BCD-996T tap problems redux

[DaveNF2G]

I was assisted in an earlier thread to get UniTrunker to "see" baseband signals from a tapped BCD-996T via my soundcard.

Now I have a similar problem with the serial port.

I'm trying to feed the DOS versions of MTrunker and ETrunker and I get "sig?inv" or "WantDSR" every time. Nothing I do makes any decoding start. It's exactly the same setup that was working with the sound card.

Now I do understand that there are differences between soundcards and serial ports. :wink: What is not clear to me is what, if any, changes I have to make to my discriminator tap on the 996. The current setup includes a capacitor and a resistor in series.

Any help will be greatly appreciated.

 

[Dewey]

I was assisted in an earlier thread to get UniTrunker to "see" baseband signals from a tapped BCD-996T via my soundcard.

Now I have a similar problem with the serial port.

I'm trying to feed the DOS versions of MTrunker and ETrunker and I get "sig?inv" or "WantDSR" every time. Nothing I do makes any decoding start. It's exactly the same setup that was working with the sound card.

Now I do understand that there are differences between soundcards and serial ports. :wink: What is not clear to me is what, if any, changes I have to make to my discriminator tap on the 996. The current setup includes a capacitor and a resistor in series.

Any help will be greatly appreciated.

How is the discriminator wired into your serial port? I see no mention of a data slicer... REQUIRED.

Dewey

 

[ka3jjz]

Again, from our wiki

http://wiki.radioreference.com/index.php/Data_Slicers

73s Mike

 

[slicerwizard]

Hi Dave, you start by running SLICER.EXE, which the goofy wiki page calls "the Hamcom Tuning Interface Application"

 

[DaveNF2G]

How is the discriminator wired into your serial port? I see no mention of a data slicer... REQUIRED.

Dewey

I can see where you might have gotten the impression that I was talking about a direct connection. Sorry about that. I'm using the generic 2-level slicer that just about everyone uses for Trunker.

BTW, I thought I had posted this in the Uniden Scanners forum. Not sure how I managed to put it here.

 

[DaveNF2G]

Hi Dave, you start by running SLICER.EXE, which the goofy wiki page calls "the Hamcom Tuning Interface Application"

I'll try that. I haven't had to use it before, but this is a different radio, so...

 

[pro92b]

You mentioned the message 'want DSR' but most slicers use the CTS pin. Did you set Trunker to CTS and try it? In any event slicer.exe will tell you which pin is active (if any).

 

[Dewey]

I can see where you might have gotten the impression that I was talking about a direct connection. Sorry about that. I'm using the generic 2-level slicer that just about everyone uses for Trunker.

BTW, I thought I had posted this in the Uniden Scanners forum. Not sure how I managed to put it here.

Ok, sorry about that. I was going to ask if you have the correct com port set, but since it appears that you've run the DOS programs before (your statement below about a different radio), I would start with Andy's (slicerwizard) advise to see what your signal through the slicer looks like.

Dewey

 

[DaveNF2G]

You mentioned the message 'want DSR' but most slicers use the CTS pin. Did you set Trunker to CTS and try it? In any event slicer.exe will tell you which pin is active (if any).

Yeah, I tried all 4 related configs - DSR Normal, DSR Invert, CTS Normal, CTS Invert. Nothing happens with CTS (which is exactly what I'd expect as I wired the slicer to use DSR). I get the error messages on DSR.

 

[pro92b]

Try a few experiments to isolate the problem if possible.

Regarding the computer, is the serial port turned on and working? I have two old Thinkpads that default to serial port off and it is necessary to specifically run a configuration utility to enable the port.

If you swap out the 996, does the setup work with another radio monitoring the control channel and feeding the slicer?

I don't know what IF chip is used in the 996, but if your slicer has hysteresis built in, is it small enough to allow switching with the signal level from the 996? If you can identify the IF chip and the slicer schematic, that can be assessed.

Do you have more than one slicer to try?

 

[DaveNF2G]

Try a few experiments to isolate the problem if possible.

Regarding the computer, is the serial port turned on and working? I have two old Thinkpads that default to serial port off and it is necessary to specifically run a configuration utility to enable the port.

The computer in question is a Dell desktop. The serial ports are on according to the BIOS setup screen.

If you swap out the 996, does the setup work with another radio monitoring the control channel and feeding the slicer?

Yes.

I don't know what IF chip is used in the 996, but if your slicer has hysteresis built in, is it small enough to allow switching with the signal level from the 996? If you can identify the IF chip and the slicer schematic, that can be assessed.

