PLC As A Receiver
Conventional serial transmission communications methods work with a
bit string that is transmitted at a fixed rate. The burden is then completely on the
receiver to synchronize it’s sampling with the transmission. This is usually done in
hardware and software is presented with an interpretation of the transmitted string.
Usually the factor that limits performance is the transmission media and much of the
effort of the receiver goes into reading a blurred signal. Again, this is usually done in
hardware.
In the patented SensorPulse system the ability of receiver is the
limiting factor. In the SensorPulse system the receiving hardware only presents an on or
off signal to software. The transmission media is not a factor and the signal is received
vitally as sharp as it is transmitted. The most important limitation is that software has
a fixed sampling interval and cannot adjust its phase or frequency to accommodate the
transmitter. The SensorPulse uses software exclusively to interpret the transmitted
string. The software algorithm deals with the probabilities of what is received versus
what is transmitted. Some of the patent claims have to do with methods of varying the
timing of the transmission to bias the probabilities of how it will be received. Several
methods have been patented that that provide varying degrees of stability and accommodate
varying abilities of the transmitter. Described in this document is the method best suited
for use with PLCs.
Probabilities
The under lying physical principal is found in the graph of
received probabilities (Figure 1). The unit of time used on these curves is the Scan Time
of the receiver. Scan time may also be thought of as the sampling interval. In general the
sampling time of the receiver is very short compared to the sampling interval or scan
time. The receiver takes a snap shot of the inputs then processes the interpretation logic
and waits. The scan time timer is started when the receiver starts to look at its inputs.
When the scan time is completed the cycle restarts. The scan timer is reset, a snap shot
of the inputs is taken, the interpretation logic is executed and the receiver waits until
the scan timer times out.
Figure 1 - Received Probabilities
If the transmitter transmits a pulse that is 3 scans long the
receiver may see one of three signal lengths. Figure 2 shows the highest probability
reception. In this case Transmission and Receiver Sampling are out of phase and the
sampling occurs some time after the start of the transmission. In this case the receiver
will see a signal that is 3 scans long which is the same as the transmission. From the
probability graph this occurs 96% of the time.
Figure 2 - High Probability Reception
Figure 3 illustrates when the Transmission and Receiver Sampling
are in phase or transmission starts during the sampling period. In this case it is
unpredictable whether sampling will see the signal. If the results of the first sample are
unpredictable then the fourth sample will also be unpredictable. If the transmission is
seen in both sample 1 and 4 then logic will perceive the pulse being 4 scans long. If the
transmission is missed in both sample 1 and 4 then logic will perceive the pulse being 2
scans long. There is also the chance that one sample may see the transmission and the
other one will not so that logic will perceive the pulse being the proper length of 3
scans.
Figure 3 - Low Probability Reception
From the received probability graph (Figure 1) one can see that the
2% of the time logic will see a short scan and 2% of the time logic will see a long scan.
This graph is the result of testing a single receiver and transmitter. Variations in the
exact probabilities may vary depending on the accuracy of the two clocks in the system.
While exact numbers may change the general principal outlined in this graph will always
hold true since no two clocks will ever match exactly.
Note from the probability graph that if the transmission is close
to or less than one scan there is a chance that the transmission may be missed completely.
Since this is unacceptable any, transmission method that calls for transmission timings
close to 1 scan or less are not viable choices.
Default Pulses
The SensorPulse works with two types of pulses that have
statistically unique lengths. These pulses are the ID pulse and Data pulse. The ID pulse
occurs only once in a message and signals its start. All other pulses lengths are data
pulses. This unique ID pulse allows synchronization between the transmitter and receiver.
In order to achieve the highest throughput we have chosen a default
ID pulse of 1.2 scans and data pulses as 3 scans. We have found this provides adequate
statistical uniqueness to provide stable operation. From Figure 4 a pulse of 1.2 scans
will be perceived as 1 scan 83% of the time and 2 scans 17% of the time. A pulse of 3 will
be perceived as 3 scans 96% of the time , 2 scans 2% of the time, and 4 scans 2% of the
time.
Figure 4 - Received Probabilities
This set of numbers may experience some short term instability at
the power. Since all the time periods and the number of the data bits are known to both
transmitter and receiver, the receiver knows when to look for the next identification
pulse. There is a chance that there may be a few misses when the system is first started
but once the system becomes synchronizes it will tend to remain synchronized with
repetitive transmissions providing long term stability.
Basic SensorPulse Message Format
The Basic SensorPulse communication protocol from Digital output on
SensorPulse unit to the PLC consists of an ID pulse followed by data pulses. The exact
order is: an off data pulse, an ID pulse, an off data pulses, and 16 bits of data. The 16
bits of data consist of a signed twos compliment 16 bit binary number capable of
representing a number between -32768 and 32767.
Figure 5-Input Basic SensorPulse Message Format
Delta SensorPulse Message Format
The Delta SensorPulse communication protocol from Digital output on
the SensorPulse unit to the PLC consists of an ID pulse, a delta type pulse and data
pulses. The exact order is: an off data pulse, an ID pulse, an off data pulses, a delta
type data pulse, and either 4 or 16 bits of data. The data consist of a signed twos
compliment 4 or6 bit binary number. If the delta type bit is off the value following is a
16 bit number capable of representing a number between -32768 and 32767. If the delta type
bit is on the value following is a 4 bit number capable of representing a number between
-8 and 7. When the delta type is received the 4 bit number is added to the last 16 bit
number to yield a new 16 bit number.
Figure 6-Input Delta SensorPulse Message Format, Delta
Bit Off
Figure 7-Input Delta SensorPulse Message Format, Delta
Bit On
SensorPulse As A Receiver
The SensorPulse as a receiver is much more flexible than the PLC.
The probability concepts of what is received verses what is transmitted are not necessary.
A more conventional method of directly measuring pulse time is possible. This method leads
to an output protocol that is easy to program in the PLC.
Basic SensorPulse Message Format
The Basic SensorPulse communication protocol from the PLC to
Digital input on SensorPulse unit consists of an ID pulse followed by data pulses. The
exact order is: an off data pulse, an ID pulse, an off data pulses, and 16 bits of data.
The 16 bits of data consist of a signed twos compliment 16 bit binary number capable of
representing a number between -32768 and 32767.
Figure 8-Output Basic Output SensorPulse Message Format
The difference between the output protocol and the input protocol
is the shape of the data pulse. For the Scan of a data bit the output of the PLC is on. In
the second scan the output is either on or off depending on the data bit. In the third
scan the output is off. This provides an off to on transition for each bit and the
SensorPulse input then measures the time to the on to off transition.
Figure 9-Data Pulse Coding
Delta SensorPulse Message Format
