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UARTBus OSI-1: Physical layer

Physical layer connection frame

i.e.: How the nodes connected together?

At the signaling level, the minimal requirement is to have a common physical phenomenon what multiple node can affect and which have a dominant and recessive state. When multiple node at the same time trying to push information to the physical layer, any and even one dominant state can override any even more recessive state (like in CAN systems).


The signal what the node emits should be detectable for all other node, and also for the sender. This loopback used

Pull-up data rail

The UARTbus originally is designed to connect nodes using only one wire (or one pair) which is pulled up with a resistor R_bus to a maximal U_bus_max voltage.

The bus data line consist of one wire pulled up to the bus Vcc voltage. This line can be pulled down by any node used to transfer data.

Every pull-up bus driver circuit should fit into this fixture:
Bus and node bus driver frame

Design requirements of the bus driver

First bus diver

driver circuit



Second bus driver

driver circuit

(During the development: Vcc = 12V; Rbus = 500Ω; Rtx = 0Ω; Rin & Ronline = 10 KΩ; 120m twisted pair for GND and Bus wire; 115,2 kbps) (Note: this BS170 tolerates ±20V of Ugs voltage so can be connected directly to the bus wire, but this might be risky when dealing with high transient voltage. Solution: Connect TVS diode between Bus and GND wire. See documentation)

This is a simple level driver which makes the byte sent on the TX port appears in the Bus line and data appears on the Bus wire translated to the RX port with the proper logic level. This also means what we send on the TX we receive on the RX. This used for transmission error and collision detection.

Bus wire and RX oscillogramm



Pull-up data rail further development options

OSI-1 further development options

Documentation TODOs