IoT-Sensor Link

Different sensors can be attached to our IoT-Stations. Our simple Sensor-Link Protocol allows the development and connection of new sensors.

The communication runs over a half-duplex RS485 connection in multi-master operation mode. So each sensor can send its measurement without the need of constant asking by some sort of "master".

The line can be sequentially extended (line topology) and contains 4 wires with differential RS485+ RS485- signals, GND and 3.3V. Our sensors have automatically switched 120 Ohm termination resistors at the line endings.

We use 4 pin 2.5mm audio jacks, because the wires are thin and the connectors are small (about Ø5mm), so a small hole can be drilled for placing the sensors inside different devices.

Notation

[name:8] Data "name" coded in 8 bits
[name:76543210] Data "name" coded in 8 bits described later
[name:32] Data "name" coded as 32 bit value

Network-Layer

This specification of the network-layer allowes each bus-member to read (without parsing) or skip packet-frames. So the bus-state can be anaysed to get a free bus-slot.

Bit-Transfer

All bytes are transferred LSb/bit0 first to be processed by RS232 UARTs contained in most microcontrollers.
All multi byte numbers are transferred MSB/higest byte first to be handled like I2C transferral of most sensor chips. So
[number:16] is transferred as [number bits 8..15][number bits 0..7] and
[number:32] is transferred as [number bits 24..31][number bits 16..23][number bits 8..15][number bits 0..7]
The UART transfer mode is 8N1, the stop-bit ensures the auto-baud feature with the sync-byte 0x55.

Packet-Frame

[sync:8] [head:4+4] [framelen:8/16] [payload:...] [crc:16]

sync=0x55 constant value hex 55 (toggling bits from startbit, syncbyte, stopbit allow to detect the baud rate - inspired by LIN protocol)
head type of packet-frame and used for start detection (should be checked for bit7=1 and then for nibble redundancy)
framelen length of [payload] including the [crc] (allows skipping whole frames) but NOT including [head]
payload bytes containing packet information and packet data
crc CRC16-CCITT of [framelen] and [payload] but NOT including [head]

The head byte is used for detection of the packet-frame and used for configuration. The 4bit nibbles are cloned to give proper redundancy on sync start.

[HEAD:7654][HEAD:3210]	DESCRIPTION
`0...` `0...`	forbidden (could result in the sync byte 0x55)
`1...` `1...`	always 1 to separate head from sync byte 0x55
`.00.` `.00.`	undefined
`.01.` `.01.`	length = 8 bit value, no crc used (validity ensured by stop-bit framing only)
`.10.` `.10.`	length = 8 bit value, 16-bit crc appended (and added to framelen)
`.11.` `.11.`	length = 16 bit value, 16-bit crc appended (and added to framelen)
`...0` `...0`	undefined
`...1` `...1`	undefined

Data-Layer

This specification of the data-layer allows any bus-member to read all types auf packages, without interpreting the content-types (syntactical parsing).

Data-Formats

All protocol interpreting devices should at least implement the size of payload to skip them while interpreting the package. Nethertheless a complete frame can be skipped by its [framelen] information.

TYPEID [hex]	FORMAT	PLAYLOAD LENGTH	DESCRIPTION
0			intentionally undefined
1	`[byte:8]`	1	1 byte integer value (LSb first)
2	`[word:16]`	2	2 byte integer value (LSb/MSB first)
3	`[long:32]`	4	4 byte integer value (LSb/MSB first)
4	`[huge:64]`	8	8 byte integer value (LSb/MSB first)
5	`[extra:128]`	16	16 byte integer value (LSb/MSB first)
6
7	`[sign:1][range:2][exponent:5][fixpoint:16]`	3	16 bit fraction value in fixpoint representation
8	`[sign:1][range:2][exponent:5][fixpoint:32]`	5	32 bit fraction value in fixpoint representation
9	`[sign:1][range:2][exponent:5][fixpoint:64]`	9	64 bit fraction value in fixpoint representation
A	`[sign:1][range:2][exponent:5][fixpoint:128]`	17	128 bit fraction value in fixpoint representation
B
C
D	`[len:8][rawdata:...]`	[len]	Byte sequence - mostly String encoded in UTF-8, [len] defines the bytecount not the (multi byte-)charactercount
E	`[len:16][rawdata:...]`	[packetlen]	Byte sequence - mostly an enveloped packet-frame where [rawdata] includes `[head] [framelen] [payload] [crc]`
F

The semantic interpretation of the value (signed, unsigned, string, time) is done in the context of the packet stream.

