FRN interface

Licence free two-way radio services that now includes both FM and digital channels. Discuss models, modifications and other similar worldwide standards such as FRS and GMRS.
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FRN interface

Post by radiosification » 15 Feb 2011, 00:37

Hey everyone
I have bought a radio to set up a gateway on FRN but it doesn't have VOX. Also, my computer doesn't have a COM port. Does anyone know what I can do other than building the circuit myself as I don't have a soldering iron.
Could I buy an interface like the one on this site:
Sorry I can't link to the product but they have some stupid set up that redirects you back to the homepage. The one I'm talking about is the last one in "PSK-Interfaces"
Would it be possible to use that one (It indicates something like that on the FRN site but doesn't say exactly which one) with a USB adapter as I don't have a COM port.
If you're interested in digital voice, check out my YouTube channel:

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Re: FRN interface

Post by mattltm » 15 Feb 2011, 13:44

Spend the money on buying a soldering iron and make your own.

Worth doing it for the experience.

As for a com port, a usb to serial adaptor will sort that out.
"Spooky action at a distance."

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Re: FRN interface

Post by thazlett » 16 Feb 2011, 00:13

I'm in the process of doing this for a CB radio for my gateway. I have a PMR446 one set up already.

I found this diagram more helpful then the one on the FRN website: ... =3&t=10651

The FRN diagram seems to miss out the PTT connection which I found a bit confusing, or was I missing something?

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Re: FRN interface

Post by rpcomms » 12 Feb 2018, 23:57

The FRN website needs updating
I too found it confusing too

if you instal the frn software,goto setting,then ull see the comms page for interface,the website shows old pictures that causes you to get confused

PTT Out use RTS or DTR Hi or Low activate
Busy detect RTS or DTR Hi or Loc activate

A usb to RS232 TTL converter thats carries CTS,CTR,DTR,RTS signals

this is a much more reliable method of interfacing the radio,than vox
also use 1:1 ratio
600:600 isolation transformers on the sound card line in/out

Im using opto isolators one for PTT and another for Busy Detect from radio
as well so radio is fully isolated from PC,NO HUM!

MIC Audio in 600ohm
Fixed Audio out from cb radio
"Busy flag" from cb radio,uses squelch switch on off of around a few volts DC inside cb ,so drive the opto isolator when channel busy,FRN will then know has a valid signal
If its in VOX,software will assign sound-card live all the time to monster for activity

Its a much better, solid way of doing it than VOX as delays,ramp up times can cause issues.

Also check squelch open and close,ensure radio interfacing has not got lazy squelch esp opening up,there no delays
Some old school cb use carrier squelch,and yes do have AGC on FM!,
sounds daft but Uniden Unlace looking at has it,daft but something to check.
Ive altered the cb radio so has faster open\close times on squelch,and no squelch chop when vary signals coming into radio.
some radio squelches can be very aggressive on the hysteresis curves

System levels references

CB 60% deviation =1.20khz
Max dev 2.00khz for 10Khz spacing
UK40 CB is exception spec can go 2.20khz max

PMR446\HAM\LMR 60% deviation=1.50khz
Max dev 2.50khz 12.5khz spacing
bear in mind
speech only 2.2khz max (never set any higher if using CTCSS)
CTCSS 0.30khz
2.2+0.3=2.50khz max and no more

HAM\LMR\Marine\GOV 60% deviation=3.0khz
Max dev 5.00khz 25.0khz spacing
bear in mind
speech only 4.25khz max (never set any higher if using CTCSS)
CTCSS 0.75khz
4.2+0.0=5.0khz max and no more

watch if using china export multi-modes (they are not CB approved specs at-5555V3,etc) often they are set up for,ive found a few where these are setup on fm wrong deviations (too high for cb over 4.0khz dev!!) and fm detectors set for FM-Wide,not narrow and may cause low audio from fm chip.
Due too incorrect resistor fitted across fm ceramic resonator on early pcbs.
I found R254=3K9 is optimized for 60% system dev=1.2khz
fm-narrow cb
some found in sets at 2K2,3K0,V6 pcb 4K3 is Okish but 3K9 seems spot on.

hope that helps

Rob 73's

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Re: FRN interface

Post by 26mb04 » 14 Feb 2018, 02:54

If you can't justify £ for isolation transformers or you just want the BaoFeng experience, you can use a low value cap in series with the audio out/in lines. It does the same DC isolation, just not as ideally as a 1:1 trans. Electrolytics are ok here on low-level ac, BUT:

Some Baofengs (including 888s) have a DC offset on the audio out! The electrolytics must being wired the correct way here. You can use the offset to drive a PTT switch without vox, and without opening up the BaoFeng! :D

I just discovered this last night, I though't I'd share it after spending too long messing with vox.

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Re: FRN interface

Post by 26mb04 » 15 Feb 2018, 17:12

Ok, it turns out the 888 doesn't have the DC offset, at least much of one. Not sure how to edit my previous post!

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