GPS Chip L86 output junk data on Serial Terminal

Thread Starter

ep.hobbyiest

Joined Aug 26, 2014
149
I am using L86 gps module. I was trying to get the module working with USB to serial converter. When i do the following connection
RX (GPS) <--> TX (USB to serial)
TX (GPS) <--> RX (USB to serial)
then nothing shows on the terminal. If i connect as
RX (GPS) <--> RX (USB to serial)
then lots of junk data displays on the terminal. how come gps throws data if connection is wrong?
Is my connection is method is wrong and How can i test the gps module.
Any help would be appreciated.
 

Papabravo

Joined Feb 24, 2006
12,848
If you connect two receiver inputs together, they will both pick up junk from the ether. What gave you the idea to try something manifestly destined for failure? The biggest problem in serial communication is getting the baudrate and the framing correct. How is that handled in your setup?
 

John P

Joined Oct 14, 2008
1,794
I'm betting you haven't got an oscilloscope. One of the most basic things you can do with one is simply to see where there's a signal and where there isn't.
 

JohnInTX

Joined Jun 26, 2012
3,976
You need to use a level shifter on the UART to convert the CMOS logic levels to RS-232 to drive the USB-serial dongle. See page 23/45 of the attached document. A MAX3222 would work.
The default setup is 9600-n-8-1.
Also, you should know that there are a lot of junk USB-RS-232 cables out there that perform poorly if at all. Get a DB9-F and connect pins 2 and 3 to make a 'wrap' plug. Then type at the terminal and make sure you get an accurate echo. If you have one with a genuine 'Prolific' chip and driver, that should work OK. Avoid others.

Good luck!
 

Attachments

Last edited:

John P

Joined Oct 14, 2008
1,794
Not clear, he said "USB to serial converter", not "USB to RS-233". It could be logic-level communication. A scope would sort this out! But mixing TTL-level and RS-232 would be a guaranteed way to lose.

Oh, regarding the Prolific chips, I just started a thread about a Prolific-based USB converter that worked for a couple of months on Windows 10 and then quit. It seems you have to have the right model of the chip to be sure it'll work with Win10. The real gold standard for USB conversion chips is FTDI, but the cheap Chinese stuff isn't likely to use that.
 

ArakelTheDragon

Joined Nov 18, 2016
1,327
The connection is not wrong. Sometimes its "RX" to "RX", sometimes its "RX" to "TX", it depends on the device. If there is garbage data, this is the correct connection. The garbage data is probably because the information is not synchronized(this is the most common reason). The serial terminal has settings, 9600kbps, NL and NC and other settings. You need the correct ones. On top of that USB to serial is not just connect the 2 wires. Sometimes you need an MCU to take the USB input and translate it to "RS232". Not to mention the USB driver for the operating system.

https://scienceprog.com/diy-usb-to-rs232-adapter/
https://core-electronics.com.au/tutorials/programming-with-usb-serial-converters.html
https://www.commfront.com/pages/3-easy-steps-to-understand-and-control-your-rs232-devices
 

Thread Starter

ep.hobbyiest

Joined Aug 26, 2014
149
Know idea why i though of connecting same direction together.
Yeah i was using USb to TTL (UART) converter ic.

The module started working after connecting second pair of uart. but the question remain that why that chip is throwing such junk data if the direction is same. and no data on correct direction.
 

Sensacell

Joined Jun 19, 2012
2,561
Serial data garbage- here is my troubleshooting list:

1) Baud rates correct? Measure the period of the signal with scope.
2) Signal levels correct - TTL? 5V? 3.3V? RS-232? etc.
3) Signal polarity - idle line high or low?

Check these step-by-step
 

ArakelTheDragon

Joined Nov 18, 2016
1,327
The co
yes.

yes

not sure.

Other UART on the same board is working with the same connection. So, i guess there is no issue with connection..
The connection is not the problem here, the problem is are your 2 devices synchronized, do you have the same settings on both the sending and receiving device.
 
Top