Open Squelch DCD,

How important is it ???


In packet radio we are generally operating on a shared channel. In order for many stations to co-exist peacefully, packet controllers use a scheme known as CSMA (Carrier Sense Multiple Access). In this scheme each controller listens to the channel prior to transmitting, and will not transmit if another carrier is detected.

Most of the modems used on packet controllers will accept "white noise", such as an open squelch, and identify it as a carrier. This results in either a continuously flickering or constantly illuminated DCD LED, which indicates that the controller has sensed a carrier on the channel. This has resulted in many individuals operating their radios in the squelched mode. While this may reduce one aspect of the problem, it enhances the overall adverse effects of the problem. Many radios are very slow to open the squelch, which tends to make CSMA fall apart, since the modem can't hear the signal while the squelch is opening . If the controller decides to transmit during this time frame, there will be a collision.

The PacComm Open Squelch DCD Accessory

The PacComm DCD Board uses the circuit designed by Eric S. Gustafson, N7CL and which also has been sold by TAPR as the "TAPR DCD Board" kit.

The PacComm Open Squelch DCD is a surface mount card that measures 1.85" x 1.25" and is designed to be installed in four different ways:

  1. Plug into a TCM3105 modem chip socket.
  2. Attach directly to the Modem Disconnect Header.
  3. Connect to any packet controller via a wire harness.

Attach directly to a TMR circuit board using a 7 pin in-line header.

The DCD-3105 Model simply plugs into the socket of a TCM3105 modem IC, and the IC is plugged into the DCD-3105.

The DCD-Header Model plugs directly into the pins of a TAPR-style Modem Disconnect Header (20 or 26 pin style).

The DCD-Universal Model may be integrated into other packet controllers (not limited to the TCM-3105 modem IC) by directly wiring it to the modem IC.

The 7-Pin Model is a factory only supplied version of the DCD circuit board.

The following PacComm products have the Open Squelch DCD circuitry built-in, therefore an optional adapter is not necessary:


Installation Instructions for PacComm Open Squelch DCD Adapters

Installing a DCD-3105 Adapter on a PacComm TINY-2 or MICROPOWER-2

  1. Remove the 4 Phillips screws from the front and rear panels. Note the grounding strap between the bezel and the panel, this reduces the RF emissions of the board.
  2. Slide the circuit board from the case and place it on a static free surface.
  3. Notice the orientation of the TCM3105 modem IC (U16) in its socket by noting the indentation in one end of the case.
  4. Remove the modem IC (U16) from its socket. A small screwdriver may be used to pry the IC out of its socket. Be careful to pry up opposite ends of the IC alternately so that the pins are not bent.
  5. Install the modem IC into the socket on the DCD-3105 adapter board with the indentation on the IC matching the indentation on the socket.
  6. Install the DCD-3105 by plugging the adapter pins into the U16 socket on the packet controller board. Remember to keep the modem IC notch in the same orientation as the original.
  7. Reinstall the packet controller in its case. If the modem IC strikes the top of the case when inserting the circuit board, it may be necessary to clip the pins of the modem IC approximately 1/16" to allow it to sit lower in the socket. This is not normally necessary.

Installing a DCD-Header Model on a PacComm TNC-320

  1. Remove the 4 Phillips screws from the front panel and rear panels. Note the grounding strap between the bezel and the panel, this reduces the RF emissions of the board.
  2. Remove the TNC-320 by sliding the PCB towards the front of the case. Place the circuit board on a static free surface.
  3. Locate the J5 modem disconnect header. This is a 26 pin header (2 x 13) located near the rear center of the board.
  4. Pin 1 of J5 is the pin nearest the "J5" label and toward the rear of the circuit board.
  5. Cut the trace on the "solder" side of the board that runs between Pin 1 and Pin 2 of J5.
  6. Install the DCD-Header 26 pin connector onto the header at J5. Orient the connector so that the end with the Pin 1 marking is aligned with Pin 1 of J5.
  7. Replace the TNC-320 board into the case and reinstall the bezels and end panels.

Note that the DCD LED of each port reflects only the state of the analog modem DCD signal. Running the radio input with open squelch will cause the DCD LED to be constantly illuminated, but the proper DCD signal will be used by the firmware for accessing the radio channel.

