A bus network for microcontrollers that is based on the concepts of token bus and polling.
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Token based hybrid network, Work in progress

The idea here is to build a protocol that can work over RS485 and any bus in general. Currently it is being developed using versions of softwareSerial, but hacked to work as common bus.

The network is set out to allow the following:

  • Any node can broadcast information to all the other nodes directly.
  • Define automatically a Master, or not have a Master at all.

Definition of the protocol

To achieve this, I will take an approach that is a hybrid between Token ring and bus polling.


nodes are the pseudonym for the units that run a program that lets them participate in the bus with the protocol. Most probably, an MCU.

  • Each node has a token input (TIP) pin and a token output (TOP) pin (later a single pin can fulfill both functions (TP)). Also a common bus pin named COM.


  • Each module has three states: Listening, Broadcasting and Connecting.
  • On Listening state any normal node is on high impedance mode, reading the serial. On Listening state, the node is also reading the TIP pin. If TIP pin is set to 1, it switches to Broadcasting state.
  • on Broadcasting state, the node sends all his available information if any, otherwise a "no information" header. Finishing this, it turns the TOP pin to 1, causing the following node in line to switch into Broadcasting state.
  • when receiving any data, a node turns the TOP back to 0 if it was set to 1.
  • every node has a unique ID, starting from 0. The node number 0 has a special role.
  • When a node has just been powered up, it will be set to Connecting state. On connecting state, the node has no address, and has the TOP on state 0. It is listening to all the broadcasts, and keeps track of the highest address in the network. When it's own TIP goes high, it sets his own address to the highest + 1, writes a connected message to the bus, and sets his TOP to 1. This should cause a chain reaction where all the modules assign their addresses incrementally, if powered all at once.
  • The TIP is pulled to the state 1, causing the first in line to start the chain reaction from 0. When a node is on Connecting mode, if it detects 1 on its TIP, and has not registered any address, it means that there is no lower module. It will set it's own address to 0. Maybe it is convenient if it waits about 10 bytes before doing so, in case the network is on some weird stuck state.

Token logic and the node 0

  • Node number 0 has the role of originating a new token when the token has reached the last module in the token line. It reads the end of the token line when there is a message with disconnected header; or in other words, a timeout.
  • last node could detect nothing connected and notify that to the node zero so it doesnmt have to wait for timeout.


  • a message consists of the following components:
  • origin is a byte representing a unique identifier that indicates what module has broadcasted that message.
  • header is a byte that indicates the mode of the upcoming data. It is divided in two nibbles: first nibble indicates the mode of the message (broadcasted, addressed, empty, offline). The second nibble indicates the length of the upcoming message in words. The maximum message length is 16*4 bytes. Maybe if the second nibble is F, the listeners will wait for a special termination byte.
  • an offline header is equivalent to a disconnected module. The idea is that a module that is offline or not connected, will generate an offline header by not sending anything. You can also think of this as a timeout of the length of a single byte.



-----\         /---------------\         /---------------\         /---------->
      \       /                 \       /                 \       /           
    |-TI-----TO---|           |-TI-----TO---|           |-TI-----TO---|       
    |    node     |           |    node     |           |    node     |       
    |-----COM-----|           |-----COM-----|           |-----COM-----|       
           |                         |                         |              

<-- "left or previous"  "right or next"-->

TI is input with internal pullup, and the logic is direct (not inverse) meaning that it defaults to 1

For easier commenting, in the code, modules are said to be on the left or right. This corresponds to the hierarchical connection of the ti/to. A module on the left is the one whose TOP is connected to the node's TIP in question. A node on the right, (also refered as the next node) is the one whose TIP is connected to the node's TOP in question.


step 1: common bus working

set up three arduinos, all connected to the same bus. Test that it is possible to address each individual arduino in the network by a hard coded address, and that the arduino can respond with his own ID plus a string.

step 2: Automatic address assignation

the arduinos are tied with the TI and TO connections. One single arduino is set to reflect in the serial all the signals that happen in the common bus. After the automatic address assignation, the arduino that is connected to the serial should be able to address individually each arduino as in the previous step

step 3: Automatic token

The arduinos should start their activity without input from the node that is connected to the serial. The message length is fixed. The activity can be seen in the serial output of one of the nodes.

  • The testing showed that I can send 43.5 messages of 8 bytes per second (the Fq of the TOP of one device was 14.5 Hz, and there were three devices on the bus)
  • The payload is 6 bytes.
  • The bus shows that there is room for duplicating the message rate, and if I can also get it working at higher baud rates, I could get some extra room aswell.
  • message rate should be enough to connect enough modules if only one is going to be sending a 12 ppqn clock.

step 4: A node can be added or removed without compromising the network

step 5: Message length is defined in the header

step 6: Pack the protocol into a library

  • There are still many global variables that should be easy to put into the header file instead

step 7: Messages with undefined length, finished with a message terminator

  • the round speed with three devices goes up to 41 hz when turning debug off. There is still space to upgrade this fw.

#further development ideas

  • the protocol should support a closed token loop somehow. It would remove the token timeout wait at the end of each round, potentially triplicating the frequency of communication. Mabe the token timeout could smaller aswell.
  • When a module detects that a message has been sent and has a token, it should release his token. This would prevent overlapping of messages in case a device took too long to answer to the token causing a token restart.
  • actually, the last module could detect that there is no following module, and thus write a restart byte, preventing the master from waiting for a token timeout