An Introduction to Packet, the IMP Modem
Tony Marchese, N2YMW

BACKGROUND:

The project described in this article is a Bell 202 compatible, FSK modem which has been dubbed the "IMP", an "Introductory Modem for Packet".  The IMP is designed to connect directly to a standard serial port and typically requires no external power.  As such, the IMP may be used with almost any computer equipped with an RS-232 serial interface, appropriate software, and an external transceiver to access 1200 baud, packet applications such as BBS and satellites.


THEORY OF OPERATION:

Digital information is comprised of pulsatile waveforms.  These pulse waveforms contain high frequency components which exceed allowable bandwidths if utilized in their original form.  A modem is a device which reduces the required bandwidth, through a process known as modulation, by converting the pulsatile data from the computer's serial port to audio frequency waveforms for transmission.  The modem also uses the reverse process, known as demodulation, to convert the audio signal back to digital information for use by the computer.

The IMP's circuitry is detailed in the schematic of Figure 1.  The TCM3105, labeled U1, performs all of the filtering, timing, and conversion functions required for modulation and demodulation of the digital and analog signals.  Q5 inverts the CLK signal to the Transmit / Receive Standard Select (TRS) input which, in conjunction with Bit Rate Select inputs (TXR1) and (TXR2) at ground potential, configure the TCM3105 as a 1200 baud, Bell 202 compatible modem.  This configuration utilizes identical tones for the audio transmit and receive signals with 1200 Hz representing a "Mark" (logic 1) and 2200 Hz representing a "Space" (logic 0), the appropriate frequencies for 1200 baud, half duplex Packet operation.  Amateur Packet utilizes a half duplex communication mode, that is one direction at a time, as radio transceivers do not typically transmit and receive simultaneously.  Diodes D1 and D2 limit the incoming audio signal to approximately 0.65V as the maximum signal level for the TCM3105's Receive Analog input (RxA) is specified as 0.78V.

The RS-232 signals used with the IMP are outlined below:

DB25    RS232  RX	TX	Signal
pin #      signal   level	level	from		Notes

   2        TxD	+10V	+10V	computer	(+10V most of the time)
   4        RTS	-10V	+10V	computer	activates PTT during TX
   5       CTS	 data	     x	modem    	data from modem during RX
   7       GND	   0V	    0V	common	signal ground
   20     DTR	+10V	  data	computer	data to modem during TX

Q2 and U2 convert the information signals between the +/- 3 to 15V, RS-232 levels and the 0 to 5V required by the TCM3105.  Diodes D4-6 logic "OR" the voltage from RTS, DTR, and TxD signal lines to typically produce a 7-15VDC power supply.  The combination of the Q4, LM78L05 voltage regulator and filter capacitor C5 reduce the power supply to relatively stable, +5VDC source which is capable of providing the 10 mA of current required to operate the IMP.


CONSTRUCTION:

No special construction techniques are required for this project.  The minimal parts compliment lends the project to most construction methods.  The printed circuit board (PCB) of Figure 2 and the parts layout of Figure 3 have been provided to allow for easy construction.  The PCB fits neatly inside a the project case specified in the parts list.  I built up my latest version of this modem, including the PCB and the case, in two evenings.  An additional cable which connects the modem to the transceiver is also required.  Please refer your transceiver's documentation for the required connector types and correct wiring.


ADJUSTMENTS:

The computer will require software at this point.  The modem is compatible with a variety of packet software programs such as "BayCom", which was written by Florian Radlherr, DL8MBT and Johannes Kneip, DG3RBU.  "BayCom" is available from many Shareware distributors.

Connect a receiver and a computer with an RS-232 port to the modem.  Turn on the radio and tune the receiver to an active packet frequency.  Start the modem software.  Do not attempt to transmit at this point.  Verify the voltage supplied by the computer's RS-232 port is a minimum of +5.5V by placing a DMM across the modem's TxD and GND pads while running the software in the receive mode.  Please note, an external battery will be required if this voltage is below +5.5V.

The modem is easily modified for 9V battery operation.  A 9V battery connector, a SPST switch, and some small pieces of wire are the only additional materials required to complete this modification.  Solder the positive battery wire to one side of the switch.  Remove the RS-232's TxD wire from the printed circuit board.  Solder a length of wire to the other side of the switch and to the pad previously used for TxD.  Complete the modification by soldering the battery connector's negative leadwire to the ground side of R15.

With the voltage at TxD is greater than 5.5V, verify Vcc by placing the DMM across U1 pins 1 and 9.  The measured voltage should be 4.5 - 5.5 VDC.  If not, check the orientation of Q4.  You should also begin to notice information on the screen as data is received.  You may need to increase the receiver's volume control slightly if nothing appears.  The receive mode is should now be operational.

The last adjustment optimizes the modem's analog output level.  Replace the transceiver's antenna with a dummy load before proceeding.  Activate the software's transmit mode while monitoring the radio's output on another receiver or scanner.  Adjust R9 until a nice clean, signal is heard on the monitor.  Discontinue transmission.  Install the modem in the case, replace the antenna, and you are ready for packet operation.

I have successfully used the IMP to access information on various packet BBS and am patiently awaiting my first satellite contact.  I hope you have as much fun with the IMP as I have had.  As a final note, I would like to thank Virgil Ansley, KF2XT, for all his inspiration, patience, and elmering.  73


PARTS LIST:

U1	TCM3105 FSK Modem IC
U2	LM358, dual, low power comparator IC
D1-7	1N914 or equivalent
Q1,2,5	2N3904A NPN transistor or equivalent
Q4	LM78L05 voltage regulator
C1-2	20 pF capacitor
C3-4	0.1 uF capacitor
C5	any value between 10 to 100 uF, 10V (minimum) axial lead capacitor, any type 
X1	4.4336 MHz crystal
R2	100K variable resistor
R3	22K resistor
R6	user selectable for radio which combines PTT and TXA lines
R7	1K, 1/4 W resistor
R8	220K, 1/4 W resistor
R12	47K, 1/4 W resistor
R13	10K, 1/4 W resistor
R14	120K, 1/4 W resistor
R15	82K, 1/4 W resistor
R16	56K, 1/4 W resistor
R17	68K, 1/4 W resistor
DB25P	 Radio Shack #276-1548 or equivalent
case	3.25" X 2.125" X 1.125" (such as Radio Shack #270-230)
misc. connectors for transceiver