Fraction-Values

Fraction vales are transferred via fixpoint representation.
The usual IEEE754 floating point representation is not exact and allowes rounding errors.
[sign:1][range:2][exponent:5][fixpoint:16] 3 bytes in total
[sign:1][range:2][exponent:5][fixpoint:32] 5 bytes in total
[sign:1][range:2][exponent:5][fixpoint:64] 9 bytes in total
1. sign 0 positive value, 1 negative value
2. range indicates the reach of the upper or lower limit of the sensors measurement range (00b value inside range, 01b (above) maximum, 10b (below) minimum, 11b undefined)
3. exponent comma position (0 to 31) in the [fixpoint] value e.g. 123456(fixpoint)/100(exp=2) results in value 1234.56
4. fixpoint integer value to be divided by 10^[exponent] (ATTN: exponentvalue is positive, but mathematically interpreted nagative by using division)
All sensors return thier precision by keeping their fraction digits by keeping the exponent constant on all results and not removing trailing zeros.
A 2 fraction digits precise sensor (A) returns "520.00" or "20.51" while keeping the [exponent] constantly at 2.
A 4 fraction digits precise sensor (B) returns "520.0090" or "20.5100" while keeping the [exponent] constantly at 4.
No rounding: sensor (A) does not detect the additional "90" fraction which can be measured by sensor (B) only.

Packet-Layer

This specification of the packet-layer allows the bus-member to walk through an interpret packets.

Information-Cascarding

The package sets states which remains active until bus-reset or the state is overwritten by the same originator. This information-cascarding allows compressing streams of values to the changed data only.

Payload

[property1:8] [data1] [property2:8] [data2] ... [propertyN:8] [dataN]

The payload is a collection of [property] and [data] pairs. Each [property] is defined for a special purpose and the related [data] is interpreted belonging to that case. The properties can be seen as data specifiers that define dataprocessing rules.

Application level functions can be described by several [property] [data] pairs containing the semantical implication. In meaningful cases, the properties can be interpreted in cascading manner. For example (written as extended plain text): [sensorname] Cooler 34 upper [measureclass] Temperature [measureunit] °C [measurevalue] 23.450

If a property is unknown, it is not possible to parse the rest of the payload because the length of each [data] segment cannot be determined. So the remaining payload can be skipped via the [len] field of the packet frame. In this case the receiver is "incompatible" (or too old).

Properties

The important protocol related [property] [data] pairs are placed at the very beginning of each payload. These should be interpreted by all devices to response accordingly.

Each device may contain several sensors/actors (temperature, humidity, devicevoltage) that could response in thier own timing.

PROPERTY [hex]	DATA	DESCRIPTION
		Package Properties (for addressing, handling, encoding)
00		intentionally undefined
01		no command, bus keep alive, "hello" message
02		reset bus state, resync bus
03		bus is powering down now, devices should shutdown immediately
04
05
06
07
08
09
0A	`[toaddress:32]`	packet is for device `[toaddress]` only (from address is 2A)
0B
0C
0D
0E
0F
		Command Properties (reveiving device should to that)
10		do reset, wakeup from sleep
11	`[centis:16]`	do sleep for `[centies]` 1/100 seconds or (if 0) sleep depending on own configuration
12		do shutdown until powered up again (bus do not need to power down yet)
13
14
15
16
17		do respond with device information (blocks 30 and 40)
18
19
1A
1B
1C
1D
1E
1F
		Status Properties (device should send that as response or for information)
20	`[lastreccrc:16]`	Acknowledge that last received CRC is OK
21	`[lastreccrc:16]`	Acknowledge that last received CRC is BAD
22	`[ackid:8]`	Acknowledge last Package with `[ackid]`
23
24
25	`[lastsentcrc:16]`	Retransmission of CRC from last package for stream chaining
26
27
28
29
2A	`[fromaddress:32]`	packet is originating from device `[fromaddress]` (to address is 0A)
2B
2C
2D
2E
2F
		Identification Device Properties (as response to a "device information" request (17)
30	`[name:STR0]`	my manufacture name
31	`[name:STR0]`	my product name
32	`[name:STR0]`	my product function
33
34	`[serial:32]`	my serialnumber
35
36
37
38
39
3A
3B
3C
3D
3E	`[name:STR0]`	my custom place (user configurable)
3F	`[name:STR0]`	my custom id (user configurable)
		Identification Sensor/Actor Properties (as response to a "device information" request (17)
40	`[name:STR0]`	my function purpose ("temperature", "pressure", "textoutput")
41	`[name:STR0]`	my measurement unit
42	`[datatype:4]`	my datatype of measured values (using lower 4 bits only)
43	`[mode:76543210]`	`7` 0 read only (sensor), 1 writeable (actor) `6` `5` `4` `3` `2` `1` `0`
44	`[value]`	Minimum measurement value, type depending on definition 0x42, min length (unsigned word) for strings
45	`[value]`	Maximum measurement value, type depending on definition 0x42, max length (unsigned word) for strings
46
47
48
49
4A
4B
4C
4D
4E	`[name:STR0]`	my custom place (user configurable)
4F	`[name:STR0]`	my custom id (user configurable)
		Indicates that the following properties refer to the specified sensor/actor
A0		sensor/actor number 0
A1		sensor/actor number 1
...
AF		sensor/actor number 15

FF	`[extproperty:8]`	Definition of extended properties (level 1) for future use

Units

To reduce convertion losses, each sensor should keep its native unit and scale, so if a sensor measures kV (kilo Volt) it should not return V (Volt).

