epc120
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.
1000
562
316
1
2
3
5
178
100
56
10
20
50
100
32
18
10
1
1.78 3.16 5.62 10 17.7831.6256.23 100
Pitch [cm]
Figure 10: Delay vs Pitch. Parameter: Line length [m]
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.
Message
Manchester
Signal on Bus
TDataRate
Figure 11: Manchester encoded signal on the bus
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 VS of the
communication signal on the bus shall match this setting. A good principle is that the voltage swing VS measured on the bus should be min.
25% and max. 150% above the CDET value.
Example:
If CDET is set to 200mV, the voltage swing VS should be in a range of 250 to 500 mV. Ideal for this setting is a swing voltage V S of 400mV.
Attention:
Make sure that the voltage swing VS is in the given tolerance range at every physical location of the bus. Due to reflections in the cable, losses
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
10
Datasheet epc12x - V2.1
www.espros.ch
EPC [ ESPROS PHOTONICS CORP ]
