Protocol for transferring data to and from the SAP...

The SAP waits for input on the USB port, and replies as necessary.

Think of it a bit like an SMTP server (or a command line).
All transactions are terminated by /n

PC side should be fairly easy - just use ordinary file interactions...
Theoretically, we could just use minicom to communicate...

commands should be limited to 256 bytes; wait for an OK back before sending more data...
any data sent after a \n and before the OK will be ignored.

General rules - Any command setting an attribute will receive the value set back, followed by an OK on the next line
If the set command is sent without a value, the current value is returned anyway.

SAP side - use an intermediary buffer...

On each run

if there is any data in the usb buffer
   copy into my_buf.
   if there is a CR in the buffer
      call service routine
      send OK if happy, KO if not
      reset buffer counter to 0.


unsigned char readHexByte(char *text, char *result) {
	 unsigned char tempa,tempb;
	 tempa = text[0] - 0x30;
	 if (tempa > 9) temp -= 7; // A..F
	 if (tempa > 15) temp -= 0x20; // a..f
	 if (tempa > 15) return -1; // invalid conversion
	 tempb = text[1] - 0x30;
	 if (tempb > 9) temp -= 7;
	 if (tempb > 15) temp -= 0x20; 
	 if (tempb > 15) return -1; // invalid conversion
	 result* = (tempa << 4) + tempb;
	 return 0;
}
      
unsigned char readHexWord(char *text, char *result) {
	 unsignec char tempa, tempb;
	 if (readHexByte(text,&tempa) return -1;
	 if (readHexByte(text+2,&tempb) return -1;
	 result* = tempa * 256 + tempb;
	 return 0;
}
      


Commands


H (Hiya)
No response - just send back an OK.
"OK"

L XX (Laser brightness)
Set laser brightness to 0xXX. if 0xXX == 0, turn laser off.
"%02hhX\n", laser brightness


D XX (Display brightness)
Set Display brightness to 0xXX (max FFh).
"%02hhX\n"

B [01] (Battery)
Turns on/off whether the battery reading is shown on startup.
"%02hhd\n"


T [01] (Temperature)
Turns on/off whether the temp reading is shown on startup
"%2hhd\n"


G[sensor] (get raw sensor readings)
sensor = G|M|T|V|I ( gravitic | magnetic | temp | volts | current)
get most recent raw sensor readings, as signed 16-bit ints, in hex format
"%04X %04X %04X\n" // for GM
"%2hd\n" // for TVI
return XXXX YYYY ZZZZ\n


S[sensor][index] XXXX(set calibration matrix)
sensor = G|M ( gravitic | magnetic )
index = [0..9|A|B]
Set grav or cal matrix item to (XXXX) / 0x4000 (gives us a max of +/-2.0)
"%04X\n"
return OK\n

R (Readings)
get current calculated readings
"%03d +02d\n" // compass, clino
OK

C
commit readings to long_term memory
"\nOK"



Advanced commands (for retrieving stored legs)
/* note lower case */
l [01]
Turn leg recording on or off
"%hhX\n"
return OK


g [index]
index = XXXX (hex address of leg to retrieve)
if no index is sent, return last leg sent...
"%03d +02d\n" // compass, clino
OK

s [index]
set index of next leg to be stored. (ie s 0000 should reset the device (but data will persist until overwritten)
will wrap at some point, not sure how big, just yet. Prolly 512d
"%04X\n"
return OK


Sample transcript....
> is sent from PC -> SAP
< is sent from SAP -> PC. One space added after > or < for clarity
; Comments

; verify connected
> H
< OK

; bad command
> V
< KO

; check current laser brightness
> L
< 8F
< OK

; set laser to max brightness
> L FF
< FF
< OK

; get current raw mag readings
> GM
< 20A6 E5F2 0034
< OK

; get current raw grav readings
> GG
< 4534 0003 FFF3
< OK

; bad message
> GQ
< KO


; set Gc[4] to 0x400/0x4000 (ie +1/16)
> SG4 0400
< 0400
< OK

; get Mc[10] - its 0x4200 (ie 1.125)
> SMA
< 4200
< OK

; bad string sent
> SG2 phil
< KO


; check current calculated bearings
; facing 35 degs upwards, headed due east
> G
< 090 +35
< OK