SYMEK FSK-9601, Руководство пользователя

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FSK 9601 Page 4
RXC (Pin 19) Receive Clock: (modem output): the output of the receive clock recovery circuit is sent to the TNC to synchro-
nise the received data signal. RXD is stable at the rising edge of RXC. The frequency of RXC corresponds to the transmit
clock of the remote transmitter. If the internal clock of the modem and the clock of the remote transmitter differ by some per-
cent, the receive clock is adjusted by 1/32 clock cycle to maintain synchronism.
Connecting the FSK9601 to general TNC2
If the FSK9601 is to be used with a TNC2, the 6 signals are connected to the Z80-SIO as follows: (Pinning refers to 40 pin
Z80-SIO)
SIO-Pin 9 ----------- Modem Pin 1 and 3 (+ 5 Volt)
SIO-Pin 19 ----------- Modem Pin 7 (DCD)
SIO-Pin 18 ----------- Modem Pin 9 (CTS)
SIO-Pin 17 ----------- Modem Pin 11 (RTS)
SIO-Pin 13 ----------- Modem Pin 13 (TXD)
SIO-Pin 12 ----------- Modem Pin 15(RXD)
SIO-Pin 14 ----------- Modem Pin 17 (TXC)
SIO-Pin 13 ----------- Modem Pin 19 (RXC)
SIO-Pin 31 ----------- Modem Pins 2,4,6...20 (GND)
Note: for 9600 baud operation, the system clock of the TNC2 should be 10 MHz or higher. The NRZ-option GALs of the
FSK9601 must be used.
As the G3RUH-modem includes a scrambler, it would make no difference in transmit spectrum if NRZ or NRZI is used. How-
ever, to keep all systems compatible, the NRZ and NRZI versions have to be observed. Using two modems with the 'wrong'
GALs would result in the same perfect communication, however other modems with the 'correct' NRZ/NRZI setting cannot
read this signals.
Connecting the radio to FSK9601
Connecting a 9600 baud FSK modem to a radio requires modification of the radio in most cases. You have to find a suited
point in the transmitter circuit to modulate the transmitter without significant distortion. The receiver circuit has to be tapped
directly after the demodulator to get an unfiltered signal.
For connecting the radio, use a 5 pin DIN connector (180 degree pinning). The five pins are numbered as shown: (view to
solder side of plug, view to front of the socket):
Pin 1: MODULATOR of radio, output of modem
Pin 2: GND ground
Pin 3: PTT transmitter keying, push-to-talk contact to ground
Pin 4: DEMODULATOR output of radio, input of modem
Pin 5: N.C.
Note: the five pins are not enumerated in turn.
Modulator (Pin 1)
Here, the modulator of the radio is connected. The maximum output voltage of the modem is up to approx. 3.5 Volt
pp
and
could be adjusted with the only trimmer resistor (accessible at the rear side of TNC3/31). The minimal output voltage, which
could be set, is about 10 to 20 mV. Below this, the adjustment becomes somewhat fiddly. The modem has a output amplifier
with a series output resistance of 2 k
Ω
.
Sometimes, the radio requires much less AF-voltage, e.g. only 10 mV for sufficient modulation. For this case, the AF voltage
must be reduced by a voltage divider, which should be placed in the radio. The output of the modem should be about 1 Volt
for good adjustment with the pot near middle position. This gives less susceptibility to hum and noise. Another fine place for
the voltage divider is the DIN-plug.
The output voltage is coupled via an electrolytic capacitor. There should be no DC voltage applied to pin 1, which is more
positive then 2.5 Volt because the output capacitor of TNC will be reversed in polarity. If the modulation input of your radio
shows more voltage, you should connect a 10µF capacitor in series, minus connection directed to the modem. The transmit
signal sounds like white noise, but on a oscilloscope you recognise pulses with a distance of about 0,1 ms. (at 9600 Baud)
IMPORTANT: the correct adjustment of the transmit AF is very important. As the radios are modulated directly (there is no
AF-amplifier or modulation limiter), you may far overdrive the modulator by applying too much AF voltage. This gives the
transmit signal an excessive bandwidth and the IF filter of the destination receiver will totally distort the signal. We advice to
watch closely the eye diagram at the receiver and adjust then the proper modulation level. If the AF is too low, you may loose
some signal-to-noise ratio, but you don't risk to distort the signal by overdriving.
Demodulator (Pin 4)
The demodulator-output of the radio is connected to this pin. The AF voltage should be 150 mV or more. The decoder works
at voltages of 50 mV or more without errors, but the DCD-LED will flicker and the modem is sensitive against signal distor-
tions.
If the output of your radio produces much more signal voltage, it should be reduced by a voltage divider, which is mounted in
the radio to get 0.7 Volt or less. The input resistance of the modem is about 100 k
Ω
in parallel with the input filter capacitance
(100 pF).
Input voltages exceeding 1 volt are limited by a diode pair, causing excessive distortion of the signal. Make sure not to over-
drive the input to avoid this.
PTT (Pin 3)
This pin is switched to ground when transmitting. With this, all common PTT-circuits of the usual radios may be operated. The
switch in the TNC is a n-channel vertical MOS-field effect transistor (VMOS-FET), which is able to switch up to 25 Volts and
up to 250 mA. In the 'on'-state, the resistance of the FET is typically 30
Ω
or less, the cut-off current when switched off is far
below 1 µA. VMOS-FETs are suited well for such applications, because the voltage across the switched-on FET is low (only a
few mV). Bipolar silicon npn-transistors have usually saturation voltages, which are much higher.
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