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USB-to-Serial TTL Adapters (`FTDI` / USBMOD3)

Technical guide to wiring, configuring, and loopback testing FTDI-based USB-to-TTL serial adapters for Honda ECU datalogging.

Intermediate

Adapted from pgmfi.org wiki

Modern laptops lack the legacy RS-232 serial COM ports historically used to connect to automotive tuning hardware. To establish a real-time datalogging connection with a Honda ECU, you must use a USB-to-TTL serial converter.

One highly reliable solution is the Elexol USBMOD3, a breakout module powered by a high-speed FTDI transceiver chip that translates the ECU's 5V TTL serial signals into USB-compliant data packets.

1. Module Configuration & Pin Map

To configure the USBMOD3 module for bus-powered (USB-powered) operation and bypass hardware handshake controls, make the following pin connections on the module's interface:

Source Pin Target Pin Purpose
Pin 4 (Vcc) Pin 8 (Reset In) Pulls the reset line high (prevents accidental resets).
Pin 9 (Enum Power) Pin 10 (Reset Out) Configures the module for bus-powered operation.
Pin 18 (Power Control) Ground (GND) Pulls power control low to enable bus power mode.
Pin 12 (VIO) Pin 13 (V+) Powers the UART I/O pins at the correct internal voltage.
Pin 23 (DSR) Pin 24 (DTR) Bridges Data Set Ready to Data Terminal Ready to bypass hardware modem handshake controls.
Pin 25 (CTS) Pin 26 (RTS) Bridges Clear To Send to Request To Send to bypass hardware flow control.

2. Loopback Diagnostic Test

Before soldering the module to your ECU harness, perform a loopback test to verify that the USB converter chip and PC drivers are functioning correctly:

  1. Bridge Serial Lines: Temporarily connect a jumper wire between Pin 27 (RXD) and Pin 28 (TXD) on the USBMOD3 board.
  2. Connect to PC: Plug the module's USB cable into your laptop. Download and install the standard FTDI Virtual COM Port (VCP) drivers from the official FTDI website.
  3. Identify COM Port: Open Windows Device Manager and expand the Ports (COM & LPT) section. Note the COM port number assigned to the USB Serial Port (e.g., COM3).
  4. Open Terminal Console: Launch a serial terminal application (such as PuTTY or Tera Term) and connect to the identified COM port at 38400 baud.
  5. Type Test: Type several characters into the terminal window.
    • Success: If you see the characters echo back on the screen, the FTDI chip is successfully transmitting and receiving data.
    • Failure: If nothing appears, check your drivers, pin jumpers, and USB cable.

3. Wiring to the ECU CN2 Port

Once the loopback test passes, remove the temporary jumper between Pin 27 and 28, and connect the module to the ECU's CN2 header:

  1. Connect Pin 27 (RXD on module) to Pin 2 (ECU TX) on the CN2 header.
  2. Connect Pin 28 (TXD on module) to Pin 4 (ECU RX) on the CN2 header.
  3. Connect Ground (GND on module) to Pin 1 (ECU GND) on the CN2 header.

Warning

Never connect the ECU's CN2 Pin 5 (12V) to the USBMOD3 board. This will instantly destroy the transceiver module and damage your laptop's USB port.

Credits and source

Authors markolson, blundar

Source Adapted from Usb To Serial Converter Second Gen on pgmfi.org wiki. Licensed under CC BY-NC-SA 1.0.