Something is different with GPS data between 436, HP1 and SDS100

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jjlongworth

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Warning - technical discussion ahead

I've worked in electronics for over 40 years and I've been scanner enthusiast for even longer. So, this isn't my first rodeo. I have a 436, which is an amazing scanner, and have successfully used the GPS feature in it since new. Rather than go the puck route, I chose to use a DeLorme PN-20. It was cheap and it allows me to still use the GPS features (i.e. speed, distance etc) while scanning. This has worked fine for years. As soon as you plug in the scanner, the GPS text appears on the display immediately regardless of the satellite status.

I also have a HomePatrol 1 that works with this setup. However, I should point out, that the GPS text doesn't appear until the GPS has a lock on the satelites.

To use the PN-20, I needed to make an adapter to breakout the necessary signals from the GPS: RXD, TXD and GND. Although I've read that only TXD and GND signals are required, I also connected RXD. The information for the gps side was well documented. The scanner side was a little trickier because it used the unique Hirose brand connector. Nevertheless, I found the pinout for the scanner and this setup worked perfectly.

Here's where it gets interesting: I recently purchased an SDS100. I'm not quite as thrilled with the SDS as I had hoped, but that is for a different post. One of my complaints would be the fact that they eliminated the Hirose connector and integrated the functionality into the mini USB (bad idea). Instead they added a Mico USB that, to my knowledge, has no functionality. But I digress...

Since the days of being able to find a good reference like SAMS Photofact are long gone for most things, I turned to the internet. I found a couple of posts on RR that mentioned the pinout for the mini USB plug on the side of the SDS. They've said that it is wired as a standard mini USB (looking at the back of the connection from left to right):

Pin 1 = GND
Pin 2 = ID
Pin 3 = D+
Pin 4 = D-
Pin 5 = V+

I've confirmed the pinout and the USB functionality works fine. I am able to program and charge the unit without any issues. I've read that since there are only two signals required (GND and TXD) for the GPS, that they used the unused ID pin for the TXD input. On principal, I hate mixing signals types on connectors to produce a "non standard" cable, but armed with this info, I made a cable similar to the one I use for the 436.

Take my word for it, I have checked my connections multiple times and I have confirmed that both devices are set for DMS:DDD:MM'SS.ss and 4800 baud. Something is different between these scanners. I could have a defective unit, but would find it hard to believe. It is supposed to be serial data, so we're not blazing a new trail here.

I could have just bought the puck and be on my way, but something is different between these two applications. Either there is a firmware issue or there is something different about the hardware internal to the scanner. Any ideas?
 

jjlongworth

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I'm sorry, maybe I should have been clearer on two points:
1) The SDS does not recognize the GPS. It doesn't matter what I try. For example, I've tried connecting the GPS lines with and without the rest of the connector powered thinking that perhaps they rely on the +5 on the USB to power the serial circuit. Doesn't matter. The scanner does not see the GPS.

2) In the case of the 436 and the HomePatrol, the firmware behaves differently when the GPS is plugged in. The 436 recognizes the GPS immediately. The HomePatrol requires a satellite lock. This made troubleshooting even more confusing.
 

kruser

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I believe you need to provide power to the GPS on the SDS100. I don't think the micro usb port supplies power but I could be wrong.

I built adapters for a Garmin GPS puck. It works for my Unidens with the funky Hirose serial connector as well as the unused pin in the SDS100 micro usb connector and then with the SDS200 via its RJ14 phone jack.
 

kruser

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P.S. I have not had a chance to put a scope on the signals. I may try and do that this weekend.
I just checked my adapters and the SDS100 uses the MINI USB port, not the Micro for the GPS signal. And it does require an external power source as the SDS does not supply power out from its USB ports.
 
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jjlongworth

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Ok. Well, I've tried powering both sides via battery and through a separate +5 supply to each. And yes, it is the mini USB, not the micro USB. I've buzzed out the cable 2 or 3 times and it looks correct. I'm stumped.
 

kruser

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Ok. Well, I've tried powering both sides via battery and through a separate +5 supply to each. And yes, it is the mini USB, not the micro USB. I've buzzed out the cable 2 or 3 times and it looks correct. I'm stumped.
Are your serial port options set correctly in the SDS100? They are under the Set Your Location menu where you set the baud rate etc.
 

kruser

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For those who might try and follow along, here is the USB pinout being discussed. This was found on another RR post:
Yep, serial data from the GPS should be on 1 - GND and 2 - ID.

V+ is an input only line. When the radio sees +5 on that pin, it triggers the Mass Storage or Serial Port Mode screen on the radio. It also provides the charge voltage of course.
 

jjlongworth

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Thanks for confirming the pin positions and the voltage status. I'll go back and look at it again (for the 4th or 5th time). Maybe I missed something. Otherwise, I have to conclude there is something wrong with the serial input. I'll keep you posted.

BTW - yes, I went to Location Setting and setup the data format and baud rate. Unless there is some other setting that turns the serial port on and off that I'm not aware of... Maybe I'll try a hard reset?
 

kruser

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One other thing just came to mind. The models with the Hirose connector use standard RS232 level signaling.
The SDS series expect TTL level and won't work with RS232 level signaling.
I remember now that I had to insert a 232 to TTL adapter board between my GPS and the data plugs that fit my SDS 100 and 200.
They are cheap like just a few bucks and are all over Amazon and eBay. I bet that's the problem.

If you have a place like MicroCenter near you or maybe a Fry's electronics, they may carry them locally if they are open but they will cost more than the Amazon route.
 

byndhlptom

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some possibles....

1- Have you tried the GPS with just the SDS hooked up? Most serial devices are not intended to drive mutiple rx ports. Experience is that 1-2 is usually OK, any more is contingent on TX driver capability. You may have to buffer the data stream to drive multiple devices.....

2- You should be able to monitor serial stream with a PC serial port. set it the same as everything else (4800,8,n,1?)

3- Looking at serial stream with scope can determine is you are loading down the TX port of GPS. I'd expect problems if signal dropped below +/- 4 v. Different devices/RS232 rx chips have different minimum signal level requirements.

$.02
 

jjlongworth

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So... just to summarize:
The serial data should be RS-232 level. Not TTL.
The only pins required on the scanner are Pin 1 - GND and Pin 2 - TXD

Hit_Factor - You said you've built a couple of these, but only the last one worked. I assume then that you connected TXD directly to Pin 2 of the mini USB and Ground. Correct?

If that is true, then I've got the cable right but the scanner still does not see the GPS. I've even done a factory reset and reinstalled the data base and user settings. I've double checked the Location Settings in Sentinel and they are set for DMS:DDD MM'SS.ss and 4800 baud.
 

jjlongworth

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I’m on the sofa thinking about this issue and had a thought: could it be the length of the cable or the wire gauge :unsure:

I used a spare USB cable to carry the TXD and Gnd between the GPS and the scanner. It’s very thin (30ga?) wire about 3’ long.
 

nessnet

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I'm surprised Jon hasn't chimed in here.....

The SDS100 uses RS-232 (BR-355S4) - Amazon.com: GlobalSat BR-355S4 GPS Receiver (Black)

I've built a number of my own cables, and didn't have any problem with any of them using the pin-out as stated.
You also said you are set at 4800, so you are good there.

As to the cable size - not really gonna matter. RS-232 isn't too sensitive in that regard. If the radio isn't getting the serial data, it's a high probability the cable isn't done correctly (or broken) somehow.
 
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