G4SZM Digital COS

So far all the Satoshi and clone boards have an analog COS/COR facility which signals the PIC that data is being received. The problem with this method is that most any signal or noise that is heard by the connected radio will falsely flag the PIC and thus fool it into trying to decode data. By the addition of a voltage comparator to the GMSK chip's S/N pin one can correctly determine whether any data is present or not and thus flag the PIC correctly.

The G4SZM Digital COS/COR is a pre-built addon board requiring 4 wires (+V/GND/IN/OUT) to be soldered to your existing Satoshi (or clone/MHS/etc) board.  Lead time is roughly 2 weeks.

Designed by G4SZM, the Digital COS board enables a Satoshi based MiniHotSpot (and others) to make use of the unused S:N facilities of the installed CMX589A GMSK modem chip. 4 wires connect from the board to your MHS; +5V , GND, COS and S:N. Follow the below guidelines to install your DigiCOS board onto your MHS.

The board can be easily constructed from the circuit diagram here.

Using your favourite hookup wire solder the following outputs from the DigiCOS board to the MHS;

    * P - U1 pin 24
    * G - U1 pin 12
    * C - U2 pin 25 (move jumper SW3 to the left. Remove SW1/2)
    * S - U1 pin 23

I would advise that you use a small length of ribon cable from an old computer drive. Solder the MHS end to the underside of the MHS board and solder the DigiCOS side to the top of the board. The DigiCOS board will then fold over and sit on top of the MHS's GMSK chip. It can be secured with a small amount of double sided tape. It should look something like the right picture above.

Calibration is simply a matter of turning the pots until the lights light up.

  1. Attach your MHS to your PC and run up the rf_read tool. 
  2. Using your D-STAR rig, key up on your MHS's frequency and adjust VR1 (left most pot) until the COS LED lights. You should also notice that the COS light on the MHS will also light up.
  3. Adjust the remaining pots in sequence until they all light up.
  4. Open up a cold one.
As your signal degrades you should notice the LEDS flicker and even turn off in sequence.

Caveats & Gotchas

During construction of the initial batch of 25 units 680R resistors were installed instead of 470R. The result was a very hit or miss COS signal. The COS line from the DigiCOS board would drop to only 1.5V. This may or may not be enough to flag the PIC processor. The fix is to replace R5 with a 470R (still available at RadioShack!!).

It has been pointed out by a few users that the COS can easily "false" when in RX mode. Investigation has discovered that this issue is related to the "floating" nature of the PIC's COS line. In the original circuit (pre DigiCOS addition) there is a 47K resistor (R9) that pulls the COS line on the PIC high. MHS users should replace SW3 in the left position and remove SW1/2. "right original" owners should do the same.


Mark Phillips, G7LTT/NI2O 10/2009