Hamcom / JV Fax Interface

Welcome to a simple RS-232 interface that transforms your PC into a radio data, fax, and SSTV communication system !. If you're a PC user, this can be a superb way to 'start out' on a wide range of data modes at low cost. This kit is designed for use with a PC running approriate 'processing' software, such as JVFAX and Hamcomm. I claim no originality whatsoever for the circuit arrangement, in fact it's very same circuit as featured by the Hamcomm 3 author in his documentation file. My only input is the design, layout, and testing of the PCB's, and a few ideas regarding component types and voltages.

Circuitry

For radio interfacing with the RS-232 port, the 'usual' 741 / TLO71 IC is employed as an audio limiter / level shifter, and on transmit a passive audio filter is used for the audio together with a
transistor PTT interface. I've given two circuits here (Annexure 1). The first, with fewer components, is for a 'receive-only' arrangement, i.e. where your receiver audio output only feeds the interface. The second is slightly larger with the addition of transmit circuirty for PTT (push-to-talk), and a multisection low pass filter to shape the harmonic-rich square wave from the PC into a 'cleaner' sine wave to feed to your mic socket. You may build / order either circuit or of course the Transmit / Receive with just the receive components populated till you obtained an amateur licensce if you're currently a listner.

I've used a 9-way boardmounting RS-232 'D' type connector in the design, so the unit can either plug straight into the back of your PC or alternatively be fitted into a case and used with a standard RS-232 pin-to-pin cable which you can obtain at most computer shops. If your computer uses a 25 way connector, a commonly available 9 to 25 way adaptor may be used, or you can easily wire up a 25 way connector as shown in Table 1. What needs to be wired, is the lines for receive and transmit audio, ground, and PTT for the Radio end with appropriate plug to match the 5 pin keyboard type connector (as shown below) to fit your requirements on your rig.

Components

I've tried both 741 and TLO71 op-amps, however I'd advise against using the 741 even though it's often more commonly available. This is because the serial card on some computers cannot supply enough current to drive a 741 - the TLO71 has a lower current requirement and this is the type I'd recommend. Also, although some documentation specifies 16V capacitors, I've used the 25V working types, especially the electrolytics, to cope with maximum possible RS-232 voltage swings. The tolerances of the components isn't critical, but I did not get tempted to use 'nearvalues' for R4, R5, R7, C5 and C6 as these give a defined filter response. I've use a 2N2222A transistor, but any common NPN switching transistor may be used in its place, such as a BC547 or similar.

The variable resistor is used to adjust the drive of the tones sending input your Tx

Software
The kit is designed for use with JVFAX v.7 and HamComm v.3, which lets you get up and running with multimode data, SSTV, and FAX transmit and receive on either the HF or VHF / UHF bands when coupled with your transreceiver as appropriate. For the uninitiated, this clever software employs the processing power of the PC for demoudulation and generation of the various communication modes, and you can send and receive full-colour Slow Scan TV etc as well as receiving and decoding Morse / RTTY , plus Weather Fax signals, even in full colour with a 'slide show' format just like you see on the weather forecast on your TV.

Ordering information :

The kit is available with us on the following address - HAM LAB, 404 Unitted Tower, Chincholi Bunder Road, Malad (W) , Mumbai 400 064. The cost of the interface is Rs.150/- (Packing & Forwarding extra). The software comes on a single 1.44 Mb floppy and is available free of charge from us. Send us a blank floppy for a copy of the software. A Receive only version of the same interface is also available for SWL's and would cost Rs.120/- only. The kit would be available from us within a week from the receipt of the dues.

Parts List

All resistors 1/4 W
All capacitors 25V working

Only Receiver unit

C1 100nF polyester
C2,C3 10uF electrolytic
R1,R2 100k
D1-D4 1N4148
IC1 TLO71
9 way D type RS-232 connector.

Transmit/Receive unit

Components as for the receive unit, but with addition of;
C4 100nF polyester
C5,C6 22nF polyester
R3 100k preset
R4,R5 15k
R6 10k
R7 1k
D5-D7 1N4148
TR1 2N2222A or similar

Table 1
25 way RS-232 connections.

Function 9 way 25 way

TxD pin 3pin 2
DTR pin 4pin 20
GND pin 5pin 7
DSR pin 6pin 6
RTS pin 7pin 4