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I have an APU2 that diverts any 'display' over a serial console (DB9F). For accessing the serial console, a null modem cable is recommended where the wires are crossed over. However, my computer does not have a DB9 port but I do have a FT232RL based USB to UART converter. Would the wires crossover apply for the FT232 module as well? For e.g., would the RX of the FT232 go to the TX of the APU2 serial port, DSR to DTR and so on?

Edit: just looked at the board again and it uses the MAX3243 for its serial comms. This appears to be a 3/5V chip.

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  • \$\begingroup\$ yes, if the USB to serial adapter is using a standard pinout \$\endgroup\$
    – jsotola
    Commented May 31, 2020 at 3:11
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    \$\begingroup\$ Make sure that your USB-to-serial adapter accept the ±15 V voltage of RS232 adapters. \$\endgroup\$
    – mguima
    Commented May 31, 2020 at 3:49
  • \$\begingroup\$ Be aware that RS232 and UART have inverted logic levels, so it would be better to use an adapter to RS232 not a UART one. \$\endgroup\$
    – Reroute
    Commented May 31, 2020 at 6:40
  • \$\begingroup\$ Please state which FTDI module it is. If it does not use RS232 voltage levels, it cannot be used. \$\endgroup\$
    – Justme
    Commented May 31, 2020 at 7:03
  • \$\begingroup\$ Your edit reaches a backwards conclusion: on "that board" (which ever end it is) you have an RS232 line driver/receiver which means it uses higher signal voltages, 3/5v would be the supply. Also your APU2 uses RS232 levels. You need an RS232 solution on both ends. Suitable cables are available off the shelf, this is essential a computer usage question not a circuit design one. \$\endgroup\$
    – Chris Stratton
    Commented May 31, 2020 at 11:02

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If your USB to Serial adapter is one that comes with its own DB-9 connector then that connector supports the typical RS232 voltage levels of the signalling and has connector pin out that matches to the standard serial pinning of a legacy PC serial port. In this case it would be a regular "cross over" cable wiring to connect the two. Alternatively there used to be available DB-9 null modem adapters that contained the cross over internally so you could then use straight wired cables to complete the connection.

Here is a picture of a typical USB to RS232 adapter of this type.

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This type of adapter typically has two chips inside. One chip is a USB to UART converter device and the other is a charge pump level translator chip to produce the positive to negative swings of the RS232 signalling. Note that this level translator also inverts the sense of the UART level signals from the USB-UART adapter.

These days with the combination of most PCs and laptops losing RS232 serial ports AND the proliferation of embedded MCU platforms like Arduino and rPi the use of simpler USB to UART adapters has come to be very common. These adapters just have one chip inside which is the USB to UART device and do not include the charge pump level translator chip. Such adapter would not be compatible for connection to a DB-9 RS232 port. Here are some pictures this latter type of adapter.

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