The EMS-57 microphone is shipped with Alinco DR-235T MkIII radio. It has a 16 key pad for generating all 16 DTMF tones, allows control of some functions of the radios, and enables direct frequency entry or memory selection.
Theory Of Operation
Note: These theories were derived from the EMS-57 schematics (available below) but by using a Jetstream JTMIC1 microphone for observation. While Jetstream and Alinco accessories are directly interchangeable, the hardware is not identical. As an example, while the EMS-57 uses a MT37531 microprocessor, the JTMIC1 uses a EM78P565AMJ processor. The latter appears to have the DTMF generator built-in, as opposed to using an external TC35219F DTMF generator. The JTMIC1 also has fewer "popcorn" comments.
The LOCK switch (SW20) determines if the keypad, UP and DOWN buttons on the microphone are enabled or not. SW20 is a double-pole double-throw (DPDT) switch. One side is connected to the UP and DOWN buttoms (SW20-A), and the other side (SW20-B) to an I/O pin on the on-microphone microprocessor.
In the OFF position, SW20-A provides a current path to ground for the UP and DOWN buttons. These are pulled high to 5V in the radio; when the UP or DOWN key is pressed, the path through SW20-A to ground signals the radio. In the LOCK position, the path to ground is interrupted, and the UP and DOWN buttons have no effect on their respective signals.
In the OFF position, SW20-B pulls the I/O pin on the microprocessor to ground. When grounded, the microphone keys operate in the mode determined by DTMF switch SW21. In the LOCK position, this signal is high, and the microprocessor ignores any key presses.
The DTMF switch (SW21) in conjunction with PTT (SW18) determines if the 16 digit keypad will generate DTMF tones when PTT is pressed, or send control data to the radio (hi/mid/low power selection, band select, direct frequency entry, etc) when PTT is not pressed.
When PTT is pressed, the PTT signal to the radio is grounded. The processor then looks at the DTMF switch (SW21-B) and determines whether key presses should be ignored (DTMF switch in the OFF position), or if the DTMF generator (IC1) should be enabled. If enabled, the processor drives the appropriate combination of DAT1, DAT2, DAT3 and DAT4 lines to IC1 to select the correct DTMF tone, then enables the TD line to IC1 to start the tone output.
If DTMF is being generated, the MUTE output from IC1 causes the audio path from the microphone element to be disabled. This is done to prevent mixing voice with DTMF output. The DTMF output goes through SW21-A, through variable resistor VR1 for level adjustment, and then to the microphone audio line to the radio.
When SW21 is in the OFF position (no DTMF) and PTT is not pressed, the processor determines what key is being pressed, and sends a 7 character string to the radio over the MR signal. The characters are sent at 9600 baud, with 8N1 formatting, standard ASCII. The table below shows the string sequence for each key. Keys do not have an auto-repeat function; only one string is sent per keypress. Values shown in brackets are actual hexadecimal values.
Based on the EMS-57 schematic, it appears that if multiple devices where placed on the MR line, they could do collision detection; note that TXD and RXD pins are tied together on the processor. The MR line is directly connected to the ring line on the 3.5mm data connector. We know that the ERW-4C programming cable only uses the ring for programming, and can do bi-directional communication. It's possible that the microphone holds off sending it's data if it sees the GPS sending data (although I have no way to test this currently).
There is also some weird relay logic that the lines on the 3.5mm jack go into. The TX and RX lines go into the wipers of a DPDT relay on the radio. One position of the relay sends the data to the radio's microcontroller. It's unclear where the other position goes to, or why. It may be something related to having the EJ-41U TNC installed. Unfortunately, the DR-235T service manual schematics are a mediocre quality scan, and not PDF output from a CAD package.
The EMS-57 also appears to have provisions for doing a "key beep" when DTMF keys are pressed. There are 5 unpopulated components, 4 of which form an oscillator that drives a small speaker or piezo element. When the DTMF generator is actively generating a tone, the speaker will beep for the duration of the key press.
- No datasheet appears to be available for the Toshiba TC35219F DTMF generator. The MUTE output behavior based on the schematic.