USB-TTL Interface board using the FTDI FT232RL

USB-Serial interfaces

Preamble

There are a number of different devices, or boards, that convert from USB to RS232 serial signals. They all carry out, more or less, the same function, the main difference being the choice of IC used to implement the interface. There are a range of ICs that are used, most commonly:

  • FT232RL
  • CH340G
  • CP2102
  • PL2303

All work with Windows. Some work with OS X, some do not, see USB-TTL interface devices on OS X. With Linux, mileage may vary.  Some implement the Rs232 interface well, others poorly.

Looking briefly at each variant in turn:

FT232RL

USB-TTL Interface board using the FTDI FT232RL
USB-TTL Interface board using the FTDI FT232RL

The IC used on this board is made by FTDI, and is generally regarded as the better of the bunch, due to its support across various OS platforms, and its rich and full implementation of the RS232 standard. It is, generally, the most expensive though (although that is only by a matter of a few cents)

Here is a schematic

Schematic of USB-TTL Interface board using the FT232RL
Schematic of USB-TTL Interface board using the FT232RL

Here is the pin out

FT232RL Pinout
FT232RL Pinout

CH340G

USB-TTL Interface board using the CH340
USB-TTL Interface board using the CH340

This clone is often used in knock off Arduino Nano boards as it is a relatively inexpensive alternative to the FT232RL. It is a smaller IC than the FT232RL, with fewer outputs.  Boards implemented with this IC rarely offer the DTR, CTS, RTS or other RS232 signals, often only implementing the Rx, Tx, ground and two power (5V and 3V3) lines, with Vcc selected using a jumper on the output pins.

There are more sophisticated versions available, such as this one with a switch to select voltage, which also implements the DTR and CTS lines

USB-TTL Interface board using the CH340 with voltage switch and CTS/DTR
USB-TTL Interface board using the CH340 with voltage switch and CTS/DTR

Here is a schematic of a board that implements CTS and DTR

Schematic of USB-TTL Interface board using the CH340G
Schematic of USB-TTL Interface board using the CH340G

Here is the pin out

CH304G Pin out
CH304G Pin out

and legend

CH340G Pin out table
CH340G Pin out table

CP2102

USB-TTL Interface board using the CP2102
USB-TTL Interface board using the CP2102

Interface boards implemented using this square IC are instantly recognisable. These boards often implement many of the RS232 lines. However, there are some that do not, such as this one

USB-TTL Interface board using the CP2102 with only Rx and Tx lines
USB-TTL Interface board using the CP2102 with only Rx and Tx lines

or this one that offers only DTR, labelled as RST, for the Arduino

USB-TTL Interface board using the CP2102 with only DTR (Rst), Rx and Tx lines
USB-TTL Interface board using the CP2102 with only DTR (Rst), Rx and Tx lines

Here is a schematic

Schematic of USB-TTL Interface board using the CP2102
Schematic of USB-TTL Interface board using the CP2102

Here is a pinout

CP2102 Pinout
CP2102 Pinout

PL2303

USB-TTL Interface board using the PL2303
USB-TTL Interface board using the PL2303

This is one of the “clone” interface boards, with limited functionality. They rarely offer the DTR, CTS, RTS or other RS232 signals, often only implementing the Rx, Tx, ground and two power (5V and 3V3) lines. In the example below, only one voltage pin is given, with the voltage selected using a jumper:

USB-TTL Interface board using the PL2303, with voltage selection jumper
USB-TTL Interface board using the PL2303, with voltage selection jumper

Here is a schematic of an implementation that offers RTS and DCD. The pinout is also illustrated.

Schematic of USB-TTL Interface board using the PL2303
Schematic of USB-TTL Interface board using the PL2303

FT232RL with ISP

There is a variant of the FT232RL board, which has an additional ISP/ICSP connector, although it lacks the breakout pins down the sides.

These ISP pins can be used for bit banging, as explained in Nick Gammon’s excellent answer to FTDI Breakout with additional ISP connector.

Here is a schematic

Schematic of USB-TTL Interface board using the Ft232RL with ISP header
Schematic of USB-TTL Interface board using the Ft232RL with ISP header
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