DDCMP Framer

This is a DDCMP synchronous link framer, connected to a host with a USB interface.

Documentation

For detailed documentation, look in the doc subdirectory:

1. api.md -- programming interface for the framer.
2. kicad.md -- instructions for assembling the circuit board.
3. software.md -- how to build and load the firmware.
4. design.md -- design notes.

Connections

To use the framer, connect the micro-USB board to a USB port on the computer that will control the framer.

There are a number of choices for connecting the synchronous link to whatever device you want to use it with:

1. To use a synchronous modem or modem eliminator, connect it with a straight through DB-25 cable to the male DB-25 (marked "DTE"), and operate the framer in "rs232_dte" mode.

2. To connect to a DDCMP device such as the DMR-11 using an RS-232 connection, connect its male DB-25 connector with a straight through DB-25 cable to the female DB-25 (marked "DCE") on the framer, and operate the framer in "rs232_dce" mode. In this mode the framer supplies the bit clocks; you specify the desiredd data rate as one of the start command parameters.

3. If you have a device such as a DMR-11 that has a line card which is meant to connect to a BC55 bulkhead panel via a 40-pin ribbon cable, but you don't have or do not want to use a bulkhead panel, connect the line unit with 40 pin ribbon cable to the 40-pin header J8 (marked "To LU"). You can use either RS-232 (with bit clock generated by the framer) or integral modem operation using this connection.

4. If you want to connect the framer to a BC55 bulkhead panel (RS-232 or integral modem), connect the BC55 with a 40-pin ribbon cable to the 40-pin header J7. Do not install termination resistors on the integral modem panel (BC55A); for details, refer to doc/kicad.md.

5. You can connect integral modem coaxial cables to BNC connectors J2 (incoming data) and J3 (outgoing data). Note that DEC specifies triax cable (two shields) but those connectors are expensive so here we just use BNC connectors, which should be fine so long as you're not operating in an extremely noisy environment. Do not install terminating resistors at the framer end, they are built in to the board.

Indicator lights

There are three indicator lights on the framer board, as well as an LED on the Raspberry Pico board itself. They are:

1. Pico board LED: "on" indicator. It lights up when the framer is started with a successful start command, and off when the framer is stopped.

2. "SYN" indicator: byte sync status. This lights up when the framer receiver has completed byte synchronization, having received four or more consecutive SYN bytes (hex 96). SYN status remains set unless the DDCMP state machine restarts sync search; it will do so if a header CRC error occurs. If the light does not come on, this may indicate no received signal, or (for integral modem mode) speed mismatch exceeding the 10% receive rate tolerance. If the SYN indicator flashes frequently, this may indicate poor signal quality, or (for integral modem mode) a speed mismatch right at the limit of what the device can handle. Note that the weak signal capability of the integral modem depends on the transformers used; refer to doc/kicad.md for details.

3. "RX" indicator: flashes when a frame is received (either successfully or with an error).

4. "TX" indicator: flashes when a frame is transmitted.

The RX and TX LEDs have pulse stretchers, so the light flashes are clearly visible even at low packet rates.

The RX and TX LEDs flash once when a framer start command is successfully executed.