High-Speed RS-232/RS-485/RS-422
Single-Mode Fiber OpticModem
User s Manual
MODEL277SM
1.0 Description:
TheMODEL277SM wasdesigned toprovide themost
versatileconnectionpossible between anyasynchronous
serialequipmentusingFiber Opticcable. It allowsanytwo
piecesofasynchronous serialequipmentto communicate full
or half-duplexover two fibers at typicaldistancesupto20KM.
Theconvertercanalsobesetupin"Repeater"modeto
createamulti-dropmaster/slaveconfiguration,allowing one
serialdevicetotalkto multipleslavedevicesaroundafiber
ring.
RS-232data signals at upto115.2K bpsandRS-422,or
RS-485datasignalsat upto 115.2Kbpsaresupported.
Differentstandards canbe mixed and matched to allow
RS-232devices toconnecttoyourRS-422or RS-485
system.ThismeanstheMODEL277SM canreplace
convertersandisolatorswhen connectingremotedevices,
while providing the EMI/RFI andtransient immunity ofoptical
fiber.
The MODEL277SM supports boththe Transmit andReceive
datalines, and provides full hardwarecontrol of theRS-422
/485driverwithautomaticSend DataControl circuit.
Timeoutsare autodetectbetween 0.10 and2.2 ms. All serial
connectionsareprovidedontheDB-9 femaleconnectoror
6-bit terminal, whilethesingle-mode fiberisconnectedvia
two SC connectors.Theunit ispoweredby9VDC at 140mA
max.
2.0 RS-232 Connections:
Connection oftheMODEL277SM issimpleandstraight
forward.TheDB-9 female serial connectoris usedfor
connectingto RS-232.The RS-232signalsare pinnedas a
DTE device (input onPin2 and output onPin3)
RS-232 Connection Diagrams
DB-9 Male
(PIN)
RS-232
2 RXD
3TXD
5 GND
3.0 RS-422 & RS-485 Connections:
RS-422/RS-485 Connection Diagrams
6-Bit
Terminal
-422RS-485
PIN1 T+ 485+
PIN2 T- 485-
PIN3 R+ NC
PIN4 R- NC
PIN5VIN VIN
PIN6GND GND
4.0 Fiber Optic Connections:
TheMODEL277SM uses aseparate LED emitterand
photo-detector operatingat 1310nm wavelength Connections
.totheemitterand detectorareonST typeconnectors.
Almostany multimodeglassfiber size can beusedincluding
50/125 m,62.5/125 m, 100/140 m,and200 m. Onefiber
isrequiredforeach connectionbetweenatransmitterand
receiver.In apoint-to-point configuration,twofibersare
requiredbetweenthetwo modems, onefor dataineach
direction.Amulti-dropringconfigurationrequiresone fiber
betweenTX and RX aroundthe loop. SeeFigure3 for typical
point-to-point andmulti-drop configurations.
Themostimportantconsiderationinplanningthefiber
opticlinkisthe “power budget”of the fiber modem. This
value represents theamountofloss in dBthatcan be
present inthe link between thetwo modemsbefore the units
fail to perform properly.Thisvalueincludeslineattenuation
aswell as connector loss. FortheMODEL277SM thetypical
connector-to-connector powerbudget is 13dB. Because
62.5/125 mcable typically hasalineattenuation of 0.6dB
per Km at 1310 nm, the 12dB
power budgettranslatesinto 20KM. This assumesnoextra
connectors orsplicesin thelink.Each extra connection
would typicallyadd 0.5 dB of loss,reducingthe possible
distance by166m(547 ft.). Theactuallossshouldbe
measured before assumingdistances.
6.0Multi-DropOperation:
Amulti-dropconfigurationis createdby formingaringof
MODEL277SM. Each transmitter is tiedtothefollowing
converter’sreceiver,startingat amaster nodeandcontinuing
aroundto eachslaveand back tothe master.By setting
JP1(onPCB Mainbroad) to the“1-2”position ontheslaves,
“2-3”positionon the Master,all data sent from themasteror
precedingslaves is echoed backout thefibertransmitterto
therestof theslavesandeventually backto the master
node.Becauseall data isechoedback,thereare special
considerationswhen constructingamulti-dropsystem. The
master will seeit’sowntransmitteddata. This means thatthe
masterdevice mustbe full-duplex (RS-232,RS-422, or
