At the moment I have a 3 wire serial system. 1 GND, one marked TX and one RX. I need to solidify this idea in text, write some code and test it in real hardware. If things don't go well the next option is to add another wire, which I still have space for if I stick to three peristaltic pumps. Which is no problem for this project. L293D's are under £2 each.
After a couple of days cogitating I have made a decision. I considered a variable length with the msb marking another byte like in UTF-8. However this is over complicated for this little job so I have decided on a fixed length scheme.
One possibility.
The minimum number size bound the amount of liquid in millilitres.
1111 1111 1111 1111 is 65535 which is 65.535 litres. This fits the bill for the 30 litres in the specification.
So the protocol, at the moment, is three bytes long
aaaa aaaP - action and parity bit
bbbb bbbb - lower bits of value
cccc cccc - upper bits of value
The next idea
I realised that it is unlikely that a pump will get a 1.01L instruction. They will usually get 10ml or 10L. However there may be the possibility that the controller has a small and large pump on the same L239D. So let's encode the magnitude in the action, I have plenty of those to go round and the amount can just be a single byte.
aaaa aaaP - action and parity bit
vvvv vvvv - value
This is the most satisfying to me. A message will then fit into a 16 bits.
The Protocol v0.1
| Action | Value | Description |
|---|---|---|
| 0000 0000 | 1010 1010 | All Off |
| 0000 0001 | vvvv vvvv | Pump 1, v ml |
| 0000 0010 | vvvv vvvv | Pump 2, v ml |
| 0000 0100 | vvvv vvvv | Pump 3, v ml |
| 0000 1000 | vvvv vvvv | Pump 4, v ml |
| 0001 0000 | vvvv vvvv | Pump 1, v L |
| 0010 0000 | vvvv vvvv | Pump 2, v L |
| 0100 0000 | vvvv vvvv | Pump 3, v L |
| 1000 0000 | vvvv vvvv | Pump 4, v L |
| 1111 1111 | 1010 1010 | All on until further notice |
The values in the All On and All Off are to make sure that a signal is being sent and not just some line noise.
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