Scan-Data C
C
TELEMETRY EQUIPMENT
SIGNAL MULTIPLEXERS
P.O. Box 17028
Encino, CA 91416-7028
Phone: 818-785-6200
Telemetry & SCADA C FLOW COMPUTERS FAX: 818-785-4415
Email:scandata@aol.com
made easy... C SUPERVISORY SYSTEMS www. scan-data.com
APP-1127
Application Note APP-1127 10-02-00
Testing RTU Communication Circuits. How do the RTUs communicate?
A. GENERAL: The RTUs convert incoming analog, pulse and
status information into a binary protocol, CAP,
Modbus, etc. These binary protocol bits have to
To test bare cable and radio circuits, use two Scan- be converted to tones before they can be
Data Telemetry Test Sets (TTS), one at each end. transmitted out over cables or over radio.
These quickly check the attenuation and the signal
All radio and cable circuits are basically voice
quality. To test existing or newly installed systems, use
grade circuits, originally designed to transmit
the procedures described in this application note. human speech. They cannot transmit pulses very
well.
Modems to the rescue! A modem typically
translates these digital bits or pulses into
frequency shifted (FSK) tones. The tones are
modulated with the digital information. It is
therefore a modulator. Going the other way, it
decodes the received FSK tones into digital
pulses. So it is also a demodulator. Therefore it
is called a modem. Clever.
With the rapid growth of computers, modem
technology has expanded far beyond what Ma Bell
had in mind when the 300 Baud Bell-103 type
modem was first designed. Bell-103 is still an
extremely reliable way of transmitting 2-way
information over a single noisy rural telephone pair
Most RTUs (and all Scan-Data RTUs) have a standard
without the need for complicated automatic error
600 ohm communication circuit interface which uses checking.
isolating transformers and surge protection devices.
One 600 ohm connector pair is the data in circuit and While more and more exotic 2-wire modems are
the other pair is the data out circuit. These two pairs mass produced for the on line computer market,
RTUs and PLCs still use the super reliable Bell-
connect directly to the cable or radio. 202 4-wire 1,200 baud modem standard where
complex end to end negotiations and speed
The RTU receives an incoming FSK (Frequency Shift fallbacks are not required. Scan-Data RTUs all
Keyed) polling signal with a level of at least -25 dbm have built in Bell-202 modems. Stand alone MDM-
202 modems are also used at the central station
over its data in terminals. It will respond to a proper computer and with PLCs and RTUs that have no
incoming polling signal by sending out an answering built-in modems.
FSK answer signal over the data out terminals and by
turning on its 'Transmit On' open collector transistor
output. You can quickly check all this by listening to the
incoming and outgoing signals with a TTS test set or, incoming signal. Third: Check that the RTU responds
for radio circuits, by listening to the communications with an outgoing signal. Fourth: In radio systems,
with a scanner. check that the RTU turns on the radio transmitter.
Central stations and signal multiplexer master units are
For detailed circuit testing, follow these steps:
tested similarly, except that the communication
procedure is reversed: The master originates the polls
First: make certain that the RTU has an adequate
and the far end RTUs answer.
power supply. Second: Check the quality of the
A. CHECKING THE RTU POWER SUPPLY:
What about db's (decibels)?
Is the RTU power supply stable? A power supply that Decibels are used to measure signal levels in
dips in voltage from time to time may cause the RTU communication circuits. They make gain and loss
to de-rail. Even if the power supply looks stable, calculations easier. If you have an amplifier with a gain
of 15 db, for example, in a cable with a loss of 13 db
sudden power surges, such as turning a radio you will have a net gain of 15 - 13 = 2 db. If you apply
transmitter ON, turning motor starter relays ON and a signal of -12 dbm at one end of this combination
OFF, etc., could cause the power supplied to the RTU you will get 2 - 12 = -10 dbm out at the other end.
to dip momentarily.