Code [hex]	Unit	Description
00	-	intentionally undefined
		Temperature
01	°C	Celsius
02	°F	Fahrenheit
03	K	Kelvin
04	°Ré	Reaumur
05	°N	Newton
06	°Ra	Rankine
		Volume
10	m³	cubic meter
11	dm³	cubic decimeter
12	cm³	cubic centimeter
13	l	liter
14	dl	deciliter
14	cl	centiliter
16	ml	milliliter
17	fl oz(UK)	fluid ounce(UK)
18	fl oz(US)	fluid ounce(US)
19	in³	cubic inch
1A	ft³	cubic foot
1B	yd³	cubic yard
1C	gal(UK)	gallon uk
1D	gal(US)	gallon us
1E	bbl	petroleum barrel
1F	pt(Imp)	pint(UK)
20	pt(US fl)	fluid pint(US)
21	pt(US dry)	dry pint(US)
		Distance
30	km	kilometer
31	m	meter
32	dm	decimeter
33	cm	centimeter
34	mm	millimeter
35	mi	mile
36	in	inch
37	ft	foot
38	yd	yard
39	nautical mile	nautical mile
		Weight
40	t	tonne
41	kg	kilogram
42	hg	hectogram
43	g	gram
44	dg	decigram
45	cg	centigram
46	mg	milligram
47	µg	microgram
48	carat	carat
49	grain	grain
4A	oz (av)	ounce avoirdupois
4B	lb (av)	pound avoirdupois
4C	cwt(UK)	long hundredweight
4D	cwt(US)	short hundredweight
4E	ton(UK)	long ton
4F	ton(US)	short ton
50	st(UK)	stone
		Area
60	km²	square kilometer
61	m²	square meter
62	dm²	square decimeter
63	cm²	square centimeter
64	mm²	square millimeter
65	ha	hectare
66	a	are
67	ca	centiare
68	mile²	square mile
69	in²	square inch
6A	yd²	square yard
6B	ft²	square foot
6C	ro	rood
6D	acre	acre
6E	nautical mile²	square nautical mile
		Speed
80	kmph = km/h	kilometer per hour
81	mps = m/s	meter per second
82	mph = mi/h	mile per hour
83	knot = nautical mile/h	knot
84	ma	mac
		Databyte
A0	b	bit
A1	B	byte
A2	KB (ex : 1 KB = 1024 bytes)	kilobyte
A3	MB (ex : 1 MB = 1024² bytes)	megabyte
A4	GB (ex : 1 GB = 1024³ bytes)	gigabyte
A5	TB (ex : 1 TB = 1024⁴ bytes)	terabyte
A6	PB (ex : 1 PB = 1024⁵ bytes)	petabyte
A7	EB (ex : 1 EB = 1024⁶ bytes)	exabyte
A8	ZB (ex : 1 ZB = 1024⁷ bytes)	zettabyte
A9	YB (ex : 1 YB = 1024⁸ bytes)	yottabyte
		Time
B0	1 year = 365 days	common year
B1	1 year = 366 days	leap year
B2	1 year = 365.2425 days	average Gregorian calendar year
B3	1 year = 365.25 days	average Julian calendar year
B4	1 year = 365.24219 days	tropical year
B5	week	week
B6	day	day
B7	h	hour
B8	min	minute
B9	s	second
BA	ms	millisecond
BB	µs	microsecond
BC	nanosecond	nanosecond
BD	picosecond	picosecond
BE	femtosecond	femtosecond
BF	attosecond	attosecond
		Frequency
D0	Hz	Hertz
D1	KHz	Kilohertz
D2	MHz	Megahertz
D3	GHz	Gigahertz
		Pressure
E0	atm	Atmosphère
E1	bar	Bar
E2	mbar	Millibar
E3	Pa	Pascal
E4	hPa	Hectopascal
E5	Psi	Pounds per square inch
E6	Torr	Torr
		Energy
F0	J	Joule
F1	KJ	Kilojoule
F2	cal	Calorie
F3	kcal	Kilocalorie
F4	Wh	Watt-hour
F5	kWh	Kilowatt-hour
F6	BTU	British thermal unit
F7	thm	Therm américain
F8	ft-lb	Foot-Pound
		Humidity
FA	%RH	percent relative humidity
FF		Definition of extended units (level 1) for future use