Baycom radio modem for PC. Scheme, description

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Baycom radio modem for PC. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Data transfer

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As you know, to work with the package, you need a device that is an intermediary between the computer and the radio station - TNC (Terminale Node Controller). In addition to converting a digital sequence into sound packets (as the Modem does, for example, with RTTY communication), the TNC converts packets formed in accordance with the AX-25 protocol into ASCII codes understandable to the computer, and also performs many other special functions.

The TNC consists of a microprocessor, RAM, permanent memory, a clock generator, etc., i.e. parts, as a rule, that make up any computer. Radio amateurs, of course, wondered: is it possible to use a computer in packet communication mode without resorting to additional devices such as TNC, but to make sure that the computer takes over all the functions of the packet controller?

German radio amateurs DG3RBU and DL8MBT developed software for the COMMODORE C-64 home computer, which they called DIGICOM-64. It allows you to work as a package, while only a small set-top box is required - a modem to connect the computer to the radio station. The DIGICOM-64 program has been very popular in Europe, where the number of users of the COMMODORE C-64 computers is very high. With the widespread use of IBM PC computers, naturally, the question arose of creating a similar program for this computer. In late 1988, Andy Payne N8KEI created a program that allows you to run a package on an IBM PC without a TNC. He named it PMP (Poor ManPacket). In early 1990, the BAYCOM program appeared, developed by the same radio amateurs as DIGICOM-64.

Thus, for those who have the opportunity to use an IBM PC, it is enough to make a small modem and connect to a radio station. A diagram of one of the options for such a modem on the TCM-3105 chip is shown in the figure.

(click to enlarge)

Baycom radio modem for PC

The modem consumes only 3.5 mA, so it is powered directly from the RS-232 interface signals. BAYCOM uses a non-standard connection to RS-232 signals: DTR signal is used as transmitted data (from computer to radio station), CTS - received data, RTS - PTT (Push-To-Talk - transmitter control), high level corresponds to transmission mode.

This circuit only works with 1200 and 2200 Hz tones, which are used on VHF at 1200 baud. To work on HF (300 Baud, tone frequency spacing 200 Hz), you need to slightly modify the circuit. It is only necessary to make a modem with not 1200 and 2200 Hz tones used on VHF, but with any tones in the passband of the transceiver's audio path (300-3000 Hz), the separation between which is 200 Hz.

It is possible to modify the modem circuitry on the TCM-3105 so that the audio tones are 650 and 850 Hz. To do this, you need to change the signals at the inputs that determine the division ratios, and reduce the clock frequency by 2 times. This mode of switching on the TCM3105 microcircuit is not entirely correct, but as practice has shown, the circuit works quite stably to receive confidently audible signals. (There are no problems with transmission, the tones are clear and stable). It is not difficult to add switching to HF mode to the VHF modem if you have a quartz crystal at a frequency of 2.217 MHz (4.433: 2 = 2.217).

On HF, you have to tune in to receive burst signals by ear (you can make a simple indication, but this is again a problem with additional power, RS232 will not work). At first, tuning seems very difficult (however, with a TNC without HF indication, such as the PK-88, this is also difficult to do). You can try to tune the frequency of the transceiver by listening to your connection request (command: Connect CALL) and comparing the tones with those whose signals you want to receive. With some skill, it turns out pretty quickly. As soon as received callsigns appear in the bottommost window of the screen, stop tuning and try to connect.

Tear off kv.4.43 from the 15th leg of TCM3105, conn. to n.z. cont. P2K. N.R. contact of this group conn. to the last conn. Dr and C \u22d 16nF, other end - to the 15th leg of the TCM. Center. cont. of this group - to the 4th leg and to ext. C = 15/XNUMX pF KPK-M.

Tear off the 12th leg of the TCM from the ground, apply +5 to it through 10Kom and connect to the NC contact of the 2nd group P2K. Break the connection 2-TSM and 13-LN2, include 10 Kom in the gap, 13 leg LN2 is connected to the nr cont 2nd gr. Wed contact - to the ground. NS - 1200, HP - 300 baud.

As you know, m / s TCM3105 cannot work on HF at a data rate of 300 baud in a typical switching circuit. Nevertheless, it is possible to use it on HF, with a slight change in the modem scheme (any "BAYCOM"-like one). There are at least two such options. The first is proposed by RW3DR and uses switching to CCITT V23 600 baud mode with a simultaneous change in the frequency of the quartz (2,217 instead of 4,43). It is inconvenient here that you either need to find a quartz at 2,217 MHz, or use an LC circuit for the frequency setting circuit of the TCM3105 internal oscillator. However, it is possible to use a different m/s mode to eliminate these problems. So, how to make the BAYCOM modem work on HF with minimal changes:

Disconnect pins 12 and 13 of the TCM3105 from ground and connect them together through a 10 kΩ resistor to + 5V (TCM switching to BELL 202 150 baud mode);
Disconnect the capacitors from terminals 15 and 16 (quartz connection point). they are not needed with a small length of wires between quartz and TCM;
Instead of quartz 4,43, turn on quartz at 8,86 MHz (used in many PAL decoders, etc.);
Turning off the inverter connected between TCM pins 2 and 5, connect these pins together.

With this inclusion, the tone frequencies are 775 and 975 Hz, the spacing is 200 Hz. Such a modem works normally, the tones are clear and stable, signals are received with confidence in case of strong interference.

In addition, the option of using m / s TCM3105 in modems for 2400 is known, despite the fact that it is not intended for this. The whole change in the circuit is to increase the frequency of the quartz and, accordingly, increase the AFSK frequencies. The choice of quartz frequency is a compromise value for 2400 on the TCM3105.

It is desirable that: the lower frequency (log 1) of the modulation in the AFSK message be as close as possible to the bit/s rate, otherwise the TCM will hardly receive such a signal - it is difficult to detect it. It is good if its frequency is such that the period of oscillation of the modulating AFSK frequency fits into the duration of the period for a speed of 2400. I.e. the lower modulation frequency should be as close to 2400Hz as possible. In addition, small values of quartz are also unacceptable for reasons of optimal shift between AFSK frequencies 0 and 1. Optimal shift for AFSK = 0.8*bit/s.

At 2400, it is necessary to strive for the shift between frequencies to be as close as possible to 1920Hz, but at the same time, the frequency (log 0) of the upper message increases accordingly and it already hardly passes through narrow-band low-frequency circuits and transceiver filters.

The frequency response of transceivers is different - therefore it is difficult to optimally select the frequency of quartz so that 2400 works with any radio station. The 8 MHz quartz frequency used by radio amateurs in Moscow is good for m/s - the lower frequency of 2160 Hz is close to 2400, but the upper becomes 3960 Hz, which is a lot for some radio stations. Moreover, both for the receiver paths and for the transmitter paths. In Europe, the variant with quartz 6.5536Mhz is more common.

To work at 2400, you need to adjust the new level (do it very carefully, slowly, with a high-turn resistor) on the RxB leg.

Publication: cxem.net

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