   Asynchronouó  seriaì   involveó sendinç  informatioî  witè preciså timinç betweeî bitó oæ á  giveî character¬ buô witè arbitrarilù lonç intervaló betweeî characters® Iî ordeò tï allo÷ thió variablå lengtè interval¬ certaiî overheaä informatioî ió requireä oî eacè character®  Normally¬  durinç  aî inter-characteò  gað  thå linå ió aô á logiã ± state®  Wheî  thå characteò starts¬ á starô biô (logiã 0© is alwayó first® Oncå thå starô  biô  haó  occurreä  thå  otheò  datá  bitó  follow¬   leasô significanô  biô  first®  Therå  arå normallù · oò  ¸  datá  bitó followeä bù á paritù bit® Iæ runninç witè "eveî parity"¬ thió biô wilì  bå  seô  oò reseô tï makå aî evån numbeò oæ  oæ  datá  bits® Followinç thå paritù biô (iæ used© arå ± oò ² "stoð bits"® Theså stoð bitó arå alwayó á logiã ± anä simplù insurå thaô thå linå ió idle  foò onå oò twï biô time(s© whilå thå hardwarå re-cycleó  foò thå   nexô   starô   biô. Á   UARÔ   (Universaì    Asynchronouó Receiver/Transmitter©  caî bå useä tï converô paralleì tï oò  froí Asynchronouó seriaì data. Synchronouó seriaì involveó sendinç characteró noô onlù witè preciså  timinç betweeî bitó oæ á giveî character¬  buô aô regulaò intervals®  Datá  ió  normallù seît iî largå blockó witè  nï  gað betweeî  characters®  Nï overheaä ió requireä oî eaãh  character¬ althougè  paritù  ió  stilì sometimeó  used®  Therå  ió  certaiî informatioî  requireä  aô  thå  starô  anä  enä  oæ  eacè   block® Synchronouó  datá transmissionó aòe normallù morå intelligenô thaî asynchronous¬  anä  maù requesô re-transmissioî oæ blockó  receiveä witè errors. ELECTRICAL CHARACTERISTICS Thå characteró transmitteä bù aî RS-23² devicå arå senô  ouô tï  thå I/Ï porô á thå bauä ratå selecteä bù internaì switcheó oò software®  Thå characteò ió made uð oæ · oò ¸ bitó aó defineä  bù thå appropriatå codå chart® Foò ASCIÉ characteò thå capitaì Á ió senô  aó  4±  hex®  Thå binarù codå ió 010° 000±  witè  ±  beinç the leasô  significanô  bit¬  anä thå lefô mosô ° beinç  thå  mosô significanô  bit®  Thå followinç diagraí ió whaô ió seeî aô  thå outpuô port® Biô flo÷ ió froí lefô tï right. ---- -------------------- ---- stop S 1 0 0 0 0 0 1 0 bits ---- ---- -------- Logic 1 is low as shown. S Denotes start bit NON-STANDARD WIRING CONNECTIONS Therå  arå severaì wayó oæ hookinç twï non-standarä  RS-232à deviceó together¬ buô thå followinç threå arå thå mosô common® Iæ Šonlù  onå waù communicatioî ió requireä á minimuí oæ twï wireó  ió alì  thaô  wilì bå required®  Alì handshakinç conditionó  caî  bå satisfieä bù loopinç bacë thå appropriatå signaló oî botè endó oæ thå  cablå  (seå  Nulì Modeí diagraí below)®  Thå  threå  typicaì connectionó arå nulì modem¬ nï handshake¬ anä fulì handshake® Thå followinç diagramó sho÷ thå necessarù wiring®  Bå carefuì tï notå thaô thå manufactureò maù wirå thå devicå eitheò aó á DTÅ oò DCE® Consulô thå manuaì foò finaì verificatioî oæ pinouts. NULL MODEM NO HANDSHAKE FULL HANDSHAKE 1********1 1********1 1********1 2--------3 2--------3 2--------3 3--------2 3--------2 3--------2 4-| |-4 7--------7 4--------5 5-| |-5 5-| |-5 5--------4 7--------7 6-| |-6 7--------7 6-| |-6 8-| |-8 6------|-8 8-| |-8 20-| |-20 |-20 20-| |-20 8-|------6 20-| NOTE: Pin 1 is optional for Chassis Ground between Terminals. This information furnished for all computer users. I have spent many hours to find the above information the hard way. LARRY KRAEMER R 2 BOX 190 JACKSON, MO. 63755