How does MAX7400 work?

[JimB]

JimB, I'm sending 8bit from a microprocessor (PIC).
Futterama

So, I assume these 8 bits just represent the status of something or other.

In that case you could invent your own transmission protocol if you have control of both ends of the link and it is a "closed system", ie it is not intended for general reception.

Maybe a good starting point for some of the "I want a project for my college course" merchants we get on this board!

JimB

 

[Nigel Goodwin]

Well, Nigel, I have read alot about the rules and regulations, and found nothing thats says that I can't send "home composed music" (data tones) through a PMR.

Except you're NOT sending "home composed music", you're sending computer data - and presumably that IS banned by the PMR regulations? - as you didn't say it was allowed!.

For modems for such purposes (regardless of legality), try searching for 'packet modem', these are EXACTLY what you need, used by radio amateurs for sending data on specific allocated legal frequencies. They simply connect to the microphone, PTT button, and speaker connections on your transceiver.

 

[Futterama]

Nigel, what is wrong with eg. 1200Hz & 1500Hz square waves?

 

[Nigel Goodwin]

Nigel, what is wrong with eg. 1200Hz & 1500Hz square waves?

You don't really want squarewaves, although the bandwidth limitations of a voice channel will chop the high frequencies off anyway - so it's not too critical - but I would suggest a low-pass filter to reduce the higher harmonics.

Those sorts of frequencies are in the region of what's normally used, although I can't remember the exact values? - if you search for 'packet modem' or 'packet radio', you should be able to find the exact values normally used.

BTW, what baud rate are you wanting to use?, usual values for simple modem techniques are 300 to 1200 baud! - above that things get MUCH more complicated.

 

[Futterama]

I only need to transmit 8 or 16 bit per second. But this should be at a higher baudrate since I want to conserve power, eg. the transmission should be kept short.

So I was thinking, transmit eg. 10 "cycles" or "waves" of eg. 1200Hz for a bit 1 and eg. 10 "cycles" or "waves" of eg. 1500Hz for a bit 0. The recieving PIC will then analyse the 10 cycles to determine whether it's a bit 1 or a bit 0.

How do you normally do this thing?

 

[Nigel Goodwin]

I only need to transmit 8 or 16 bit per second. But this should be at a higher baudrate since I want to conserve power, eg. the transmission should be kept short.

So I was thinking, transmit eg. 10 "cycles" or "waves" of eg. 1200Hz for a bit 1 and eg. 10 "cycles" or "waves" of eg. 1500Hz for a bit 0. The recieving PIC will then analyse the 10 cycles to determine whether it's a bit 1 or a bit 0.

How do you normally do this thing?

You normally use a PLL in the receiver, a simple NE567 (tone decoder) will do, it accepts the incoming audio and provides a logic output to your PIC.

In the transmitter something like a 555 timer is often used, with a logic input from your PIC that switches between the two frequencies. The number of 'cycles' of each frequency is then dependent on the baud rate employed. Obviously you could easily generate the tones in software and save a 555 - but it's not really a big deal to add a 555?.

If you're switching your transmitter OFF when it's not used?, then you need to design accordingly, with suitable delays on the transmitter side (to give it time to settle down), and extra work on the receiver side so it doesn't try and decode data when the transmitter isn't ON.

Again, I suggest you try looking at packet radio systems, which are bidirectional half duplex, as you already have two way radios - packet switches the transmitters ON and OFF as required.

 

[JimB]

Nigel,
"Packet" is really intended for text and data transmission in long streams.

To repetitively send a collection of 8 or 10 bits of data, what is being proposed is possibly a better approach.

The use of a standard 300bps or 1200bps modem is a good idea, if a suitable IC is available at a reasonable price.

JimB

 

[Nigel Goodwin]

Nigel,
"Packet" is really intended for text and data transmission in long streams.

Yes, but the packet size is usually set in the PC software, NOT in the packet modem. The hardware though does EXACTLY what he wants, it's a radio modem link with full control over the transmit and receive switching.

If you don't want all the functionality you can simply use the parts you want?.

Incidently, one of my first Turbo Pascal projects was to write my own DOS software for running a Kantronics TNC2 Packet Modem on the 2m band (and yes, it was a LONG time ago).

The website http://www.baycom.org gives designs and software for radio modems, and is worth a look - it provides the information that 1200Hz is a mark, and 2200Hz is a space - it uses a modem IC type TCM3105. The same site also gives a packet modem purely in PC software, using the soundcard to do all the work - that might be worth studying as well?.

Another useful site would be http://www.qsl.net/yo5ofh/projects/ham radio projects.htm, which has a number of simple radio modem designs.

 

[Futterama]

Why, oh why, waste components (and board space) on a radio modem when it can be done in the PIC software?

But thanks for the links anyways ;-)