Difficulty transmitting to PMB-648 GPS
Phil Pilgrim (PhiPi)
Posts: 23,514
I need to be able to reconfigure my PMB-648 GPS so it will not latch onto a fixed position when it's not moving. I tried using the command strings provided here by Diego Potones to no avail. Further investigation shows that the logic-level receive line (blue wire) on the PMB-648 is either being driven high to +5V or pulled up with a very stiff pull-up resistor (although no resistance can be measured when the unit is powered down). When powered, a 1K pull-down results in only a 1V decrease in the voltage on the pin. I have two of these GPS units, and they both exhibit the same behavior, so I don't think it's due to a defective part. Both units function properly transmitting NMEA sentences with the correct coordinates.
I would like to be able to communicate via the PropBOE's servo headers, which have series protection resistors, but have thus far been prevented from doing so. Has anyone else experienced this issue? If so, how did you overcome it? I'm reluctant to force the issue with a transistor buffer until I know that it won't harm the GPS.
Thanks,
-Phil
Footnote: It occurs to me that the logic-level input may be being driven by the RS232 input interface. Perhaps there's something that needs to be done with the RS232 input line to disable the input driver? It's hard to find much technical info about this module -- especially from Polstar.
Footnote 2: Yep, this is definitely the case. If I connect the RxD pin to +3.3V, the Rx pin goes low.
I would like to be able to communicate via the PropBOE's servo headers, which have series protection resistors, but have thus far been prevented from doing so. Has anyone else experienced this issue? If so, how did you overcome it? I'm reluctant to force the issue with a transistor buffer until I know that it won't harm the GPS.
Thanks,
-Phil
Footnote: It occurs to me that the logic-level input may be being driven by the RS232 input interface. Perhaps there's something that needs to be done with the RS232 input line to disable the input driver? It's hard to find much technical info about this module -- especially from Polstar.
Footnote 2: Yep, this is definitely the case. If I connect the RxD pin to +3.3V, the Rx pin goes low.
Comments
About 2/3 of the way down the post by 'jerry_s - PostPosted: Aug 14, 2009 - 09:39 AM' has something about 'static navigation'. Maybe just search the page for 'static navigation'. Hope that helps!
EDIT: This part inparticular - "Unfortunately, my GPS unit doesn't go into this mode after powerdown and powerup, so it seems like I always have to switch to binary sirf mode, turn off static navigation and jump back into NMEA mode."
The problem is that the Logic Data In pin is connected to the output of the RS232 level translator, so it has a very low impedance, which I'm not sure whether I should breach. I tried the RS232 input, but the Prop output through the PropBOE's 3.9K series resistor is too wimpy to drive it. If I knew it was safe to drive the Logic Data In pin low while the level translator is driving it high, I could use this emitter follower circuit, soldered right onto the servo plug:
-Phil
-Phil
I am having the exact same problems. I am using a PIC24F microcontroller and have tried hundreds of things to get this to work with NO AVAIL!!! I am going to try to connect this to an RS-232 port on an old computer to figure out if I can reprogram this offline before placing it back into the enclosure in which it belongs.
AFAIK, you have have either to use the transistor circuit shown above to drive the TTL pin or to go the RS232 route.
-Phil
Do you by chance have the program that you used to change the PMB-648 from static to constant nav mode? I am looking to do the same with an identical setup to the one that you have. Thanks!
-Nick
It's in this program that I used to record GPS data:
CON _clkmode = xtal1 + pll16x _xinfreq = 5_000_000 'SD Card pins: DO_PIN = 22 CLK_PIN = 23 DI_PIN = 24 nCS_PIN = 25 'Serial I/O pins: GPS_IN = 16 GPS_OUT = 17 PC_IN = 31 PC_OUT = 30 'Write indicator: LED_PIN = 26 'Unit ID: UNIT = "0" 'ASCII constants: CR = 13 LF = 10 OBJ sio : "FullDuplexSerial" gps : "FullDuplexSerial" sd : "fsrw" PUB start | ch, i gps.start(GPS_IN, GPS_OUT, 0, 4800) sio.start(PC_IN, PC_OUT, 0, 9600) sd.mount_explicit(DO_PIN, CLK_PIN, DI_PIN, nCS_PIN) filename[7] := UNIT dira[26]~~ waitcnt(cnt + clkfreq) gps.str(string("$PSRF100,0,4800,8,1,0*0F", CR, LF)) 'Switch to SiRF binary protocol at 4800,8,N,1 'Diable Static Navigation waitcnt(clkfreq + cnt) repeat i from 0 to 9 gps.tx(DisStatNav[i]) 'Switch back To NMEA Protocol Message ID 129, leave only GGA and RMC strings and keep speed at 4800 waitcnt(clkfreq / 2 + cnt) repeat i from 0 to 31 gps.tx(BackToNMEA[i]) repeat waitstr(string("$GPRMC,")) getstr(@time, 10) 'if (time[5] == "0" or time[5] == "5") gps.rx if (gps.rx == "A") gps.rx getstr(@lat, 9) waitstr(string("N,")) 'Change to "S" for southern hemisphere. getstr(@lon, 10) sio.str(@time) outa[26]~~ 'Turn LED on during write, so it won't be interrupted by flipping off the power. sd.popen(@filename, "a") 'Open the file to append data. sd.pwrite(@time, strsize(@time)) 'Write the time and position line to the file. sd.pclose 'Do a close to stat the file. outa[26]~ 'Turn off the LED. else sio.tx("*") PUB waitstr(strptr) | ptr, match ptr := strptr repeat while match := byte[ptr++] if (gps.rx <> match) ptr := strptr PUB getstr(strptr, len) repeat len byte[strptr++] := gps.rx DAT filename byte "gpsdata_.txt" time byte "dddddd.ddd," lat byte "ddmm.mmmm," lon byte "dddmm.mmmm",13,0 'SiRF Disable Static Navigation, Message ID 143 DisStatNav byte $A0,$A2,$00,$02,$8F,$00,$00,$8F,$B0,$B3 'Switch back To NMEA Protocol Message ID 129 at 4800 bauds, leave on GGA and RMC strings updating oce per second. BackToNMEA byte $A0,$A2,$00,$18,$81,$02,$01,$01,$00,$01 byte $00,$01,$00,$01,$01,$01,$00,$01,$00,$01 byte $00,$01,$00,$01,$00,$01,$12,$C0,$01,$61 byte $B0, $B3-Phil
I think it's because it's still early, (at least that's what I'm telling myself!), but how did you connect both pins 16 and 17 on the PropBOE to the GPS cable/transistor circuit that you made? From the picture of the cable, it looks as if you would only be able to connect on pin to the GPS. I know I am missing something here, but I cant quite grab at what it is!
Here's the complete schematic for the cable I built. The BOE end is a 2x3 header receptacle that connects to both P16 and P17:
-Phil