epc120
4. 2-Wire Bus
The 2-wire bus and the power supply utilize the same two wires. The data is transmitted by modulating the current on the power-line. The
modulated current, together with the resistor in the power supply, produce a voltage signal on the line. All devices receive this signal. The
system is designed to operate with a line impedance of 50Ω (±5%). An inductor in parallel of the resistor or a DC regulator with a lowpass
feedback shape the pulses and keep the the DC voltage drop over the resistor low. The required corner frequency of this L/R-filter is listed in
the table below.
The communication interface has been designed to be used for line lengths of up to 100m and with up to 1023 sensor devices. For line
lengths of up to 3m it is possible to operate the line without termination2. Above this length the line has to be terminated by a resistor of 50Ω
(±5%) which is equal to the line impedance and a capacitor of 100nF in series.
The data rate on the 2-wire bus is set by the parameter DRATE. It also defines TSCANmin (refer to Chapter Error: Reference source not found on
page Error: Reference source not found) and the required inductor according to Table 1. The maximum data rate allowed on the 2-wire bis is
depending on the bus length. The longer the bus wire, the lower the data rate. Table 1 shows the possible bus wire length according to the
data rate.
DRATE
k
Data Rate on the
2-Wire Bus
Minimal Data Rate
Required on SPI
Interface
Corner
Frequency L/R
Inductor
Bus Wire
Length3
00
01
10
11
8
4
2
1
250 kbit/s
500 kbit/s
1 Mbit/s
300 kbit/s
600 kbit/s
1.2 Mbit/s
2.4 Mbit/s
0.5 MHz
1 MHz
2 MHz
4 MHz
16µH
8µH
4µH
2µH
12 … 100m
6 … 12m
3 … 6m
≤ 3m
2 Mbit/s
Table 1: Data rate of the 2-wire communication
The default value of DRATE is 00. The parameter DRATE has to be identical for all devices on one physical 2-wire bus.
The SPI bus should be faster than the 2-wire bus, otherwise the communication does not work. Since the command length dependent on the
command type, the delay time to the next command has to be adjusted to the previous command. The time delay can be calculated with the
given data length in Table 7 on page 19.
The parameter CDET defines the optimal signal amplitude for the receiver. The maximum rate at pin VDDR (5.5V) should not be exceeded
and signals which are smaller than 70% of the recommended values are not detected.
Since the command length is dependent on the command type, the delay time to the next command has to be adjusted to the previous
command. The time delay can be calculated with the given data length in Table 7 on page 19. The data handling chain of the 2-wire
communication channel is shown in Figure 6.
Transmitter
Receiver
Command
Data
Command
Data
Original
Message
Received
Message
Parity Bits
added
Error Correction
Manchester
Decoder +
Error Detection
Manchester
Encoder
Current
Sink
Line
Filter
A/D
Converter
Figure 6: Data handling
2
3
Dependent on the electro-mechanical design and the bus location of the edge, the termination network can be necessary. It is in the
responsibility of the system designer that the data integrity on the bus is guaranteed. Data integrity can be tested by readout bus
transmission errors. It is strongly recommended to do that during type qualification during EMI qualification tests .
The effective length is dependent on the electro-mechanical design of the edge. The values in the table are indicative only.
© 2011 ESPROS Photonics Corporation
Characteristics subject to change without notice
8
Datasheet epc12x - V2.1
www.espros.ch
EPC [ ESPROS PHOTONICS CORP ]
