Globalsat BR355 GPS

Hi all,

I have a Globalsat GPS receiver (NMEA 0183) which I have been told is capable of connecting to a PLC using RS232.

It has 4 cables:

+5VDC
0V
TX
RX

I have connected up to my RLC CSMSTRSX and configured the port as raw serial and as 4800/8/N/1. The TX of the GPS is connected to the RX of my RLC device.

However I only get junk coming in the string. I have been on the Globalsat forum and some people suggest connecting my 232 common to the 0V. Which I do not want to do.

Anyone have any ideas?

thanks

I recently did the same thing but my (maybe lousy) recollection was that the Globalsat model I used output RS485 levels, not RS232. The RS485 was used so the NMEA could be sent longer distances to multiple receivers, as opposed to RS232 which is only good for short distances (12 ft is the spec, if I recall) and just one receiver. Which model Globalsat and options are you using?

nwboson said:

Which model Globalsat and options are you using?

Click to expand...

BR-355S4

I thought it was RS232 - I can try 485.

some people suggest connecting my 232 common to the 0V. Which I do not want to do.

Click to expand...

RS232 MUST HAVEE A COMMON. If you don't want to hook that up then give up on this project.

bernie_carlton said:

RS232 MUST HAVEE A COMMON. If you don't want to hook that up then give up on this project.

Click to expand...

I don't mind hooking it up but will the 0V of my PSU be ok?

DLMUK said:

BR-355S4

I thought it was RS232 - I can try 485.

Click to expand...

No luck with 485! pretty sure it is 232.

It appears your unit provides the 'GGA' message. The beginning of it is the UTC. Reference here page 9

bernie_carlton said:

It appears your unit provides the 'GGA' message. The beginning of it is the UTC. Reference here page 9

Click to expand...

Sorry I don't understand the relevance of that link - might be me being stupid though.

I currently have no readable comms coming from the device...

So that model is RS232, not RS485.
You would need the PS2 connector of the GPS wired as follows:

GND - Pin 1 - To GND wire of 5VDC Power source
and also to Red Lion RS232 connector COMM pin(s).
RS232 (of Red Lion) requires this gnd reference.

VCC - Pin 2 - To 5VDC Power Source

Rx - Pin 4 - To Red Lion Tx pin

Tx - Pin 5 - To Red Lion Rx pin

The GPS device will provide a string in ASCII terminating with a Carriage Return and Linefeed.

It will begin '$GP' then 'GGA' and the rest of the message as shown. You (the device) will have to parse out the time, the 'hhmmss.ss' section. (hours - hh - minutes - mm - seconds - ss - hundreths of a second - .ss)

The '$GP' identifies the source of the message as a GPS device. The 'GGA' identifies this type of message.

This is UTC or as formerly known Greenwich Mean Time. You will have to provide your own local timezone correction.

[Edit: For some reason I got the impression that you wanted time (I don't know why, you said nothing about that in your original post) but this message has location informations also in the string. Good luck.]

nwboson said:

So that model is RS232, not RS485.
You would need the PS2 connector of the GPS wired as follows:

GND - Pin 1 - To GND wire of 5VDC Power source
and also to Red Lion RS232 connector COMM pin(s).
RS232 (of Red Lion) requires this gnd reference.

VCC - Pin 2 - To 5VDC Power Source

Rx - Pin 4 - To Red Lion Tx pin

Tx - Pin 5 - To Red Lion Rx pin

Click to expand...

I have connected like this but the comms is still all over the place. I have created a small program to simply read the string and separate out the header.

Every now and again I get $GPGGA or GGA or $GP. I seem to be half way there but cannot quite get full comms.

I have check the manual, it is 232, 4800/8/N/1. Tried other settings anyway and get even worse results.

Connected a screen on the cable to see if it was a noise problem, GPS has is stuck out the window facing skyward.

At a bit of a loss as to what the cause is.

Thanks

Its interesting that the manufacturer calls this 'GPS with PS2 Interface'. The PS2 uses a TTL serial connection rather than RS232. And its also interesting that it runs off a single 5V supply. It may simply be this device outputs TTL serial rather than true RS232. TTL serial swings between 0V and 5V (approximately), whereas true RS232 is +12 to -12 or at least +3 to -3V, referenced to that COMM pin. You may occasionally be seeing good data because you are so close to the mark/space thresholds, but not quite there.

If you have a fast DMM with a peak function or an oscilloscope handy, what does the signal look like? Is it swinging at least above +3V (a high) and below -3V (a low)? If its TTL serial (not going below -3V) you may need to build or buy a TTL Serial to RS232 level converter.

@Nwboson

I do not have the means to test at the moment. However I do remember somewhere in the piles of reading I have done that the signal from the device is 0V to 6V (i think it was 6).

Which would tally up with what you are saying.

Cheers,