DSK-MODEM V1.01 DSK-MODEM (c) Johan Forrer KC7WW 1993,1994 Author: Johan B. Forrer KC7WW 26553 Priceview Drive Monroe, OR 97456 United States of America CONTENTS 1. Requirements 2. DSP modem background. 3. RS 232 connections. 4. Special modems 5. Disclaimer 1.0 Requirements DSK-MODEM is part of PCTOR and provides support implementing DSP code on a TI 320C26 DSK target. In its simplest form, the supplied DSP modems will work on an unmodified DSK unit. It is, however recommended to add the included interface circuit for effective tuning display and full transmit capabilities. PCTOR version 3.05c or higher is required. Please contact the author if operation with other software tnc's is required. If this program is to be used with PCTOR, please include the following two lines to the "TOR.CNF" file. dskmdm:2 .... this will indicate that a DSK is attached dskbaud:1 ... this will determine the baudrate that the DSK will use 1 = 115200 2 = 115200/2 3 = 115200/3 etc. The first line tells PCTOR that support for the DSK is requested. PCTOR version 3.05c and higher will automatically download the appropriate modem as the operating mode changes. The user, however, may still manually select any modem using the ALT-M key. 1. DSP modem background The TI 320C26 digital signal processor starter's kit (DSK) is used as a platform for implementing a number of DSP modems. For those not too familiar with this little unit, the DSK is a 2" x 3" PCB containing a 40MHz DSP and associated circuitry. The DSK module is available from TI distributors as part no. TMDS3200026, and costs only $99. Please note that the schematic for a simple interface containing a tuning display and PTT driver is included as a separate plot file in this package. IMPORTANT NOTE: Soldering on the DSK PCB will void your warrantee. The author is not liable in any way for damages that may result in such modifications. Also please note that there is a minor hardware fault on the DSK PCB. A simple fix for this fault is described in the TI DSK newsletter. The fault amounts to a missing trace that connects a resistor SIP package to VCC. Without this trace, external interfaces will not work properly. The DSK connects to a PC computer's COM port and communicates with the PC at rates up to 115200 baud. DSK-MODEM contains all the code necessary to communicate with the DSK. Once a particular modem is selected, DSP code is downloaded silently to the DSK's on-chip memory. This operation only takes a second or so to complete. The DSP then performs all signal processing operations in real time using its 10 MIPS engine. The resulting output from the DSP is a stream of data bits that is sent to the PC for further processing by programs such as PCTOR or PC-PACTOR. In addition to the RS 232 connection, an audio signal from your transceiver is fed to the DSK's analog to digital convertor. If PCTOR is to be used to transmit data over the air, additional wiring to the transceiver PTT and FSK control is also required. All the DSP modems used in this package uses non-coherent detection principles to recover digital signals. The operation may be summarized as follows: The audio signal is first digitized by an A/D convertor and then applied to two parallel discriminator filters. The envelopes of the outputs of the discriminator filters rectified and then low-pass filtered. The output of the low pass filter are thresholded and the corresponding 0/1 output applied to the TxD serial line (the FLAG output of the 320C26). In addition, the output of the low pass filter is written to a D/A convertor. This signal is useful as a tuning aid when an oscilloscope is used. Just tune for maximum positive to negative transitions. 2. RS232 Connections For a minimal interface, Rd, Td, DTR, RTS, DCD, and Ground, is required. This will allow operation on RTTY and AMTOR. For a 25-pin serial connector: pin 2 (TD) - output data bits (mark -12V, space +12V) pin 3 (RD) - input data (also connected to pin 8 (DCD)) pin 4 (RTS) - PTT (off -12V, on +12V) pin 8 (DCD) - input data bits (mark -12V, space +12V) pin 7 (Ground) For a 9-pin serial connector: pin 3 (TD) - output data bits (mark -12V, space +12V) pin 2 (RD) - input data (also connected to pin 1 (DCD)) pin 7 (RTS) - PTT (off -12V, on +12V) pin 1 (DCD) - input data bits (mark -12V, space +12V) pin 5 (Ground) If this program is to be used with the popular BAYCOM packet program, an additional wire link between RD and CTS is needed. 3.0 Special modems Five modems are provided for usage on RTTY, AMTOR, PacTOR, and HF Packet. The main features are summarized in the following table Tone freq's BW Data LPF -------------------------------------------- RTTY 2125/2295 85 50 AMTOR 2125/2295 100 100 PacTOR 2110/2310 160 100 HF Packet 1600/1800 160 150 RTTY 1275/1445 85 50 -------------------------------------------- 4.0 Disclaimer The author, Johan Forrer KC7WW is not responsible for any damage, injury, loss of profit or gain associated with the use, installation, or application of this software. December 1993 J.B.Forrer KC7WW 26553 Priceview Drive Monroe, OR 97456 United States of America e-mail: forrerj@FRL.orst.edu CompuServe: 70730,3472