Microchip COM20020i Manual

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Side 1/14
TECHNICAL NOTE 7-5
Rev. E, May 1994
RS-485 CABLING GUIDELINES FOR THE COM20020 UNIVERSAL
LOCAL AREA NETWORK CONTROLLER (ULANC) AND
EXPERIMENTAL PROCEDURE FOR VERIFICATION OF RS-485
CABLING GUIDELINES
INTRODUCTION TO EIA RS-485
EIA RS-485 is a specification for the support of a multi-drop differential serial digital data network. RS-485 came about as
microprocessors and the use of distributed intelligence became popular in the design of industrial systems. The
implementation of such concepts created a demand for a standard method of communicating serially in such environments.
The actual RS-485 specification came about as a result of the shortfalls of the RS-422 standard. RS-485 is virtually
identical to RS-422 except in two respects, increased receiver sensitivity and support of longer line lengths.
The basic RS-485 specification standardizes the electrical characteristics of each transceiver and provides some basic
guidelines for establishing a network. By definition, a basic transceiver shall have a minimum input resistance of 12 Kohms
and handle a +/-7V common mode voltage regardless of whether power is applied or not. When the transceiver is
powered it must present the minimum input resistance and present less than 50pf of capacitance at its input terminals. In
addition, each driver must be capable of providing a minimum level of 1.5V in the presence of 32 transceivers and two 120
ohm terminating resistors. The 120 ohm termination results from the use of twisted pair cable, the preferred media in many
industrial applications because of its wide availability and low cost. Another critical requirement of RS-485 is that the
receiver must be capable of detecting levels down to 200mV which is of great advantage when long line lengths are
needed. RS-485 does not specify a modulation method or a maximum data rate. This gives the system designer great
flexibility in creating a low-cost high-performance network.
The combination of long line length, high node count (32 nodes), and edia has made the RS-485 support of low-cost m
specification the primary choice as a data communication standard in industrial applications.
CABLING GUIDELINES FOR RS-485 INTERFACE WITH THE COM20020
The following cabling guidelines provide a basis for establishing a low cost Local Area Network (LAN) based on the
ARCNET protocol for use with the COM20020 Universal ARCNET Controller with a differential RS-485 driver. The
guidelines presented are for unshielded twisted pair cable modulated with the COM20020's backplane encoding scheme.
All testing and experiments were performed using a 24AWG copper twisted unshielded 2 pair cable with a characteristic
impedance (Zo) of 120 Ohms. The topology used in all experiments was a daisy-chained configuration with no stubs (i.e.
no drops).
TRANSMISSION LINE EFFECTS IN LOCAL AREA NETWORKS
Transmission line effects often present an obstacle in obtaining high performance in data communication networks. Among
the problems that plague high data rate LAN's are reflections, signal attenuation, and D.C. loading. Taking into account all
three parameters when designing a network can result in a faster and more reliable network.
2
A) REFLECTIONS IN TRANSMISSION LINE
A reflection in a transmission line is the result of an impedance discontinuity that a travelling wave sees as it propagates
down the line. To eliminate the presence of reflections fr st terminate the line at its om the end of the cable you mu
characteristic impedance by placing a resistor across the line as shown in Figure 1.
TERMINATION OF A NETWORK
- Reflections are caused by a discontinuity in the line
Z = 120 ohm
Z = 120 ohm
R = 50 OHM
INCIDENT WAVE
Z = 120 ohm
Z = 120 ohm
DIRECTION OF PROPAGATION
REFLECTED WAVESOURCE
Z = 120 OHM
R = 120 OHM R = 120 OHM
INCIDENT WAVE
DIRECTION OF PROPAGATION
DRIVER
DISCONTINUITY
PROPERLY TERMINATED NETWORK
Figure 1
It is important that the line be terminated at both ends since the direction of propagation is bidirectional. In the case of
unshielded twisted pair this termination is 120 ohms. Note that all reflection measurements were made with no stubs on
the network (i.e. no drops).
Theoretically, a properly terminated transmission line would produce no reflections at all. However in a real network, small
reflections are produced since the characteristic impedance of the cable cannot be met exactly due to variance in the
manufacturing process of the cable. Another primary source of reflections is the impedance mismatch between a data
transceiver and the line, which can cause problems on a data network by creating perturbations on the line during an
otherwise idle state. Reflections affect the network by triggering false transitions (bits) on the line receiver's input translating
into possible framing errors and CRC errors.
A measure of the relative strength of the reflection generated by discontinuities along the line is called the Reflection
Attenuation Factor (RAF). This is a measure of the strength of the reflected wave to its incident wave. The RAF can be
obtained by comparing a reflected wave to its incident wave. The magnitude of the reflected wave can be measured by
sending a burst of sine waves down a transmission line and observing at the sending end the magnitude of the wave after
the burst has ended (see Figure 2). The reflection measured at the sending end is the reflection generated by a
discontinuity at the receiving end of the line. The magnitude of the incident wave can be measured at the receiving end of
the cable, since this is the wave from which the reflection is generated. It is important to compensate for line loss when
measuring the reflected wave because the reflected wave, measured at the sending end of the cable, has lost some
amplitude due to line loss.
10
NODES RT= 120
2.5Mhz
Z = 120 Ohm
SINE WAVE
Z = 120 Ohm
Z = 120 Ohm
Z = 120 Ohm
MEASURING REFLECTIONS IN A NETWORK
MEASURE REFLECTION HERE
MEASURE INCIDENT WAVE HERE
Figure 2

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