I'll get back to you on that. I built the "standard" slicer whose schematic has been floating around online for at least a couple of years. I've been using it for about that long.

Do you have more than one slicer to try?

Nope, I only built this one.

 

[pro92b]

I opened my 996 and measured the DISC point with a scope so I know what kind of signal is present. The IF chip must be under one of the metal shields so there is no point looking for it. There are many slicer variants so I need the exact circuit you are using.

 

[DaveNF2G]

I've been searching for it because I want to build another one. Haven't found it yet.

 

[DaveNF2G]

OK, I tried SLICER.EXE and DUMPER.EXE on my 996. Dumper shows sig?inv no matter what. Slicer shows DSR as the active pin with interrupt rates varying from 0 to 15 per second.

It looks like the discriminator is being loaded down by the slicer, or else the 996 just doesn't produce much power in the baseband audio circuit.

My setup: DISCRIMINATOR -> CAPACITOR -> RESISTOR -> SLICER

As I understand it, the capacitor decouples the DC path between the discriminator and the slicer and the resistor is supposed to prevent the slicer from loading down the discriminator output. So, gurus, what's wrong with this picture?

 

[SCPD]

My setup: DISCRIMINATOR -> CAPACITOR -> RESISTOR -> SLICER

What is the value of the resistor?
What is the value of the capacitor?
What IC was used in the circuit?
How is your slicer powered?

Check the power. Get out your VOM and measure the voltage at the V+ and V- pins on the IC. You'll want at least +6 and -6 volts - maybe more depending upon the IC used.

 

[pro92b]

OK, I tried SLICER.EXE and DUMPER.EXE on my 996. Dumper shows sig?inv no matter what. Slicer shows DSR as the active pin with interrupt rates varying from 0 to 15 per second.

It looks like the discriminator is being loaded down by the slicer, or else the 996 just doesn't produce much power in the baseband audio circuit.

My setup: DISCRIMINATOR -> CAPACITOR -> RESISTOR -> SLICER

As I understand it, the capacitor decouples the DC path between the discriminator and the slicer and the resistor is supposed to prevent the slicer from loading down the discriminator output. So, gurus, what's wrong with this picture?

I doubt that the slicer is loading the discriminator significantly. If it was you would notice a problem with the radio like no audio. The 996 doesn't produce much voltage at the tap point and that may be the root of the problem. The power delivered from the tap to the slicer should be vanishingly small in any case, otherwise it will load the discriminator.

The setup is correct conceptually but the problem is in the details. Component values and the slicer schematic are required to determine what is wrong.

The capacitor blocks DC and passes AC. The capacitance value must be large enough to pass the required AC bandwidth. Since your slicer probably has a capacitor at its input, in this application the capacitor in the radio is redundant and serves no purpose. The case may be different for direct sound card connections although they should be AC coupled as well in nearly all cases.

The resistor isolates the tap from the outside world and if placed at the tap point rather than the jack on the rear panel, it helps keep noise from being injected into the tap. The slicer should not load the discriminator if it is designed properly even if there is no resistor in the radio.

 

[DaveNF2G]

I opened up the 996 and a PRO-2067 that I have used successfully with this slicer. I verified that the '2067 is able to feed a trackable signal to the computer where the BCD-996T is not.

The PRO-2067 is tapped with a 1.0 uF tantalum capacitor.

The BCD-996T is tapped with a 1.0 uF tantalum capacitor and a 10 kOhm resistor in series. This combination provides a trackable signal to UniTrunker via soundcard. However, it fails to feed a trackable signal to Trunker via 2-level slicer.

The slicer in question is based on a LM741 and is powered from the serial port.

While I had everything open, I tried jumpering past the resistor and capacitor in turn, and then bypassing both of them. According to both SLICER.EXE and DUMPER.EXE, there was no difference among the test configurations.

 

[pro92b]

What are the values of the resistors in the slicer and what do they connect to?

 

[DaveNF2G]

I guess I'll have to reverse engineer the damned thing because I can't find the exact diagram I used when I was building it. I don't think there are too many different ways to build a 741-based 2-level slicer, though. Like I said, I built the thing years ago so the design has been around for a while.

 

[mikeATtleccdotnet]

That’s strange, I’m using a tapped Uniden 996t directly into an old IBM Thinkpad’s Mic input and it work’s great. The tap consists of a 10µF Tantalum Cap and 10k Ω resistor to an RCA Jack on the back. UniTrunker decodes at 95% with a rather poor control channel signal level, less then -95dbm.

-Mike

I've also taped a 800mhz MTS2000 and had about the same results.