In many cases, it is preferable to minimize the
number of the electrical connections used for communication between an
electronic device and the outside. However, a connection of the electric power
to the device has to be provided regardless. Therefore, it would be convenient
to use the power connection for data transfer.
A simple method for unidirectional or bidirectional
digital communication through the DC power connection is described. The circuit
does not create any additional voltage drop. The communication can run at the
speed up to several kilobits per second, depending on the parameters.
The basic circuit for unidirectional communication
is presented in Figure 1. The over voltage protection, fuse, power on/off
switch and other evident elements of the practical schematics are omitted for
clarity.
Figure 1.Unidirectional
communication through the connection of the DC power.
The transmitter side acts as the source of the
variable current. The current is modulated with the information signal by the
MOSFET Q2. On the receiver side, the adjustable Zener regulator D1 is keeping
the constant voltage across the power terminals by controlling the current
through the transistor Q1. The voltage drop on the resistor R1 is proportional
to the current. Thus the variation of the input current results in the
variation of the R1 voltage, which is used as the received information signal.
The R2 and R3 resistors are setting the Zener voltage threshold, and the
capacitor C2 is required to keep the regulator stable.
Since the circuit acts as the voltage source, there
is no voltage variation between the power terminals. Therefore, the power
decoupling capacitor C1 should not affect the communication. However, the
non-ideal voltage source has finite output resistance, so the circuit exhibits
the low pass filtering effect. The cutoff frequency is determined by R1 and C1
time constant and the correction capacitor C2. For the given values, the cutoff
is approximately at 3 kHz. That allows for RS-232 communication at the rates of
up to 4800 bps.
Using the same idea, the circuit can be modified to
allow for communication in both directions: to the device and from the device
(Figure 2). The D2 and Q4 are operating as the Zener voltage source with the
threshold of 0.5V lower then D1 and Q1. The CPU U3 signal TX controls the
direction of the transmission.
Figure 2. Bidirectional communication
through the DC power connection.
When TX is active, Q3 is ON, disabling the Zener
voltage reference D2.The power voltage
is limited by D1 and Q1, and Q5 is modulating the current. The circuit
transmits the information to the device like in the previous example.
When TX signal is in zero state, the voltage source
D2 Q4 is active. Since the Zener threshold of D2 is set lower then that
of D1, the D1 Q1 source is off. The device modulates the current with
the transistor Q2, and the D2 Q4 acts as the receiver of the information.
The R10 is added to avoid the current surge when the circuit is switching
from "Transmit" to "Receive" state.
For normal operation of the circuit, it is
required that the current consumption of the device is constant. The
distribution of the current is determined by R1, R5 and R12, R13. The values
given in the schematics assume that the CPU drains up to 25mA. However, those
values can be modified for any given current consumption, as well as the
parameters of the current source.
A similar approach can also be used for the initial
setup and calibration of the battery operated devices such as smart sensors or
transducers. During the setup time, the
test fixture with the source of the current is connected in place of the
battery. The threshold of the Zener regulator is set to be lower then the
battery voltage; thus, in normal operation of the device from the battery, the
communication circuit is OFF, not affecting the operation.
