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Wednesday, 4 March 2015

DSTAR GPS Connections - Really?


You may recall last time I was musing about connecting a GPS receiver to my IC-7100 to complement the DSTAR mode.

Anyhow, I tested the electronics I threw together and found my voltage regulator to be highly suspect so I threw it in the bin. I hadn't got any 7805 regulators, so I had to construct a 5V supply using a LM317 I had. There's a really handy calculator here:

And I soon concluded that a value of R1 =  330R and R2 = 1000R would give me a 5.04V regulated output.

I built the 5.0V regulator and coupled it up to the GPS receiver; a GPS antenna that has been sitting on the porch roof for some time was connected to the antenna socket and a scope to the data output line.

Here's what I saw:

Now, clearly I can't read or interpret this stream of pulses, but it certainly looks like a RS-232 type data output to me.

So, I decided to connect it to the radio and see what happens.

Guess what? Nill, zero, zip, nada.

Now, there are a few variables here that I have to fiddle with. Firstly the data jack on the back of the radio, what is the pin out? There's nothing in the manual to tell me. I assume the ring of the connector is the ground, so we have two options there. The TXD and RXD could be one of two ways round. Lets' try the two options....

Guess what? Nill, zero, zip, nada.

Additionally there is the baud rate of the GPS data - is it 4800 baud? 9600 baud? Again I am not too sure.

I think the best thing for me to do is to hook the GPS receiver up to an Arduino board and see what the data actually looks like before going any further.

So here is a very Basic Arduino Scetch to read a byte from the GPS receiver and write it out to the Serial Monitor:

And this is the output:

So, this confirms that the GPS receiver is spitting out GPS data at 4800 baud.

So now I think I need to focus on the connection to the radio. There is NOTHING in the manual at all to tell you what the pin out of the DATA1 connection on the radio rear panel is. Time for some research....

I dropped Icom UK an email asking about the connections to the DATA1 port and very quickly received this image from then by means of reply:

Now, the radio is definitely configured for an External GPS connection at 4800 baud.... but still Nill, zero, zip, nada.

More head scratching required me thinks....


It's just come to me in a flash! My GPS receiver output is TTL and the IC-7100 needs an RS232 input. I need to convert one to the other!

Earlier on today by some total coincidence, I stumbled across the TTL to RS-232 converter I made back here for the FT1000D:

Not that finding this made me think that this could be the problem here! Anyhow, I've hooked the board from that project to the output of my GPS:

And this is the input and output of the converter on the 'scope:

And low, and indeed behold, this is the output connected to the IC-7100:

It works!

Local conditions.

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