Update README.md

DeanCording · Apr 7, 2018 · 45ee9a9 · 45ee9a9
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 # owonb35
 
-Bluetooth Client for Owon B35 Multimeter
+Bluetooth Client for Owon B35 Multimeter for newer (post mid 2017) multimeters with the Semic CS7729CN-001 chip.
+
+Older multimeters that use the Fortune Semiconductor FS9922 chip should use [inflex/owon-b35](https://github.com/inflex/owon-b35).
 
 The [Owon B35](http://owontme.com/products_owon_3_5%7C6_digital_multimeter_with_bluetooth) series of multimeters use Bluetooth Low Energy (BLE) to transmit readings to remote monitors such as smartphone apps.  This Linux client allows to to caputure measurement data to you computer to record in a file or display on charting software such as gnuplot or KST.
 
@@ -77,7 +79,98 @@ Single value - `omonb35 -n -b | mosquitto_pub -t voltage -l`
 JSON format - `omonb35 -T -b -j | mosquitto_pub -t measurement -l`
 
 
+## Protocol
+
+The multimeter uses the Bluetooth Low Energy Generic Attributes [(BLE GATT)](https://www.bluetooth.com/specifications/gatt/generic-attributes-overview) to transmit measurements.
+
+Measurement data is output as a BLE notification with a 0xfff4 UUID. The packet consists of three uint16_t numbers.
+
+The first number encodes the function, scale, and decimal places.
+```
+Overload                                1 1 1
+Decimal--------------------------------------
+                                          | |
+DCmV         1 1 1 1 0 0 0 0  0 0 0 1 1 x x x  f0 18
+DCV          1 1 1 1 0 0 0 0  0 0 1 0 0 x x x  f0 20
+ACmV         1 1 1 1 0 0 0 0  0 1 0 1 1 x x x  f0 58
+ACV          1 1 1 1 0 0 0 0  0 1 1 0 0 x x x  f0 60
+                                             
+DCuA         1 1 1 1 0 0 0 0  1 0 0 1 0 x x x  f0 90
+DCmA         1 1 1 1 0 0 0 0  1 0 0 1 1 x x x  f0 98
+DCA          1 1 1 1 0 0 0 0  1 0 1 0 0 x x x  f0 a0
+                                            
+ACuA         1 1 1 1 0 0 0 0  1 1 0 1 0 x x x  f0 d0
+ACmA         1 1 1 1 0 0 0 0  1 1 0 1 1 x x x  f0 d8
+ACA          1 1 1 1 0 0 0 0  1 1 1 0 0 x x x  f0 e0
+                                            
+Ohm          1 1 1 1 0 0 0 1  0 0 1 0 0 x x x  f1 20
+kOhm         1 1 1 1 0 0 0 1  0 0 1 0 1 x x x  f1 28
+MOhm         1 1 1 1 0 0 0 1  0 0 1 1 0 x x x  f1 30
+                                            
+nF           1 1 1 1 0 0 0 1  0 1 0 0 1 x x x  f1 48 
+uF           1 1 1 1 0 0 0 1  0 1 0 1 0 x x x  f1 50
+                                            
+Hz           1 1 1 1 0 0 0 1  1 0 1 0 0 x x x  f1 a0
+Duty         1 1 1 1 0 0 0 1  1 1 1 0 0 x x x  f1 e0
+                                            
+TempC        1 1 1 1 0 0 1 0  0 0 1 0 0 x x x  f2 20
+TempF        1 1 1 1 0 0 1 0  0 1 1 0 0 x x x  f2 60
+                                            
+Diode        1 1 1 1 0 0 1 0  1 0 1 0 0 x x x  f2 a0
+                                            
+Continuity   1 1 1 1 0 0 1 0  1 1 1 0 0 x x x  f2 e0
+                                            
+hFE          1 1 1 1 0 0 1 1  0 0 1 0 0 x x x  f3 20
+                                            
+             +------+---+--------+-----+-----+
+                F     0  Function Scale Decimal
+                
+                
+Function
+    0 0 0 0 - DCV
+    0 0 0 1 - ACV
+    0 0 1 0 - DCA
+    0 0 1 1 - ACA
+    0 1 0 0 - Ohm
+    0 1 0 1 - Cap
+    0 1 1 0 - Hz
+    0 1 1 1 - Duty
+    1 0 0 0 - TempC
+    1 0 0 1 - TempF
+    1 0 1 0 - Diode
+    1 0 1 1 - Continuity
+    1 1 0 0 - hFE
+
+Scale
+    0 1 0 - micro (u)
+    0 1 1 - milli (m)
+    1 0 0 - Unit
+    1 0 1 - kilo (k)
+    1 1 0 - mega (M)
+    
+Decimal Places
+    0 0 0 - 0
+    0 0 1 - 1
+    0 1 0 - 2
+    0 1 1 - 3
+    1 0 0 - 4
+    1 0 1 - 5
+    1 1 1 - Overload
+```
+
+The second number bit encodes the reading type.
+```
+Hold------------------------   01/05 Autorange
+Delta--------------------- |   02/06 Autorange
+Autorange--------------- | |   04
+Min-----------------   | | |   10
+Max--------------- |   | | |   20
+                 | |   | | |
+   00000000  X X X X X X X X
+             8 4 2 1 8 4 2 1
+```
 
+The third number is the measurement digits as a signed magnitude binary number (msb is sign bit). It is converted to the floating point by using the number of decimal places as specified in the first number.