USB Rig Interface - SARCNET

School Amateur Radio Club Network
School Amateur Radio Club Network
School Amateur Radio Club Network
School Amateur Radio Club Network
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USB Rig Interface
This project is a USB Audio, Serial and PTT interface for rigs (Amateur Radio transceivers), which don't directly support digital modes. Plug these interface devices into your Windows or Linux laptop or Raspbian Raspberry Pi and you can send and receive digital tones from your rig and automate the frequency and mode control (for rigs which support a serial interface) and PTT control.

The following schematic diagram shows the use of the USB Audio Interface and the USB Serial Interface described below.


  • If the rig supports VOX operation, via the selected audio port (VOX is often not provided on auxilliary ports), only the USB Audio Interface is required as the tones when present will automatically key the radio.
  • For direct microphone and speaker connection to the USB Audio Interface, an additional in-line, audio attenuator may be necessary. This is often just a suitable series resistor, as impedance transformation is not critical.
  • If the rig does not support VOX operation, or remote control of frequency and mode is needed, the USB Serial Interface is required. In this case configure the software for the serial port used by the USB Serial Interface.
  • If the rig does not support PTT operation via its serial remote-control port, an additional optoisolator is required for PTT operation. In this case the software must support PTT via toggling the DTR or RTS signals.
  • The additional LED in the optoisolator circuit is required to accomodate the TTL levels.

USB Audio Interface
The USB Audio Interface uses a CMEDIA CM108 7.1CH USB Sound Card, which is one of the few devices which actually works on Windows PCs, Linux PCs and the Raspberry Pi. The original case and the 3.5mm phono jacks and LEDs are removed from the PCB. Two ETAL P2781, 600 Ohm, microprofile, audio line transformers are then glued onto the PCB and connected to the original PCB pads using 0.4mm solderable enamelled copper. Shielded cables, with suitable connectors for your rig, are then soldered to the transformers. The whole assembly is then enclosed in 19mm, dual wall, heatshrink tubing. Note: There are two different sound card PCB layouts available, type 1 and type 2. Please check which one you have.

USB Sound Card

USB Rig Audio Interface with Type 1 Sound Card

USB Rig Audio Interface with Type 2 Sound Card
VOX Circuit
This VOX circuit is for PTT control of transceivers which do not have serial or USB CAT ports. Note that, while some CM108 sound cards permit the PTT signal to be derived from GPIO3 on pin 13 of the CM108, not all CM108 sound cards actually expose this pin. In this case, a simple VOX circuit can be used for PTT instead. It detects the digital-mode tones from the USB sound card output and then activates the rig's PTT line, causing transmission of tones when they are present. It is powered by the 5v supply from the USB port. It uses an opto-coupler to provide PTT isolation. When used together with the transformer-coupled USB sound card, the computer and the rig will be galvanically isolated.  

Here is how it works: Q1 amplifies the audio signal. D1, D2 and C3 detect the tone waveform. Q2 drives the opto-coupler and a handy TX LED when a tone of above about 0.25Vpp is present on the input.

VOX Circuit Schematic
The performance of the VOX circuit is adequate for most digital modes: The attack time is under 1ms. The hold time is about 5ms. It works well with WSJT-X and FT8.

VOX Circuit Performance
The unit was constructed by wiring the CM108 sound card to a PTH prototyping board, before adding the surface-mount components required for the VOX circuit and the isolation transformers for the sound card. PTH components could be used instead. All wiring was performed using solderable enamelled copper wire. A 9-pin D connector was used to connect to a variety of rigs, using specific cables for each one. The pins on the D connector were standardised, as follows: Pins 1, 2 & 3 are reserved for 12V/RXD/TXD. Pin 4 PTT. Pin 5 GND. Pin 6/7 TXA+/TXA-, Pin 8/9 RXA+/RXA-.

USB Rig Interface with VOX Circuit
USB Serial Interface
This USB Serial Interface is for rigs which don't have a USB port, but use a TTL or RS-232 serial port for remote control purposes. It is used by a software application to send remote-control commands to the rig. The rig may support remote-control protocols, such as ICOM CI-V and YAESU CAT.

You could use this cable with software for:

  • Digital Modes such as FT8 (and dozens of others) where the software controls the radio frequency and mode and PTT
  • Satellite tracking where the software controls the radio Doppler frequency correction
  • Logging software with automatic frequency and mode recording

You will also need a desktop or laptop computer with:

  • A USB Type A Port
  • Remote control software specifically for your rig, which can address the computer's USB Port. For example:
    • FLDIGI
    • WSJT-X
    • Ham Radio Deluxe
    • Another application, which supports the HAMLIB Remote Control Library

Genuine FTDI USB-RS232 Converter
This USB Serial Interface uses a USB to serial converter cable. All you have to do is to buy a suitable device and wire up the cable with a connector to suit the remote control port on your rig. However there are some important caveats:

  1. Some rigs support RS-232 levels for remote control. Some will only work with TTL levels and could be damaged by RS-232 levels. Some will work with both. RS-232 uses at least +/-7V levels while TTL uses +5/0V or +3.3/0V levels. However, the logic sense for TTL (1-0, mark-space) is inverted. It is important to determine which technology (and voltage levels) is used by your rig. The only option is to consult the manual, schematic or to use a multimeter on the rigs TXD line to check.   
  2. Some USB to serial converters sold as "USB to RS-232" only support TTL. If you need true RS-232, always look for devices which specifically quote RS-232 and don't ever mention TTL. Most RS-232 devices will have a DB-9 connector.
  3. Some USB to RS-232 converters use counterfeit chips, which are not supported by the genuine USB driver software used on Windows. Notoriously, there are cables using counterfeit Prolific PL-2303 chips on the market. These are typically much cheaper than the ones using genuine chips. Avoid these cable as they may not work. Buy genuine FTDI (or the much cheaper CH-370) chips, which all seem to work OK.

For RS232 compatibility, it is recommended to construct this USB Serial Interface by making an additional cable with a mating DB-9F connector at one end and a connector suitable for your rig at the other. Just connect this cable to a commercial USB to RS-232 adapter cable. For TTL compatibility, just wire a cable and connector for your rig to a USB to TTL converter module. Some 5V USB-TLL converter modules are integrated into a cable, which is convenient. Check if you need and RTS line, though, as some cables don't provide it.

An optoisolator, if required for rigs with no serial PTT control, can be enclosed in the backshell of the rig interface cable, with appropriate insulation, such as covering it with heat-shrink tubing.

CH340 USB to TTL Converter
Note: The loop-back jumper used for testing

CH340 USB to TTL Converter
Note: For 5V or 3.3V TTL Levels, but no DTR or RTS for PTT

CH340 USB to TTL Converter Cable
Note: 5V only and no DTR or RTS
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