1682 Logic level fiber optic serial interface

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Documents: Schematic (Shown with 1528/2528 (51.1ohm), now using the 1529/2529 (100ohm))

Description:  The 1682 fiber optic to logic level RS232 serial interface uses Broadcom transmitters and receivers to provide a high speed full duplex communication channel with the ultimate in galvanic isolation.  This interface is perfect for remote control for high voltage systems or control of ground-isolated RF systems.  A complete computer controlled data link can be implemented when this module is paired with the 1505 USB to fiber module which enumerates as a virtual com port.


Serial specifications:

  • Full duplex operation using duplex fiber.
  • Protocol agnostic, i.e. start, data parity and stop bits are not controlled 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.
  • The module consumes 20mA max during idle, the transmit LED is off during data idle (High logic level).

Transmit power:

  • The transmitter LED current is set to the maximum current of 30mA using a 100 ohm 2010 (1/2W) resistor at R1.
  • The current can be set to lower values to support shorter cable runs or lower baud rates by changing the value of R1.
  • R1 = (5V-1.5V)/IF:  For 15mA R1 would be 230 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 20mA (Idle) and 50mA (Transmitting) with the standard 100 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

  • Provided with a 12 inch ribbon cable terminated with the required PHR-6 connector
  • 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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