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I'm going to be pulling several functioning DIU3000s out of service. They will have no other purpose, so I was contemplating using them as standalone ASTRO decoders in conjunction possilbly with cheap, analog trunking scanners or other devices. I've used the diagnostic routines in the manuals during servicing and the handset to loopback on the DIU standalone and know that this works, but I'm wondering if anyone has tried pushing ASTRO from a straight receiver into a DIU. I'm thinking that the audio may need to be unfiltered, but perhaps IMBE doesn't require this. Has anyone tried this?

Your post brought up something that I'd been thinking about, in regards to the maxtrac transparent repeater mod. My understanding is the DIU's can, as you say, decode an Astro signal and put out analog audio. What's to stop someone from using it or something similar to activate the transmitter side of the maxtrac repeater? In other words instead of the recieving maxtrac being responsible for keying the transmitter, the external unit does, and at that point it retransmits whatever the recieving maxtrac is hearing. Does this make sense given the equipment and does it sound doable?

I'm not familiar with the term "transparent" repeater. Is the point of this system to reach into areas of poor coverage, like a BDA, or are you trying to pass securenet or something? I guess once I understand that, I might be able to answer more of your question.

I googled the term and found a hit on this site but it was blown away. Can you point me to a valid forum post?

Here's the post:

http://batboard.batlabs.com/viewtopic.php?t=45645

The idea is you could mod two Maxtrac radios to be a poor man's Astro/P25 repeater. The down side is they operate in CSQ and will pass ANYTHING they hear. So, if there was some way of externally controlling either the squelch on the reciever or the keying of the transmitter, it would solve that problem.

i wish i was lucky to be able to pull some DIUs and have no use for them

"...i wish i was lucky to be able to pull some DIUs and have no use for them..."
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ME TOO!

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With reference to the original post, yes, a DIU decodes IMBE into analog and encodes analog into IMBE. The problem I think you will find is that the DIU is intended to interface to a Quantar or other infrastructure device. It uses what is called Astro signalling protocol.

This protocol is intended to communicate control commands to the station and so on. Depending upon how motorola implemented that format, there may be significant differences from the "over-the-air" P25 CAI format.

In that case, that the DIU might not understand the over-the-air signal.

No doubt the IMBE voice data is embedded into the DIU-to-host protocol, but that protocol is intended for a dedicated connection [e.g V24 link] where there wouldn't be a need for the sort of error correction that is part of the over-the-air CAI. They probably did away with that to make room for the infrastructure control data.

It's still worth a try! You would have to build a widget to interface to the radio's raw discriminator audio, recover the four level channel data, decode the four-level symbols into two-level 9600 bps, and interface that to the V.24 levels the DIU wants.

xmo... Yes on the signalling... However, in test mode, you can loop the audio back into the DIU. As you may know, it involves creating a couple of jumpers on the COM port per the service manual. I believe this imitates the console signalling. Essentially, it's in a full-time pass-through mode.

Before I spent the time messing with it, I just wanted to see if someone had done this using an external ASTRO source of cheap quality.

liverdog wrote:I'm going to be pulling several functioning DIU3000s out of service. They will have no other purpose, so I was contemplating using them as standalone ASTRO decoders in conjunction possilbly with cheap, analog trunking scanners or other devices. I've used the diagnostic routines in the manuals during servicing and the handset to loopback on the DIU standalone and know that this works, but I'm wondering if anyone has tried pushing ASTRO from a straight receiver into a DIU. I'm thinking that the audio may need to be unfiltered, but perhaps IMBE doesn't require this. Has anyone tried this?

Do you need my address? I can find a use if you'd like

Sounds like a cool experiment.

I haven't done anything with DIU's, however, you could probably figure out what signaling the Quntar sends to it easily, and then replicate something to do that. Would be pretty easy to do with the right equipment.

I doubt if the loopback mode changes the line protocol - other than telling the box to listen to itself - but it will be up to you experimenters to find out!

A protocol analyzer would be the tool of choice. Ideally, you would capture the same transmission simultaneously on two instruments - one monitoring the RF channel and the other monitoring the link between a Quantar and a DIU [while receiving the same transmission].

A comparison of the captured data will tell you how much difference there is.

Well, that's what it's going to boil down too... An experiment. I'll post again after I've had a chance.

The "digital" side of a DIU is really just a balanced, differential data stream. In the case of a Quantar, received digital voice is demodulated in a DSP, and formatted into a synchronous data stream. Some of of the data can be decoded from that point in the repeater - network ID - but the voice data cannot. In in cabinet repeat, the received data stream gets re-digested by the DSP to be re-encoded into C4FM, and off to the exciter it goes. For outside the box apps, the DSP sends and receives either (both) differential (RS-422) data, or it's sent to a modem on the wireline card for transport over an analog path to a console or Astro Tac.

Bottom line.... The conversion to and from C4FM is done in the DSP in the Quantar (or other compatible radio), not in the DIU. The DIU is only half of the solution.