Designing the DP8392 for Longer Cable Applications Designing the

Designing the DP8392 for Longer Cable Applications Designing the
National Semiconductor
Application Note 621
Mohammed Rajabzadeh
July 1989
The IEEE 802.3 standard is designed for 500 meters of
Ethernet cable and 185 meters of Cheapernet (RG58A/U)
cable. To extend such segments to 1000 meters of Ethernet
cable and 300 meters of Cheapernet cable requires utilization of Transmit mode collision detection. This method is
described below.
Receive Mode: The Receive mode scheme has a very simple truth table. However, the tight threshold limits make the
design of it difficult. The threshold in this case has to be
between the maximum DC level of one station (b1300 mV)
and the minimum DC level of two far stations (b1581 mV).
Several factors such as the termination resistor variation,
signal skew, and input bias current of non-transmitting
nodes contribute to this tight margin. On top of the
b 1300 mV minimum level, the impulse response of the internal low pass filter has to be added. The CTI incorporates
a 4-pole Bessel filter in combination with a trimmed on
board bandgap reference to provide this mode of collision
detection.
Transmit Mode: In this case, collision has to be detected
only when the station is transmitting. Thus, collision caused
by two other nodes may or may not be detected. This feature relaxes the upper limit of the threshold. As a result of
this, longer cable segments can be used. With the CTI, a
resistor divider can be used at the Collision Detection Sense
(CDS) pin to lower the threshold from receive to transmit
mode.
COLLISION DETECTION SCHEMES
The collision circuitry monitors the coaxial DC level. If the
level is more negative than the collision threshold, the collision output is enabled.
There are two different collision detection schemes that can
be implemented with the CTI; Receive mode, and Transmit
mode. The IEEE 802.3 standard allows the use of receive
and transmit modes for non-repeater node applications. Repeaters are required to have to receive mode implementation. These different modes are defined as follows:
Receive Mode: Detects a collision between any two stations on the network with certainty at all times.
Transmit Mode: Detects a collision with certainty only
when the station is transmitting.
Table I summarizes the receive and transmit mode definitions:
TABLE I
Mode
Receive
COLLISION LEVELSÐTRANSMIT MODE
Table II shows the parameter values that are used in calculating the collision levels in transmit mode.
Transmit
No. of Stations
0
1
2
l2
0
1
2
l2
Transmitting
Non-Transmitting
N
N
N
N
Y
Y
Y
Y
N
N
N
N
Y
M
Y
Y
Y e Detects Collision
N e Does Not Detect Collision
M e Might Detect Collision
Designing the DP8392 for Longer Cable Applications
Designing the DP8392 for
Longer Cable Applications
TABLE II. Assumptions and Definitions
e Termination Resistor at 20§ C
e Temp. Coef. of the Terminator
e Maximum Segment Length
RDC
e Maximum Cable DC Res. at 20§ C
tc
Tm
SR
e Temp. Coef. of Copper
e Maximum Cable Temp.
e Step Response at Max Cable Length
RC
e Max. Connector Res./Station
IB a
IB b
Imax
Imin
Ro
N
e
e
e
e
e
e
SK
e Skew Factor, Effect of Encoder
RS
RL
SEO
e
e
e
e
Max. Positive Bias Current
Max. Negative Bias Current
Max. DC Drive Current
Min. DC Drive Current
Non Transmitting Output Impedance
Max Nodes per Segment
Skew on DC Level
(SKEW c 4)/100
Max. DC Loop Res. of a Segment
Load Resistance Seen by a Driver
Sending End Overshoot
C1995 National Semiconductor Corporation
TL/F/10445
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
50 g 1§ %X
0.0001/Deg.
300m
1000m
0.0489X/m
0.0100X/m
0.004/§ C
50§ C
0.97
0.94
0.0034X
0.0001X
2 mA
25 mA
45 mA
37 mA
100 kX
100
100
Cheapernet
Ethernet
Cheapernet
Ethernet
Cheapernet
Ethernet
Cheapernet
Ethernet
Cheapernet
Ethernet
802.3
ASSUMPTION
802.3
802.3
BELDEN
BELDEN
PHYSICS
ASSUMPTION
NATIONAL
NATIONAL
MIL SPEC
ASSUMPTION
802.3
802.3
802.3
802.3
802.3
802.3
802.3
802.3
e 0.02 for 0.5 ns Skew
e 0.10
e 0.14
Cheapernet
Ethernet
DEFINITION
DEFINITION
ASSUMPTION
ASSUMPTION
RRD-B30M105/Printed in U. S. A.
AN-621
RT
tT
L
minimum DC voltage generated by one minimum transmitting station and another minimum transmitting station at the
other end of a maximum length cable.
