Master is always address 0
Slaves are address 1 - 998
Broadcast is address 999

Master initiates all communication and slaves only ever respond to Master requests for interaction (STAT or POLL).
Most packets consist of a 4 character verb and then the payload.

All devices must be on the same "channel". It was explored that each slave might exist on its own channel but this was abandoned
early on because:

Separate channel: Pro
slave does not need to parse the addresses from the packet - if it hears the packet then it must be for that slave.
Con
Broadcast becomes impossible.
Device addresses limited to 127.
Cheap RxTx modules have no concept of channel.

Single channel: Pro
Slave address is encoded as part of the packet header with a theoretical maximum of 998 devices. Different channels
permit single master with multiple slave zones if required, thus giving < 128000 devices (but broadcast and use of
cheapie modules will be impacted).
Broadcast is simple - all devices hear the broadcast and pick the addresses from the packet as normal.
Can use cheapie 433MHz TxRx pairs with no onboard processing (see preamble).
Con
HC-12 module is more expensive


Command Source Details
-----------+-------+------------------------------------------
STAT: Master Master STATs address.
Slave MUST respond within 1.5 seconds and may send back data, thus a STAT contains an implicit POLL.
Nil response will result in the slave marked as not available in the status register (master will not POLL).
Slave must rep-y with ACK0 only - an INIT is pending on first discovery. If the Slave has more to say, it must wait for a poll
POLL: Master Master POLLs address.
Slave may respond but must do so within within 1.5 seconds if it has something to say but keeps quiet otherwise.

ACK0: Slave This is the terminating response to a STAT or POLL. This packet will cause the Master to accept the response and
move on to the next section of the cycle.
ACK1: Slave This is the response to a POLL only but signifies the slave has more to report. This packet will cause the
Master to accept the response and then immediately issue another POLL to the same slave. Multiple ACK1's may be
sent and the Master will continue to POLL and accept packets until receipt of ACK0 or a timeout occurs, at which
point the master will move on to the next section of the cycle. The slave must pause for 1.5 seconds between each
ACKx packet.
INIT: Master The slave must restart.
SAFE: Master The slave must go to its default/safe/power-up state.

Broadcasts: Any packet with destination address 999 is assumed to be a broadcast. Thus there is no specific broadcast verb.
Usually only the Master Broadcasts. No slaves may respond.

TIME: Master The payload is hh:mm:ss,dd/mm/yyyy All slaves synchronise their clocks to the payload.
INIT: Master The slave must restart.
SAFE: Master The slave must go to its default/safe/power-up state.
FIRE: Master The slave must go to its FIRE mode state.

Note: Slaves must never reply to broadcasts under any circumstances.


Slave must respond to STATs. The response (or lack of it) updates the internal status register for that slave. Slaves that do not
respond to a STAT will be skipped in the POLL cycle.

Master: STAT (nn)
Slave: Must reply with ACK0 (or ACK1 if there are multiple return packets) to the Master. The payload may be empty.

Master: POLL (nn)
Slave: May reply with ACK0 (or ACK1 if there are multiple payloads) to the Master. The payload may be empty but it is
recommended the slave not respond to a POLL if it has nothing to report. This reduces the cycle time and processing load


Packet format (master and slave):


[pp.pp.]STX<checksum>GS{ENCRYPT<dest address><src address><Payload>}ETX

the data between the GS and ETX is encrypted before transmission.
RC4 is a complex algorithm with a moderate data requirement (0.5K). A simpler encryption algoritm XORing a
variable length key is available if the simple nature of the slaves cannot support it i.e. simple PIC/Atmel chips

so a typical Tx from the master to device 05 looks like



Preamble When using cheap TxRx modules with no onboard processing, it is necessary to send a stream of 1's to the Tx so the Rx
can set it's levels before the actual data arrives. Because of this, some of those bits mat be lost so anything before
STX must be discarded. Chr FE has a large amount of "1" space and the trailing 0 ensures the levels are set on Rx devices
immediately before they are required to be accurate. A string of at least 2 FE chars is effective.
Using devices like HC-12 where there is onboard processing to assure data integrity, a preamble is not necessary as it is
taken care of by the radio module.

Preamble [FE FE ...]

Typical Packet structure
02 = STX, sync marker
CHK_HI ASCII Hex of checksum of packet between GS and ETX
CHK_LO
1D = GS group separator
[ 30 = attention device 005 in ascii
[ 30
[ 35
[ 30 = from the master 000 in ascii
[ 30
[ 30
[ [
[ 50 STAT
[ 49
[ 4E
[ 47
[ | or
[ 50 POLL
[ 4F
[ 4C
[ 4C
[ ]
[ any message here
03 = ETX, end of transmission

Any response must begin transmission in the next 1.5 seconds