AFSK1200 Users Manual

Issue 28 July 2000

SYMEK GmbH, Datentechnik, Ulf Kumm, DK9SJ, D-70597 Stuttgart (Sonnenberg), Johannes-Krämer-Straße 34
Phone +49  711 76 78 923, Fax: 76 78 924, Hotline: 76 54 911, eMail  Internet:

Technical Data AFSK1200

Power Supply:  5 V DC,  ca. 20 mA.

Dimensions: ca. B=40, T=120, H=25mm, Mass ca. 40 Gramm

Radio Interface: 5 pin DIN receptacle, modulation AFSK, Audio-Frequency-Shift-Keying 1200/2200 Hz according Bell 202, baud rate 1200 bit/s for TX and RX. AF-output is muted while receiving. Output: adjustable 20 mVss to 0,4 Vss, Ri < 1 kW decoupled from DC. Output voltage may be increased to 2 Vss erhöht werden. PTT (transmitter keying):  max. 16V 0,2A to ground. Input 200 mV to 10 Vss at 25 kW || 1 nF, DC-decoupled for direct connection to speaker output of radio. Sensitivity may be increased to 20 mV.

Modem-Circuit: Texas-Instruments Modem-Chip TCM 3105. All frequencies are derived from a 4,433 MHz quartz.

Modem Interface (Digital interface to TNC) CMOS -level 5 Volt. TXData, TXClock, RXData, RXClock, RTS CTS, DCD, + 5 Volt, reset, ground. Connection via 20-pin ribbon cable (High-Speed-Modem-Disconnect as used with TNC3, TNC31, TNC4, RMNC)

LED display: DCD (carrier detect), PTT (transmitter keying)

Acessible trimmers: Adjustment AF output voltage, modem-receiver decider/symmetry, Carrier detect center frequency

Transmitter keying: VMOS-FET to ground. Supervised by a watchdog circuitry with 30 seconds time limit for continuous transmission

Carrier detect (DCD): the open collector DCD can be connected 'wired-or' together with other SYMEK modems

Measuer-points: DCD-PLL-measuring pin for DCD center frequency and demodulator trigger.

Jumpers: DCD-source  (from TCM3105 or PLL-DCD XR2211), select NRZ/NRZI, watchdog-disable

Digital Interface (Modem-Disconnect)








+ 5 Volt

power supply from TNC





+ 5 Volt

power supply from TNC






not used with AFSK-1200)






carrier detect from modem to TNC






transmitter is ON (modem to TNC)






transmitter keying (TNC to modem)






transmit data (TNC to modem)






receive data (modem to TNC)






transmitter clock (modem to TNC)






receiver clock (modem to TNC)




Measure pins and adjustment

MP A1 "MODEM RX": apply a test signal to receiver input which toggles the two tones 1200 tof 2200 Hz (diddle-mode). Adjust P2 (3105) to a 1:1 duty cycle at A1.

MP A2 "DCD": apply a test signal to receiver input which toggles the two tones 1200 tof 2200 Hz (diddle-mode). Adjust P1 (2211) to a 1:1 duty cycle at A2. Signal looks like one way rectified AC. Adjustment is not critical.

A3 "DCD ext": If this pin is pulled to ground, the channel is recognised busy. However, the yellow LED doesn't light. Diode CR3 must be soldered, cathode direction to 20 pin connector, or must be short circuited. If the modem recognises the channel busy, this pin is pulled down by the modem itself. (Diode CR3 must be soldered anode to connector or shorted. Recommended type:  BAT46. (Schottky-Diode)

A4 "DIN 5": available for access to pin 5 of DIN connector (n.c. pin)

A5  "PTT ext": is pulled LO when  PTT of  AFSK-1200 is LOW. So, other modems may de disabled from transmitting via the DCD-Extwire as long as the AFSK-1200 is transmitting. The pin is pulled to +5 v via 47 kW.

Higher output voltage: Short circuit pin 3 + 4 of resistor network RN1 verbinden to get up to 2 Vss output. Replace the output trimmer 1 kW -Ausgangstrimmer by 10 kW Trimmer.

Increase input sensitivity:  AFSK-1200 is made for direct connection of speaker outputs. To increase sensitivityby factor of 10, increase  R1 (3,3 kW ) to e.g. 47 kW or remove it. 

LED display:

LED1 (yellow) "DCD":: carrier detect, LED2 (red) "DCD":: transmit

Connection of the radio

Use a 5 pin (180 Degree) DIN-connector:

Pin 1: MIC Mikrophone of radio, AF output of the modem/TNC
Pin 2: GND ground
Pin 3: PTT push to talk signal, shorted to ground while transmitting
Pin 4: SPK Audio outout of radio (Speaker), AF input of modem/TNC
Pin 5: n.c.

Note: the 5 pins are not enumerated in turn

Cables between modem and transceiver

Please refer to the German manual for a list of over 100 amateur and cb radios and their connection cables.
Here some help for reading the German expressions:

Kabel = cable, Kontakt=contact, innerer = inner, mittlerer = middle, äußerer = outer
Stift = pin, verbinden = connect, Handfunk = handheld radio
Klinkenstecker = earphone jack
colour suggestions: <ws> = white, <br> = brown, <gn> = green, <ge> = yellow

cable 0: this transceiver is not suited for packet-radio

cable 1: ICOM handheld radio IC2E, IC4E, IC12E, IC02E, IC04E, IC32E, µ2E, µ4E, Yaesu FT209, FT470, FT709, FT727, FT23, FT73, FT76, Standard C201, C401, C408, C500, C520, C528, C558 etc., Alinco LPD EC10, stabo xh8082
Pin 1 (MIC) ---<br>--- 2,5 mm earphone connector MIC inner contact
Pin 2 (GND) ---<ws>--- 2,5 mm earphone connector MIC outer contact
Pin 2 (GND) ---<ws>--- 3,5 mm earphone connector SPKR outer Kontakt
Pin 4 (SPK) ---<gn>--- 3,5 mm earphone connector SPKR inner contact
Pin 3 and Pin 1 connected via a 2,2 k
W resistor in the connector

cable 2: Kenwood handhelt radio TH21, TH41, TH25, TH45, TH48, TH55, TH77 etc.
Pin 1 (MIC) ---<br>--- 3,5 mm stereo earphone jack  MIC middle contact
Pin 2 (GND) ---<ws>--- 2,5 mm earphone jack SPKR outer contact
Pin 3 (PTT) ---<ge>--- 3,5 mm stereo earphone jack MIC outer contact
Pin 4 (SPK) ---<gn>--- 2,5 mm earphone jack SPKR inner contact
<n.c.>-- 3,5 mm stereo earphone jack, inner contact

(etc. as shown in the German issue)