The calculations below explain how the values for the resistor divider in Figure 1 are obtained. First, collision levels
Vmax and Vmin must be calculated. The Vmax or ‘‘no detect’’
level is the maximum DC voltage generated by one node.
The worst case here occurs when the transmitting node is
at the center of a maximum length cable, and the collision is
being detected either by itself or by a station right next to it.
On the other hand, the Vmin or ‘‘must detect’’ level is the
The filter impulse response is not included in these calculation since it is mutually exclusive with the Sending End
Overshoot. If the impulse response is larger than the Sending End Overshoot, the exceeding portion should be added
on to the limits.
Maximum Non Collision Level VMax (NO DETECT)ÐTransmit Mode
TL/F/10445 – 1
RTmax
RS
RL
VMax
e RT c 1.01 c [(Tm b 20) c tT a 1]
e RDC c L c [(Tm b 20) c tc a 1] a N c RC
e (RTmax a RS/2)/2
RL
VMax
e [IMax c (1 a SK) a (N b 1) (IB b )] c RL c
(1 a SEO)
e (50.652 a 16.770/2)/2
e 29.519X
e [45 c 1.02 a 99 c 0.025] c 29.519 c 1.10
e 1571 mV
ETHERNET Cable, 1000 Meters, 100 Stations:
RTmax e 50 c 1.01 c [(50 b 20) c 0.0001 a 1)
e 50.652X
e 0.01 c 1000 [(50 b 20) c 0.004 a 1] a 100 c
RS
e 11.21X
0.0001
e (50.652 a 11.21/2)/2
e 28.129X
RL
VMax e [45 c 1.02 a 99 c 0.025] c 28.129 c 1.14
e 1551 mV
CHEAPERNET Cable, 300 Meters, 100 Stations:
RTmax e 50 c 1.01 c [(50 b 20) c 0.0001 a 1]
e 50.652X
e 0.0489 c 300 [(50 b 20) c 0.004 a 1] a
RS
e 16.770X
100 c 0.0034
2
Minimum Collision Level VMin (MUST DETECT)ÐTransmit Mode
TL/F/10445 – 2
RP
e Near End Shunt Resistance
e [Ro/(N b 2)]URTmin
ETHERNET Cable, 1000 Meters, 100 Stations:
e [100k/98]U(50 c 0.99) e 1020U49.5
RP
e 47.209X
VS1(1) e 37 c 0.98 c [47.209U(11.21 a 49.5)]
e 963 mV
VS2(2) e 37 c 0.98 c [49.5U(11.21 a 47.209)]
e 972 mV
VS2(1) e 972 c [47.209/(47.209 a 11.21)] c 0.94
e 738 mV
e 1701 mV
VMin e 963 a 738
RTMin e RT c 0.99
VS1(1) e Station 1’s DC Voltage at End 1
e IMin c (1 b SK) c [RPU(RS a RTmin)]
VS2(2) e Station 2’s DC Voltage at End 2
e IMin c (1 b SK) c [RTminU(RS a RP)]
VS2(1) e Station 2’s DC Voltage at End 1
e VS2(2) c [RP/(RS a RP)] c SR
VMin e VS1(1) a VS2(1)
CHEAPERNET Cable, 300 Meters, 100 Stations:
e [100k/98]U(50 c 0.99)
RP
e 1020U49.5
e 47.209X
VS1(1) e 37 c 0.98 c [47.209U(16.770 a 49.5)]
e 1000 mV
VS2(2) e 37 c 0.98 c [49.5U(16.770 a 47.209)]
e 1012 mV
VS2(1) e 1012 c [47.209/(47.209 a 16.770)] c 0.97
e 724 mV
e 1724 mV
VMin e 1000 a 724
CIRCUIT IMPLEMENTATION
Table III summarizes the design parameters.
TABLE III
3
Parameter
ETHERNET
CHEAPERNET
L
N
VMin
VMax
R1
R2
1000 Meter
100
1701 mV
1551 mV
125X g 1%
10 kX g 1%
300 Meter
30
1724 mV
1571 mV
150X g 1%
10 kX g 1%
Designing the DP8392 for Longer Cable Applications
Circuit implementation is shown in Figure 1
TL/F/10445 – 3
FIGURE 1
To check the design, subtract the additional offset generated by the resistor divider from these levels (VMax and VMin)
and make sure that the internal 8392 collision levels
(1450 mV to 1580 mV) are within this window. The supply
voltage is assumed to be 9V g 5%.
Cheapernet
1571 mV b 8.55V (150X/(10 kX a 150X) e 1445 mV
1724 mV b 9.45V (150X/(10 kX a 150X) e 1584 mV
These calculations show that the resistor values are properly selected.
Ethernet
1551 mV b 8.55V (125X/(10 kX a 125X)) e 1445 mV
1701 mV b 9.45V (125X/10 kX a 125X)) e 1584 mV
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AN-621
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