A protocol of your own

A chip in the control room should tell a chip in the pump station what to do: pump on, setpoint 250, toggle the light. A logic memory between the two holds exactly one number. To turn that into a conversation, you need an agreement – a protocol.

Messages from id and value#

A message has three parts, packed into one number:

You define the ids and write them into both chips:

The sender packs with ins:

move msg value
ins msg id 16 8
ins msg seq 24 8
sbn MEMORY HASH("Postfach") Setting msg

Acknowledgement#

The receiver reads the mailbox, executes the command and writes the message number into a second memory, the acknowledgement. From that the sender sees that its message has arrived.

Ordering and double execution#

A logic memory keeps its value. So the receiver reads the same message again every tick. For “pump on” that does not matter, for “toggle light” it does: the light would toggle twice per second. Hence the number: the receiver remembers the last one and only executes when a new one arrives.

ext seq msg 24 8
beq seq last start     # already handled
move last seq

For the ordering there is one rule: the sender only sends the next message once the acknowledgement shows its number. Otherwise it overwrites a message nobody has read yet. After 255 the number starts at 0 again – it only has to differ from the last one.