I've got a couple of TTL USB/serial converters. I think they're FTDI chips.

As well as pins marked TX and RX, there are unpopulated holes marked TXD, TXL, RXD and RXL.

I've previously thought that TXD and RXD were aliases for TX and RX. Is this true?

What could TXL and RXL be?

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    \$\begingroup\$ I think they're FTDI chips. You mean that they use FTDI chips. Look at the model numbers on the chip and find the datasheet. TXL and RXL might be level shifted versions (for example 3.3 V) of the other signals which are often 5 V. But without knowing what actual chips are used, this is just a guess. \$\endgroup\$ May 30, 2018 at 8:04
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    \$\begingroup\$ TXL = TX LED and RXL = RX LED. You will need a resistor and the LED, of course. Might be "active low". \$\endgroup\$
    – Turbo J
    May 30, 2018 at 11:06

1 Answer 1


If the SparkFun FT232RL Breakout is anything to go by, then the TXD and RXD pins are physically connected to the TX and RX pins. This is because the breakout board can support two configurations, then 6-pin edge connector, and the full 18-pin PCB arrangement.

The TXL and RXL are likely status LED pins used to indicate when a TX packet is being sent and an RX packet being received respectively. Sparkfun calls them TXLED and RXLED respectively, which have the same pin locations as the clone boards that call them TXL/RXL.

In summary:

  • TXD = TX = Transmit
  • RXD = RX = Receive
  • TXL = TX LED = Transmit In Progress LED
  • RXL = RX LED = Receive In Progress LED

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