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Open source robust passive two-wire bus

Published: 04 April 2020
Last updated: 24 November 2023
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Abstract

To reduce the cabling on an ebike, I have developed a simple, but very robust, 2-wire control bus. The signal and the power supply go together with in total two wires: GND and 5V. 
This article describes the application of the 2-wire bus on an ebike, for which the bus was originally designed, named: cable killer.

The principle of an analog, resistance driven, 2-wire bus is not new, it is a well known solution to save on cabling. For example, in a VCR, this principle was used to reduce the wiring to a button panel by connecting the many buttons through different resistors to a single control wire. From the resistance of the control wire it can then be deduced which switch has been pressed.

See also

Open source

I give everyone permission to use my two-wire current-controlled bus circuit and the software, if you mention my name. Contact me for the software.

What is the benefit of cable reduction on an e-bike?

With this bus we can wire among other the throttle switch, brake sensor, pedal assist sensor and the speed sensor. Because the number of cables has been reduced, very robust 3-pin RC-model connectors can be used. Also, the many vulnerable thin control cables can be replaced by only two thick, AlphaWire EcoWire cable.

Each sensor or switch only needs one resistor to work. In addition, the sensor can also be powered by 5V.

Robust RC-model connectors used for an ebikeRobust RC-model connectors used for an ebike

Here is a view of the rat's nest of cables coming out of a motor controller and the reduced cabling with the two-wire current controlled bus:

Rat's nest of cables coming out of a motor controllerRat's nest of cables coming out of a motor controller

E bike cabling reduction with 2-wire current controlled busE bike cabling reduction with 2-wire current controlled bus

Motor controller water leakage through wiringMotor controller water leakage through wiring

Just one resistor is required per switch and sensor

For example, see the the throttle switch with just one resistor per switch:

Throttle switch wiring with the simple 2-wire busThrottle switch wiring with the simple 2-wire bus

How it works

I have called the current controlled bus the “Cucobus”. It is a bus with the least number of wires possible, only two: GND and 5V. Maximal 7 switches can be connected to the Cucobus. When a switch is pressed, the 5V is loaded with a resistor, see the circuit. The cable killer measures the current from the 5V and deduce from this which switch has been pressed. Two current controlled busses are used, one for the electronics at the bicycle frame and one for the handle bar. Just a single 3 pole connector is needed for the total control of an ebike:

E bike cable killer wiring minimalRobust two-wire (GND, 5V) current controlled bus for ebikes


Cable killer first version from 2015

Here is the first version of the cable killer from 2015, it uses an ATSAMD21G micro controller:

2-wire control bus for ebikes2-wire control bus for ebikes

2-wire control bus for ebikes2-wire control bus for ebikes

Cable PCB killer first version from 2015

2-wire control bus for ebikes2-wire control bus for ebikes

Simple 2-wire control bus for ebikesSimple 2-wire control bus for ebikes

A simple alternative to the two-wire LIN bus

There is a standardized 2-wire control bus: LIN (Local Interconnect Network). This is a serial network protocol used for communication between components in vehicles. But with LIN, each sensor requires its own microcontroller board, which reduces reliability and also takes up too much space. My 2-wire current controlled bus is a simple alternative to this. Each sensor only needs 1 resistor.

2-wire bus circuit description

Here is the circuit of the 2-wire bus controller part, it used in the Maxun One solar bike.

Passive 2 wire current controlled busPassive 2 wire current controlled bus

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Control of additional components

The cable killer has two also two extra connectors, JM4 and JM5, for the control of components such as lights, torque sensor, temperature sensor, shift servo etc:

E bike cabling reduction with 2-wire current controlled busE bike cabling reduction with 2-wire current controlled bus

Robustness against leakage currents and current variations

For safety reasons, no errors are allowed. As a result, the development of the cucobus took a lot of time and is quite complicated. The bus has been made interference-resistant by software, so that it is also suitable for automotive applications. The electronic has proven to be very stable, even when currents drifts occur. It is used since 2015 without any problems.

2-wire controlled bus software

  • Hall sensors can be also powered by the Cucobus 5V, its supply current is compensated by software.
  • The bus is continuously calibrated.
  • When threshold values are exceeded, an error is generated and the system will be shut down.
  • The error message is showed by a LED on the front.

2-wire control bus development

Here are photos that give an insight into how the Cucobus works:

Current controlled bus developmentCurrent controlled bus development

Software oscilloscope used for measuring intern Arduino variables

Current controlled bus developmentCurrent controlled bus development

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