Welcome to MakecircuitsMarch 2, 2015
BayComm packet modem
There are many articles about the Baycom softw1are and many quite different diagrams on the net. Here is another but this time you can also download the complete Baycom v1.7 softw1are as well as a PCB foil pattern and component layout. It`s all here on Harry`s Homebrew Homepages. First of all, here is the circuit diagram:
There is nothing particularly dificult and construction is quite straight forward. I have made up this PCB so as not to use any PCB wire links at all. Be sure to use the correct 9-pin `D` type connector so the pins fit in the board. The tw1o outer holes for mounting the 9-pin `D` type connector should be 3mm Dia. and the holes for the pins are 1.2mm Dia. The PCB is also tailored for a PCB mounting 5-pin DIN socket. These holes should also be 1.2mm Dia. All other holes on the PCB are 0.9mm Dia. The 22K resistor close to the 5-pin DIN socket is only fitted when your transciever PTT switch is by means of a DC short on the MIC lead (most hand-held radio`s). Here is my finished modem.
To align the modem, using an oscilloscope, connect it to the receiver and open the squelch on an unused channel and watch the waveform on pin 6 of the 74HC04. Adjust the 47K pot so that the noise is centered on the screen and not all at the top or bottom of the screen - i.e. the noise should be just as much positive 5v as it is 0v. If you do not have an oscilloscope then adjust the 47K for a DC voltage of 2.5v on pin 6 of the 74HC04.
Full instructions for setting up the softw1are are given in the softw1are ZIP package. The PCB foil, component overlay and Baycom softw1are are given in the DOWNLOAD section of my homepages. If you want a PCB ready made then just look at my KITS page. I hope to have a ready made PCB available soon. The TCM3105 is supposed to be obsolete but you can still buy it from N R Bardwell, see my Component suppliers page.