PCM-2074 EVB User Manual
Revision 1.0 (03-31-06)
USER MANUAL
SMSC PCM-2074
12
9.3.1
Calculating the Value of the Offset Resistors
The discussion explains how the Offset Resistor values are calculated, and refers to
In order to simplify the following discussion, assume the sink and source currents are zero (I
A
= I
B
= 0).
The voltage V
AB
between node A and B of the Receiver is:
Since V
TH,
the threshold voltage, ranges from -0.2V to +0.2V, and V
AB
> | V
TH
|, we can write an
equation for R
l
as follows:
However, we must guarantee that under worst-case conditions, the differential voltage exceeds the
required threshold voltage with some margin. These conditions, which occur when V
cc
and R
t
are at
their minimum values set a maximum value for R
l
. The equation for the worst-case can be written as
follows:
For the worst case conditions, we assume that V
CC
(min) = 4.75V, that R
t
(min) = 124 x 0.95 = 117.8
Ω
,
and that the threshold margin is 0.05V, therefore V
TH
(max)=0.25V. Putting these together, we get:,
Therefore, R
l
(nominal) = (1060.2 / 1.05) = 1009.7
Ω
To summarize, R
l
(nominal)= 1.0K
Ω
when Rt (nominal) = 124
Ω
, the threshold margin is .05V, and all
resistors have 5% tolerances. R
l
changes with the threshold margin. For a threshold margin of 0.01V,
R
l
= 1.2k
Ω
; for a margin of 0.1V, R
l
= 820
Ω
.
Since the actual I
A
and I
B
are not equal to 0, V
AB
may be less than 0.2V. Further tuning of the resistor
values may be required. To do this, measure the voltage between the receiver inputs (V
AB
) during an
idle period using a digital voltmeter or oscilloscope.
Figure 9.4 Offset Resistor Value Calculation
D
DE
R
A
B
Vcc
GND
R
1
RS485
R
1
IA
The transmission line
side is in an open, or
floating state.
R
t
IB
Receiver portion
R
R
t
x (V
cc
)
(2 x R
l
)
+R
t
V
AB
=
R
t
x (V
cc
- V
TH
)
2 x V
TH
R
l
=
R
t
(min) x (V
cc
(min)- V
TH
(max))
2 x V
TH
(max)
R
l
(max)=
117.8 x (4.75 - 0.25)
2 x 0.25
R
l
(max)=
=1060.2
Ω