Simple. But what is the difference between db and
dbm? Well, db is a relation. 15 db gain in an amplifier
If you periodically have to turn the power supply OFF means that the difference (or relation) between the
and ON again to re-set the RTU, check for power input and the output signals is 15 db.
supply dips. Use a heavier power supply, use
separate power supplies or connect a storage battery On the other hand, dbm is an absolute measurement,
like volts or gallons or degrees Fahrenheit. 0 dbm, by
across your power supply. definition, means that the signal level is 1 milliwatt
(0.001 watt) into 600 ohms. That corresponds to about
0.77 volts RMS, should you want to measure it with a
voltmeter.
B. CHECKING THE RTU INCOMING
SIGNAL: If you drop a 0 dbm signal to -6 dbm (a 6 db loss in
your transmission line) you would measure half of 0,77
volts or 0.385 volts. A good thing to remember.
The RTU will respond to a proper polling signal with an Halving or doubling the voltage lowers or raises the
answering signal. If the RTU responds to an incoming level by 6 db.
polling signal with an answer signal, the incoming
signal is OK. In a system with several RTUs each RTU You may not have to worry about this. Level meters
such as the instantaneous reading Scan-Data
will only respond to its own polling signal. If it does not Telemetry Test Set (TTS) handle this automatically.
respond, check the following: Just make sure you have your equipment and your
lines properly terminated, into the proper impedance
(normally 600 ohms).
1. Does the incoming polling signal arrive at the RTU
600 ohm input terminals with the proper level?
2. Is the incoming polling signal designed for this
RTU? The RTU will not answer polling signals
intended for other RTUs. In multiple RTU systems,
each RTU will only answer its own signal.
3. Is there distortion or are there other signals present
in the polling signal? The RTU will not respond to a
polling message containing errors.
4. The easiest way to check the incoming FSK polling
signal is to check it with the high impedance Scan-
Data TTS Telemetry Test Set. It does not load the
circuits and it has an instantaneous level indicator and
a loudspeaker that allows you to hear the quality of the
incoming polling signal. After hearing a few good
signals, you will be able to tell a good signal from a
bad signal just by listening to it.
APP-1127, Page 2
5. If you check the incoming FSK signal with an The signal level into cable and leased cable systems
oscilloscope you should see the clean FSK sine wave, is normally not critical. There is no accepted standard
shifting rapidly between 1,200 Hz and 2,200 Hz. for levels used on cable systems.. Many large utilities,
such as SCE, specify -15 dbm in and out of their cable
6. You can use computer generated test polling systems.
signals to check the RTU. Scan-Data can supply, on
special order, programs for IBM PC type computers Good four wire cable systems have line amplifiers and
that will send out polling strings and display the RTU should normally be adjusted to transmit the signals
answers on the screen. Connect the RS-232 with no loss. If you input -10 dbm at one end, you
connector on the PC to an FSK modem such as the should get -10 dbm out at the other end.
Scan-Data MDM-202 and connect the MDM-202 to the
RTU over a TTS test set (see the drawing). All Scan-Data modems has a level adjustment
potentiometer. Be careful not to change any one of
the many other potentiometers on the board.
C: CHECKING THE RTU OUTGOING
The signal level into FM radio systems is critical. If the
SIGNALS: level is too low, the far end radio receiver may not
demodulate the signal properly and if the level is too
The RTU will respond to a proper polling signal with an
high, the transmitter will over modulate and splatter.
outgoing (answering) FSK signal. Upon receiving a
The levels on all Scan-Data RTUs and radios are
correct and error free polling signal the RTU
correctly adjusted when they leave the factory.
immediately proceeds as follows:
If you ever need to adjust the level in radio systems
1. It turns on its internal modem FSK carrier and also
yourself, proceeding as follows: Check the radio
turns on the 'Transmit On' open collector transistor.
manufacturer's instructions on what level to input into
This, by the way, is a good indication that the RTU is
the transmitter. Many radios, designed for voice
alive and responding (see below).
communications, are sometimes not specific on what
input level to use.
2. It waits for about 100 milliseconds. This is called the
'front porch' or 'warm up'. It allows the transmission
You will have to observe the received sine wave in the
circuits to stabilize.
radio receiver with an oscilloscope. Set up two radios
on your test bench with dummy antennas.
