4608X-102-102 Datasheet, PDF (47/62 Pages) Bourns Electronic Solutions – Resistor Networks Product Selection Guide

RC Terminator Networks 700 Series
For designers developing high performance systems, exact
termination resistances which account for line loading may be
desirable. This resistance (or equivalent resistance) can be cal-
culated using the formula:
Rterm =
V
Zo
1 + Cd / Co
where Zo is the characteristic impedance of the line, Cd is the
total capacitance associated with the receiving devices (typically
5 pF per input gate) or other loads off the line, and Co is the
intrinsic capacitance of the line.
The series termination technique suppresses reflections at the
driving device should any waveforms be reflected back from the
driven end of the line. Series termination preserves power since
there is no current path to ground or Vcc as in the other meth-
ods. However, this technique results in incident signals that
transition slowly. It is also not appropriate for distributed loads
due to the half-amplitude waveforms which exist at intermediate
points along the line.
RC termination represents a compromise between power con-
sumption and effect on performance. Its principle of operation is
similar to parallel termination, but the capacitor blocks the DC
component of the signal, thus reducing power consumption.
However, the effectiveness of this method depends on the fre-
quency and duty cycle of the application. RC termination also
can be an expensive technique if implemented using discrete
components rather than a network.
Typical Usage
While not every address, data and control line may require
pull-up/pull-down or termination as part of the system’s design,
the table below shows common practice for some popular
devices.
MICROPROCESSORS
68000 68020 80286 80386
No. address lines 23
Termination
0
32 24 32
32
0 32
No. data lines
16
32 16 32
Termination
0
32
0 32
No. control lines 21
Termination
0
27 13 15
27
0 15
Total I/O lines
60
91 53 79
Termination
0
91
0 79
MICRO-
CONTROLLERS
68HC11 8051
38
32
0
0
701
1
8
1
NO. OF LINES BOURNS P/N
7
4608H-701-RC/CC
8
4609H-701-RC/CC
9
4610H-701-RC/CC
9
PACKAGE
High Profile
Conformal
SIP
702
1
10
NO. OF LINES BOURNS P/N
8
4610H-702-RC/CC
PACKAGE
High Profile
Conformal SIP
For all RC terminators, standard R values are 50, 68, 75 and 100
ohms. Standard values for C are 47, 100, 500 and 1000pF. See
data sheet to select custom combinations of R and C.
REFERENCES:
1. Blood, W.R., MECL System Design Handbook, Motorola, Inc., 1983.
2. F100K ECL Data Book, Fairchild Semiconductor Corp., 1986.
3. MECL Device Data, Motorola, Inc., 1987.
Specifications are subject to change without notice.
323