Maxon Protocol Converters

Procedure for Adding a Protocol Converter to the Maxon DM2550 Data Radios
By ZL1SWW
  Click for above citcuits in PDF format
Intro..

As a normal data radio, the unit has the capability to switch between 1  of two channels that are pre programmed or programmed by the user. Channel selection is done by toggling a pin on he DB9 connector to change channel.  While this is fine for some applications – extra flexibility can be added by adding a “protocol converter” that intercepts the internal CPU within the radio  and  the PLL chip that controls the radio frequency.

Control of the radio may be by several methods:-
 Direct control from PC by HyperTerminal or similar.
 LCD Display Board.
 PC Control Program ( Future Planned)
 

Prerequisites

The digital (top board) in the Maxon contains a “house keeping” CPU that controls many aspects of the radio. If this radio is to be used  with APRS, for instance, you will need to disable the battery saving option as this pulses the audio output and makes APRS tracker units see DC shifts on the RX line that comes up as a Carrier Detect erroneously.  It may check to see what setup is in the radio before continuing.

How to Install.

Firstly obtain the following:-

 68HC908-QT1 Protocol Converter Chip
 8 Pin DIL IC socket
 10k resistor
 100n Bypass Capacitor.

Milestones….

Cutting Pads on the  ground plane (top side) of the  digital board.

Lifting the 3 pins on a level translator chip (MC14504)

Installing the Protocol Converter

Testing!!
 

The protocol converter can be mounted directly on the ground plane side of the board as the height of the lid governs how high the Chip can be  before it hits the lid.

By the two preset pots, there is room to make some pads by carving out the pads in the  bare areas of copper. 
Things to note are that Pin 8 on the chip is ground so you only need to carve out 3 pads on  the pin 8 side of the chip and 4 pads on the other as pin 8 is ground and can connect directly to existing ground.
There is a through hole plate that will most likely be in between pin 5 & 6  that cannot be disturbed  as it feeds ground to the other side of the board.

Try and scrape away as much as you can between pads to avoid any shorts..

Once all is done and  isolation is tested by OHM Meter, scrape away the resist on the pads and a part of the goundplane where pin 8 sits. 

Solder down the IC socket with socket pins bent out 90 degrees to the board. The notch in the socket should point to the bottom edge of the board.

Next, turn the board over and we need to lift the pins 3, 5 & 7  for the control lines into the PLL buffer chip.(MC145404).

This can be done by getting a pin and starting with pin 5 (its in the middle of the other 2 and is easier and stops you inadvertently wiggling the pin 3 or 7  too much and will break easily after a bit of flexing).
Use the pin to lever the chip leg off the board while heating with the iron. Make sure there are no “dags” under the legs to  avoid shorts to the board.

 Connect, using fine wire like Kynar to each pin with enough length to get to the protocol converter chip socket on the other side.
Once done, it is possible to feed the wires up through existing not used though hole plates. Wire up as shown in the accompanying schematics.

Pin connections for the buffer chip MC14504 are as follows:-

Pin 3 Enable
Pin 5 Clock
Pin 7 Data
 

PTT and Squelch lines are available off the expansion connector holes at the edge of the board. Please see the photos for details. Please be sure to short LK8 which is near the option connector, this is to link the squelch line to the option board hole. This is seen in the photo.

5V for the chip can be picked up off the trough hole plate as shown in the picture. Gently scrape off resist before soldering.
 
 
 

 

Install the chip and Test.

Good Luck – De Simon ZL1SWW