USB to RS-485 Converter

Replacement: BOB-09822. This board has been replaced with a new version which fixes labeling and other issues. Once this board sells out, it will be gone! Get it cheap! Check comments for fixes and such. It has problems, but for $5, you should be able to find a use for it still. The revision is here.

This breakout board pairs an SP3485 RS-485 transceiver with an FT232RL USB UART IC to convert a USB stream to RS-485. The SP3485 is a half-duplex transceiver, so it can only communicate one way at a time, but it can reach transmission speeds of up to 10Mbps.

The RTS pin of the FT232RL is connected to the transmit and receive enable inputs of the SP3485, this line is used to control the transmission mode of the RS-485 transceiver. With the proper drivers installed, the FT232RL will enumerate as a virtual COM port; the drivers are available for Windows, Mac and Linux.

This breakout board includes the SP3485, FT232, TX/RX/RTS LEDs, miniB USB connector, filter capacitors, and other components shown on the schematic. We've broken out the RS485 output to three different connections: (1) an RJ-45 connector, (2) a 3-pin 3.55mm screw terminal, and (3) a 3-pin 0.1" pitch header; none of these output connectors come populated.

  • Fully equipped with SP3485 RS-485 transceiver and FT232RL USB UART IC
  • Operates from USB supply
  • RX, TX, and RTS LED indicators
  • miniB USB connector
  • USB power and RX/TX lines broken out to a 0.1" pitch header
  • RS-485 input/output broken out to RJ-45 connector, 3.5mm screw terminal, and 0.1" pitch header
  • Driver/Receiver Enable connected to RTS line of FT232RL
  • -7V to +12V Common-Mode Input Voltage Range
  • 1.55x0.9 inches


Looking for answers to technical questions?

We welcome your comments and suggestions below. However, if you are looking for solutions to technical questions please see our Technical Assistance page.

  • smee / about 15 years ago / 2

    It might be nice to have room for a header that connects to the unused pins of the RJ45. In the past, I've run RS485, power, and 1-wire through the same cat5 cable with good results. A header connecting to those pins on this board would make that easier.

  • Ocean Controls / about 15 years ago / 2

    This uses the RTS line to drive the Transmit enable, which means the controlling application must set the RTS line for long enough before sending out the data and turn it off straight after the last character, otherwise the first couple of characters might get scrambled in transmit and receive.
    It would be more sensible to use the TXDEN function (defaults to pin 13) in the FT232RL, which is designed to drive a 485 transceiver and will be set long enough before any data goes out the Transmit line, without the need for flow control on the controlling application.
    Maybe in Version 2?

    • Eeek! I'm sorry I didn't see this sooner. We'll get this board updated as quickly as possible.

  • Garak / about 14 years ago / 1

    If I wanted to use these for TTL serial do I need to cut the trace from the output of the SP3485 to the FT232RL?

    • AaronCarlton / about 14 years ago / 1

      You should not have to cut any pins, you should be able to use JP3, with the board as is.

  • GlennE / about 14 years ago / 1

    Cannot get this module to work with my existing 485 adaptors.
    I can recieve with this module but cannot transmit from this module. I noticed the RTS line did not come on. Also, tried setting windows terminal program to use hardware flow control but made problem worse.
    Please Help

  • Thanks Jeremy! I've updated the design to connect TXDEN to DE and kept RTS connected to the LED.

  • This is a working hack to bypass RTS and use TXDEN from the FT232RL pin 13:
    I even fixed the LED. This thing is fairly useless without the hack -- getting RTS working for this purpose is nearly impossible. SFE: Any idea why this wasn't set up like this in the first place?

  • Ocean Controls / about 15 years ago / 1

    Also, the board is labelled with the "correct" A=Inverting (D-) and B=Non Inverting (D+) signal lines, but it is quite commonly used the other way around, (like in the data sheet for the SP3485), so it can be confusing as to which wire needs to be connected where.

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