TINY-2/MICRO-2 Technical Ref., 2. Ed 1989
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Modem: The TCM3105 modem (U16) employs a Bell 202 modulator and demodulator in a sin-
gle CMOS package. The modulator transmits either 1200 or 2200 Hz depending on the state of
the SIO TXD line (U14 pin 14). The modem tone output (U16 pin 11) goes through a 10k trim-
pot to allow amplitude adjustment to achieve proper transmitter deviation.
Since the TCM 3105 always produces tones when powered, there is an audio cut-off circuit em-
ployed using a VN10 FET (Q3) and a 2.2k resistor that swamps most transmit audio to ground
when the RTSA line is high (receive). This allows normal voice use of the radio while the TINY-
2/MICRO-2 is connected.
The demodulator contains a switched capacitive filtering network, and is sensitive to audio lev-
els down to 10 mV. The receive bias adjustment (R9) allows optimizing the bias level for a par-
ticular TCM3105 modem IC . Received data is output on U16 pin 8 and is routed through the
clock recovery circuit before the SIO (U14).
NOTE: The on-board radio modem is only capable of 1200 baud operation as installed. For
higher (or lower) radio data rates, attach an external modem to the J5 modem disconnect
header.
Battery backed RAM (bbRAM): The entire 32k of RAM (U8) is backed up with a 3 Volt lithium
battery. The 74HC132 IC (U3) is also battery powered keeping pin 20 (CE) of the RAM disabled
(high) during periods of low power to the TINY-2/MICRO-2 (typically 7.5 Volts and below).
Power Supply: The 7805 regulator (U5) supplies the main portion of the board with clean, sta-
ble 5 Volt power. The incoming positive voltage is routed through a choke (L1) and diode (D6)
for reverse polarity protection and to the power switch. It feeds the 7805 regulator and pin 14
of the MAX231 IC (U15). The MAX231 has internal oscillators which allow it to generate a neg-
ative voltage to support the RS232 levels. A voltage approximately one Volt below the incoming
supply voltage can normally be measured. The battery circuit is isolated by diodes to keep the
RAM and the 74HC132 powered whenever JP-B is installed. The TINY-2 requires approximately
220 mA at 9 – 14 VDC [MICROPOWER-2 = 40 mA.]
Modem Disconnect - J5
The modem disconnect on the TINY-2/MICRO-2 PC board (J5) is provided for using higher-speed
modems, or more sophisticated, higher-performance modems for satellite or other uses. This con-
nector is the same as the TAPR TNC-2 header.
A physical connector for J5 is installed on the MICROPOWER-2, but not on all versions of the TINY-
2. Any Standard 20-pin header for use with IDC cable connectors should be suitable for installation
on the TINY-2. Parts are available from PacComm. When installing the connector, be sure to line
up the marked pin (pin 1 ) of the header with the the PC board pin 1 marking.
To use an external modem, it is necessary to cut the traces between pins 1-2 and 17-18 on the
bottom of the TINY-2/MICRO-2 PC board with a sharp knife or MotoTool. If RX Clock is provided by
the external modem, then cut 13-14. If CTS is provided, cut 9-10.
CAUTION: DO NOT CUT ANY OTHER TRACES WHICH PASS THROUGH THE J5 AREA AND WHICH
ARE NOT TO J5 PINS!
Once the traces are cut at J5, you must install push-on jumpers or a shorting connector on the
above pin pairs to use the on-board modem. This also applies to the MICROPOWER-2.
