1682 Logic level fiber optic serial interface

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Documents: Schematic

Description:  The 1682 logic level RS232 to fiber optic serial interface uses Broadcom transceivers and a duplex fiber cable to provide high speed full duplex communication with the ultimate in galvanic isolation.  This interface is perfect for high voltage control or completely isolated RF systems.  When paired with the 1505 USB to fiber module, the user has a simple to set up fiber optic link.


Serial specifications:

  • Full duplex operation using duplex fiber.
  • Protocol agnostic, i.e. start, data parity and stop bits are not limited by this interface.
  • Interface optimized for 115.2kbps with a 100M cable run with properly terminated optical cable/connectors.
  • Longer runs at lower baud rates or shorter runs with higher baud rates possible (See transmit power notes).
  • The transmitter consumes no power during idle.

Transmit power:

  • The transmitter LED current is set by default to 60mA using a 51.1 ohm 2010 (1/2W) resistor at R1.
  • The transmit current may be increased up to 80mA to support longer cable runs or to a lower value to conserve power during transmit by changing the value of R1.
  • R1 = (VCC-1.5V)/IF:  For 80mA the value of R1 would be 43 ohms and for 15mA R1 would be 221 ohms.
  • Maximum data rate in all cases is limited to 10Mbps although the practical limit for asynchronous RS232 is 460.8kbps to maintain the specification's sample point requirements.

Input power:

  • The logic interface that powers the RXD and TXD buffers operates independently of VCC and VIN and has an input range of 1.65 to 5.50V.
  • VCC may be used to power the optical interface (VIN open) with a range of 4.50 to 5.50V.
  • VIN may be used to power the optical interface (VCC open or loaded) with a range of 5.40 to 26.0V.
  • When powered from VIN, VCC is a regulated 5.0V that may be loaded up to 300mA.
  • VIN is tolerant of inputs down to -20V and transients up to +60V.  Positive transients must be limited to <100mS and <1% duty cycle typical of an automotive load dump situation.
  • Input current during idle is 15mA and 75mA during transmit with the standard 51.1 ohm R1.

VIN considerations:

  • The on board regulator from VIN to VCC has a thermal resistance θjc of 15°C/W with a maximum junction temperature of 125°C.
  • Input voltage and load current at VCC must be limited to ensure the junction temperature does not exceed the limit during the worst case transmit duty cycle.
  • The regulator is thermally protected and will shut down if its junction temperature is exceeded.
  • The regulator is load protected and will fold back its voltage if its current limit is exceeded.

Interface connector: JST PHR-6

  • 1     Ground
  • 2     RXD (Received bits out of module into processor RXD)
  • 3     TXD (Transmitted bits into module out of processor TXD)
  • 4     VIO  (1.65 to 5.50V for RXD and TXD interface)
  • 5     VCC (4.50 to 5.50V for optical interface.  May be loaded to 300mA when VIN supplied)
  • 6     VIN  (5.40 to 26V alternative input to supply VCC with 5.00V)

Mechanical outline:


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