I've been studying the schematic of an audio modem, which takes/sends data as audio signal over an audio jack via FSK encoding:

enter image description here

Does anyone know what the solder jumper SJ1 is supposed to do? My reading is that it's a 3-way jumper, but shorting all 3 pads would effectively short the MJ-2135's pins 1 and 4. Shorting SJ1's middle and bottom pads effectively produce nothing more since MJ-2135's pin 4 is already in parallel with R5.

Am I missing something?


1 Answer 1


This arduino FSK library and modem interface can operate either in full-duplex or half duplex mode:

  • In a full duplex connection, the signal to be received by the device is carried by one communication channel, while the signal transmitted by the device is carried by a second, independent channel. In this case the device is the arduino, and the two channels are two conductors in an audio cable (outgoing (TX): microphone, incoming (RX): left audio channel). These audio cables aren't standardized, but that modem interface apparently assumes the rightmost style:

    The advantage of full duplex is that the arduino can both receive and transmit data at the same time since the two signals are independent.

  • In a half duplex connection both incoming and outgoing signals are carried by the same communication channel. Since the arduino just receives its own outgoing data if it tries to transmit while receiving, the two devices have to take turns transmitting and receiving. The advantage of half duplex is that just a single channel is sufficient.

The solder jumper in that schematic is for selecting if the arduino operates as a full duplex or half duplex modem. Notice how the jumper just shorts the outgoing and incoming channels together, creating a half duplex connection.

If the analog interface is set to operate as half duplex, you must take care to not transmit while receiving data from the other end, as the library doesn't include any kind of "protocol" or flow control to arbitrate which device transmits or receives at any given time.

As Ignacio already stated, half duplex also allows the modem library to test itself: Any data you transmit is also be received by the modem and will end up in the receive buffer. Be aware that for some arcane reason there appears to be no way of disabling reception while transmitting data (so the receive buffer will always fill up whenever you transmit) and the flush() function (which should clear the receive buffer) isn't implemented. This migth change if the library is updated.

  • \$\begingroup\$ Thanks. Does that mean not shorting any pad of SJ1 gives full duplex? Then how come 3 pads are used when shorting SJ1 is designed to give half duplex? Wouldn't 2 pads do the job by shorting just pins 1 and 4? \$\endgroup\$
    – John M.
    Commented Oct 6, 2016 at 3:58
  • \$\begingroup\$ @JohnMunroe Yes, not shorting gives full duplex. I guess the 3 pad solder jumper allows explicitly selecting half or full duplex with a blob of solder, where full duplex is realized by redundantly shorting out the already shorted pair of pads. I really have no idea. \$\endgroup\$
    – jms
    Commented Oct 6, 2016 at 4:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.