3. It transmit the answering FSK message.
Connect a PC computer running your polling software
4. It waits for about 100 milliseconds. This is called the
(or running an RTU test software) over an MDM-202
'back porch' or 'warm down'. It allows the receiving
modem to one radio. In signal multiplexing systems,
software to act on the 'message complete' character
simply connect the master RTU to the radio (you need
(among other things).
no modem, the RTU already has one). Connect your
slave RTU to the other radio. The RTU should now
5. It turns off its internal modem FSK carrier and turns
answer the polls from the PC. Connect your
the 'Transmit On' open collector transistor off.
oscilloscope to the receiver audio output at one end.
Adjust the level into the transmitter at the other end
All this happens only if the input polling signal is
until the received wave just starts to distort ('double
acceptable to the RTU, if it is completely error free and
hump)', then back off a db or so for safety. Do this for
the RTU is working.
both transmitters.
If the answer signal is transmitted correctly by the RTU
Note the level into the transmitter and use this level on
and not accepted at the far end, check the following:
all the radio transmitter inputs into your system. It helps
Is the level of the outgoing signal sufficient? The RTU
to use an instantaneous level meter such as the Scan-
normally sends a signal with a level of from -5 dbm to
Data Telemetry Test Set (TTS) to accurately measure
-15 dbm which should be sufficient for most cable and
the levels of these short RTU signal bursts.
radio systems. Use the higher level on questionable
Note that there is no need to adjust levels in Spread
cables.
Spectrum Data Radios.
APP-1127, Page 3
D. CHECKING THE RADIO TRANSMITTER that each RTU turns on the radio transmitter with its
open collector output transistor when it is its turn to
TURN ON FUNCTION: transmit.
When the RTU responds with an answer signal, it will
turn on its Transmit On open collector transistor
output. Check the RTU instruction manual. You will F. RADIO REPEATERS:
find that this transistor normally has its emitter
connected to ground and its collector connected to an A store and forward radio repeater such as the Scan-
output terminal. This transistor is designed to short the Data SAF unit can be considered, from a signal level
radio Transmit On terminals, just as you do when you and trouble shooting point of view, as an RTU
press the 'press to talk' microphone switch. connected to a radio. The SAF has a modem input pair
from the radio receiver, a modem output pair to the
To check the action of this transistor, connect a 2K (or radio transmitter and an open collector transmit turn
so) resistor to +12V. The collector voltage should drop on connected to the transmitter. Follow the steps
to near zero when the RTU transmits. Connecting an outlined above when servicing an SAF radio repeater
LED in series with the resistor makes this a handy station.
Transmit On indicator. This transistor circuit is active
on all RTUs, even those used on cable systems.
E. CONNECTING TWO OR MORE RTUs
TOGETHER ON ONE LINE:
Virtually any amount of RTUs can be connected to one
cable circuit. You can use a 4-wire loop type circuit
where all the RTUs are connected along the cable
loop like Christmas tree lights or a 4-wire star circuit
where multiple cable pairs all arrive a the central
station. In either case, you must connect the cable
pairs so that the input and output impedances are not
compromised. You cannot just wire them together.
The SAF repeater board stores and examines all
incoming messages. If correct, and if pertinent to the
RTU(s) the SAF serves, the messages are re-
transmitted on the same frequency. Message
distortion is eliminated and there is no need for a
second frequency.
G. SUMMARY:
Scan-Data makes a 600 ohm signal combiner and
splitter, the FSD-202A, which combines two cable
This application note, together with other handy
circuits into one. Add one more FSD-202A for each
application notes describing different SCADA and
additional RTU.
Telemetry techniques, are immediately available on
the Scan-Data 24 hour FaxInfo line, 818-989-0450.
Although the FSD-202A is designed for cable circuits,
Call it with the phone on your FAX machine or with
with 600 ohm isolation transformers in all branches, it
your FAX software and ask for document #01 to get a
also works on radio circuits when you need to have
list of the many technical documents you can receive.
more than one RTUs share a single radio. Simply
diode-or all the transmit turn on outputs together so
APP-1127, Page 4
