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Optically Isolated USB to Serial (TTL) Interface - Used to isolate equipment grounds



  • USB to TTL level Serial (FTDI) Interface
    • Genuine FTDI FT230XS part
    • Standard FTDI drivers
  • Buffered lines on the TTL level side
    • Schmidt Trigger
      • Voltage level hysteresis
      • Improved slew rate
  • OptoIsolated Connections
    • Transmit, Receive, RTS, CTS
    • Configured for High Speed Serial
  • The Host USB connection is independent of the state of the FTDI side of the interface
    • Host side can communicate with the USB host whether or not the target is even powered up
    • Target power on/off doesn't cause a USB disconnect - handy for debug
  • Fast operation with 5V targets
  • Transmit/Receive LED Connectors
    • On-board LED resistors
  • ODAS 49mm Form Factor
    • 49x49mm card
    • (4) #4 Mounting Holes

Block Diagram


Separate Power Domains

  • The OPTOUSB card has two distinct power domains.
    • One of the domains is the USB side where the power comes from the USB connector/cable.
    • The other power domain is on the TTL side where the power comes from the FTDI connector.
  • This is unlike "normal" FTDI interfaces where the USB connection can power the FTDI side.
    • This is the purpose of the optical isolation - to isolate the ground of the USB side from the target (FTDI) side.


J1 - Mini USB Connector

  • Standard USB Mini connector

J2 - Transmit/Receive LED connections

  1. PUP0 - LED Pullup to USB Power
  2. TXLED - Transmit LED
  3. PUP1 - LED Pullup to USB Power
  4. RXLED - Receive LED

J3 - FTDI Connector

  • Signals are references to external system
  1. GND
  2. RTS (In to card)
  3. VCC (5V) - Power must be supplied to this pin
  4. Receive (Out from card)
  5. Transmit (In to card)
  6. CTS (Out from card)




Transfer Rate Characterization

  • LTSpice Simulation results
  • 8.6uS per bit = 115,200 baud
    • May not work in all applications at 115,200 baud but should work at 57,600 baud
    • For example, loopback does not work at 115,200 baud but we have run it with read UARTs on the other end and it did work

OPTO-USB LTSpice Simulation.png

Card Test


OPTOUSB Tester Card-Annotated-720pxV.jpg

  • BLUE LED is Tx from FTDI card
  • GREEN LED is Tx from Tester card


  • UUT
  • Dedicated Tester Card
  • 5V Power Supply
  • Cables
    • 6-Pin 1:1 cable (FTDI)
    • 4-Pin 1:1 cable (LEDs)
    • USB A-Mini cable
  • PC that can run puTTY
  • Run Device Manager to determine COM port
  • Run puTTY
    • Set port from Device Manager
    • Set baud rate to 57600
    • Set Hardware Handshake to RTS/CTS
  • Tester card settings
    • Left Jumper Block - Data loopback
      • Left position (Opens tx/rx connection) - data should not be echoed
      • Right position (Closes tx/rx connection) - data should be echoed
    • Center Jumper Block - Controls Loopback High/Low - Works when right switch is in the right position
      • Left position - Ground handshake back to PC (allows host transmit
      • Right position - Pulls handshake to PC up to VCC (shuts off host transmit)
    • Right Jumper Block - Controls handshake loopback or signal insertion
      • Left Position - Select voltage from center switch
      • Right Position - Loopback RTS/CTS from the FTDI connector directly


  • Connect cables between UUT and Tester
  • Set jumper block (l-r) to right, left, right)
  • Power up Test Card (5V)
  • Connect USB cable to UUT
  • Run puTTY
  • Type and verify data comes back
  • Verify Green and Blue LEDs on the Tester light with data being typed
  • Move left jumper block to left position
  • Type and verify data does not come back
  • Verify the BLUE LED is lit and the GREEN LED is not lit
  • Move left jumper block to right position
  • Move the right jumper block to the left position
  • Type and verify data comes back
  • Move the center jumper block to the right position
  • Type and verify data does not come back
  • Lights should also not light

Rework - Rev X1 PWBs


Parts List / Assembly Sheet

OPTOUSB PL and Assembly Sheet