Installing a DCD-Header Model on a PacComm PC-320

  1. Ensure the power switch on the computer is OFF.
  2. The DCD-Header will be mounted to the modem disconnect header on the PC-320 card, adding 10mm (3/8") to the thickness of the card. Open the computer case and determine if sufficient clearance exists between the component side of the PC-320 card and the neighboring card.
  3. Remove the PC-320 card.
  4. Locate J5, the modem disconnect header. It is a 26 pin header (2 x 13) located near the rear of the PCB.
  5. Pin 1 of J5 is closest to the "J5" label located toward the bottom of the board.
  6. Cut the trace on the "solder" side between J5 Pins 1 and 2.
  7. Install the DCD-Header 26 pin connector onto J5. Orient the connector so that the end with the Pin 1 marking is aligned with Pin 1 of J5.
  8. Reinstall the PC-320 card into the computer being careful to watch for clearance with adjacent boards.

Note that the "simulated DCD LED" for each port reflects only the state of the analog modem DCD signal. Running the radio input with open squelch will cause the DCD LED to be constantly illuminated, but the proper DCD signal will be used by the firmware for accessing the radio channel.

Installing a DCD-Universal Model on a TNC-220

  1. Remove the 2 Phillips screws from the rear panel and remove the rear panel and plastic bezel.
  2. Remove the screw attaching the voltage regulator to the case.
  3. Remove the TNC-220 board from the case.
  4. Locate J5, the modem disconnect header. It is a 20 pin (2 x 10) header located near the center of the board between the Z8530 and AM7910 ICs.
  5. Pin 1 of J5 is on the end of the header opposite the "J5" label. Pins 1, 19, and 20 are labeled.
  6. Cut the trace on the "solder" side of the PCB between Pins 1 and 2 of J5.
  7. The TNC-220 DOES NOT switch clock signals to the modem IC when ports are select as the newer models do. Therefor you must install the DCD adapter for operation at either 300bps (Port 1 HF) or 1200bps (Port 2 VHF). For open squelch DCD operation on the HF port, attach a wire from U11 Pin 14 to J5 Pin 12. For open squelch DCD operation of the VHF port, attach a wire from U11 Pin 13 to J5 Pin 12.
  8. Solder the end of the RED wire from the DCD board to a source of regulated 5 volts.The most convenient location is the 1/4" long heavy trace directly to the rear of and centered between the 74HC157 and 1489 ICs.
  9. Solder the BLACK wire from the DCD-Universal to J5 Pin 15.
  10. Solder the BLUE wire to J5 Pin 12.
  11. Solder the YELLOW wire to J5 Pin 18.
  12. Solder the GREEN wire to J5 Pin 2.
  13. Solder the ORANGE wire to J5 Pin 1.
  14. Use some double-sided foam tape to secure the DCD board to the top of one of the large ICs.
  15. Reinstall the TNC-220 into the case. Reinstall the regulator attachment screw. bezel, and end panel.

Installing the DCD-Header Model on other packet controllers with a "TAPR-style" Modem Disconnect Header

  1. Remove the packet controller board from it's case or remove the case cover as appropriate.
  2. Locate the TAPR-style modem disconnect header. It may be only a dual row of circuit board holes (2 x 10). Install a 20 pin header if needed. Consult the packet controller manual for instructions. Do NOT follow any instructions in the manual for cutting traces.
  3. Cut the trace on the "solder" side of the circuit board between pins 1 and 2 of the header.
  4. Install the 26 pin connector on the DCD-Header board on the modem disconnect header.
  5. Connect a power lead from Pin 8 of JP1 to a source of regulated 5 volts.
  6. Reinstall the packet controller board in it's case.

Installing a DCD-Universal Model on other packet controllers withouta "TAPR-style" Modem Disconnect Header

The DCD-Universal Adapter should work with any AX.25 packet controller. If the unit does not have a provision for a modem disconnect header, or the pinout of the header is not compatible with the TAPR standard, follow the instructions below.

  1. Remove the packet controller from its case, or remove the case cover as appropriate.
  2. Locate the source(s) of the signals required by the DCD-Universal Adapter on the packet controller and attach the appropriate wires from the DCD-Universal. Solder the end of the RED wire from the DCD board to a source of regulated 5 volts. Solder the BLUE wire to a source of a 16x clock. Solder the YELLOW wire to received data. Solder the GREEN wire to the modem's carrier detect signal and disconnect this signal from the DCD input of the TNC. Solder the ORANGE wire to the DCD input of the TNC.
  3. Use some double-sided foam tape to attach the DCD-Universal board to the top of one of the large ICs.
  4. Reinstall the packet controller board into it's case.

The connections to the DCD-Universal board are as follows:

JP1 Pin Number

Required Signal

Wire Color

1

N/A

N/A

2

X16 Clock from TNC

Blue

3

MCD, Modem CD from TNC

Green

4

MCD, Receive data from TNC

Yellow

5

DCD, CD from DCD board to TNC

Orange

6

N/A

N/A

7

Ground

Black

8

VCC (+5 VDC)

Red

 


 

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Last updated: 7/19/2004