EPC120 EPC [Espros Photonics corp], EPC120 Datasheet - Page 10

• Manufacturer Part Number: EPC120
• Description: Fully integrated Light-Barrier Chips with 2-Wire Bus Interface
• Manufacturer: EPC [Espros Photonics corp]

Another aspect is the distribution velocity of the electrical signals on the 2-wire bus. Since the bus wire itself as well as the individual elements
on the bus present a significant capacitance, the distribution velocity decreases with the number of elements and the pitch between the
elements.
It is important that the overall delay is less than 50% of the clock period of the transmission. E.g. if the system is operated with 2 MBit/s data
rate, the max. accepted delay must not be more than 125ns. Figure 10 shows, that a system operated at the full speed of 2 Mbit/s, a cable
length of up to 5m are possible with an element pitch down to 1cm.
Example:
If we have a system that contains 100 elements in a pitch of 10cm, the total bus length is 10m. According to Figure 10, the delay time of a
proper terminated bus is a little bit more than 100ns. Thus, such a system can be operated with the full speed of 2MBit/s.
Bus Signal Waveform
Figure 11 pictures the Manchester encoding and the signal on the bus. The signal on the bus can be monitored with an oscilloscope and
should look like in the drawing.
CDET is the threshold set in the 2-wire communication receiver of the chip to detect the communication signal on the bus. This parameter,
described in Table 6 and Table 10, can be adjusted to mach the specific system requirements. Thus, the voltage swing V
communication signal on the bus shall match this setting. A good principle is that the voltage swing V
25% and max. 150% above the CDET value.
Example:
If CDET is set to 200mV, the voltage swing V
Attention:
Make sure that the voltage swing V
of the wires (capacitive, inductive, and resistive), and the high bandwidth of the communication signals, significant differences can occur.
© 2011 ESPROS Photonics Corporation
Characteristics subject to change without notice
1000
Signal on Bus
Figure 10: Delay vs Pitch. Parameter: Line length [m]
562
316
178
100
Message
Manchester
S
56
32
18
10
is in the given tolerance range at every physical location of the bus. Due to reflections in the cable, losses
1
1.78 3.16 5.62
Figure 11: Manchester encoded signal on the bus
S
should be in a range of 250 to 500 mV. Ideal for this setting is a swing voltage V
T
DataRate
Pitch [cm]
10 17.78 31.62 56.23 100
10
1
2
3
5
10
20
50
100
S
measured on the bus should be min.
Datasheet epc12x - V2.1
S
www.espros.ch
of 400mV.
epc120
S
of the