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Separate Transmitter and Receiver Functional Integrity and Verification AC Power Loss and Generator Operation Interference between Multiple Units Mounting Inside Steel Enclosure Cordless Phone I have a ‘spread spectrum’ cordless phone at the house, it does not operate more than 100 feet from the base station and it certainly does not work ¼ mile away. I understand that certain versions of the RFScada devices are also ‘spread spectrum’, how can they operate 1, 5 or even 40 miles away? The main reason is power. The term ‘spread spectrum’ covers a general method of transmission, where transmit and receive frequencies are constantly changing or ‘hopping’ through various channels. This method was devised as a means to allow many devices to effectively share a bandwidth. There are several classifications allowed by the FCC to cover different devices and applications, these classifications also govern both the RF power and antenna systems that may be used. These classifications all use the generic term ’spread spectrum’ as a transmission method. The FCC designates your cordless phone and similar consumer devices as a ‘portable device’ with a severely limited RF power output and antenna. The RFScada is not designated in this group, and therefore may transmit both with a much higher power (up to 1 Watt in the 902-928 MHz ISM band) plus it may also use an FCC approved directional antenna that provides more system power gain in certain directions. In fact, because of the higher RF power output emitted by the RFScada unit the FCC mandates that it must maintain a separation distance of at least 20 cm (about 8 inches) to any nearby persons.
True Range The range is mentioned as 0.25, 5 and 40 miles etc. What is the real range? This is a very difficult question to answer, since it will vary in every installation. The actual range will depend on many factors, including the device location, height, shape of the terrain, terrain surface, obstacles, the antenna used, proximity to similar devices, radio model etc. As a very general guideline in a typical, outdoor location, using the lowest power radio option available (-SS), where each RFScada unit may be visible from the other and using the internal antennas supplied with the standard product a range of at least two miles is to be expected, and five miles is usually possible. A similar ‘line of sight’ installation, using the optional 13 dB gain Yagi antennas at each end should produce a range of 25 miles with the -SS version, 40 miles using the -SS1 version. Using the internal antenna and inside a typical factory, with moving and stationary machines, obstacles, metal walls, interfering devices and no line of sight the range may be reduced to a few hundred yards with ISM devices. The VHF and UHF models have much better coverage in non line of sight applications, plus have higher RF power output so coverage with those models is substantially higher. The range depends on many factors. Please consult with the factory for your specific application; If you are unsure of the best model for your application please call the factory for application assistance. If the exact site locations (GPS co-ordinates) are known a suitable solution may be recommended using topology programs at Data Delivery Devices LLC. Evaluation units may be available for testing at your location.
900 MHz vs 2.4 GHz It seems that everywhere you look another ‘spread spectrum’ 2.4GHz devices comes out, including cordless phones, local wireless computer networks for homes and offices, video cameras, video links, toys etc. Isn’t the very popular 2.4GHz frequency band the way to go, almost all low cost wireless devices appear to be a 2.4 GHz devices. For a long range, high reliability product such as the RFScada device the 2.4 GHz band is not the best choice. For short range (typically less than fifty feet), generally indoor devices such as toys, video links or wireless computer networks the 2.4 GHz band has some advantages, such as using a smaller antenna than required by 900 MHz devices. Many public places like airline terminals, hotels, hospitals, coffee houses, bookstores etc. are installing these wireless networks, and so the band has quickly become saturated with these devices, especially in urban locations. This often results in interference and poor (or no) operation for devices that compete in this frequency range; and many users are now finding that additional 2.4GHz devices will not operate satisfactorily in the vicinity of other 2.4 GHz devices. The biggest advantage of the 900 MHz band is the greater range (typically at least 3 times that of 2.4GHz) and reduced attenuation from rain when compared to 2.4GHz devices. The 900 MHz band is the best choice for devices such as the RFScada products, which are designed for the highest reliability, interference rejection and longest range in hostile, industrial environments.
Spread Spectrum Signals I have a ‘spread spectrum’ cordless phone at the house, it has trouble operating more than 100 feet from the base station and it certainly does not work ¼ mile away. I understand that the RFScada is also a ‘spread spectrum’ device, how can it operate 1, 5 or even 20 miles away? The main reason is power. The term ‘spread spectrum’ covers a general method of transmission, where transmit and receive frequencies are constantly changing or ‘hopping’ through various channels. This method was devised as a means to allow many devices to effectively share a bandwidth. There are several classifications allowed by the FCC to cover different devices and applications, these classifications also govern both the RF power and antenna systems that may be used. These classifications all use the generic term ’spread spectrum’ as a transmission method. The FCC designates your cordless phone and similar consumer devices as a ‘portable device’ with a severely limited RF power output and antenna. The RFScada is not designated in this group, and therefore may transmit both with a higher power plus it may also use an FCC approved directional antenna that provides more system power gain in certain directions. In fact, because of the higher RF power output emitted by the RFScada unit the FCC mandates that it must maintain a separation distance of at least 20 cm (about 8 inches) to any nearby persons.
Separate Transmitter and Receiver I ordered a transmitter for my tank and receiver for the well control. There appears to have been a mistake as both units look to be identical. Shouldn’t one be a transmitter and one a receiver? The units are identical except for the configuration stored inside each unit and any options that may be installed. Every unit is both a transmitter and a receiver. This is the only way that units are able to positively verify correct operation of all other units. The units continually communicate with each other to ensure constant and valid data exchange. If for some reason a unit needs to be replaced (for instance it may be been damaged or stolen) then another standard unit may replace it.
System Data Verification How do I know that the output data I receive is valid and is not interference from another device? There are many levels of sophisticated data encryption and protection incorporated in the device. Lets follow a typical signal to see what actually happens. First of all analog and digital input data is collected at each unit. The data is encrypted in a format that incorporates a constantly changing rolling code and is then mixed with other data pertinent to the unit, such as the local DC voltage. A 16-bit cyclic redundancy check (CRC) value is then calculated for this coded data and added. Now the encrypted plus CRC coded data is passed to the RF section and transmitted using 25 constantly changing frequencies and using yet another, completely independent 16-bit CRC with a different algorithm from the first. The remote unit receives the RF data and verifies that the 16-bit RF CRC is correct. It then further analyzes the received data and verifies that it also passes the second CRC encryption check. The remote unit then analyzes the transmitted data plus other information from the first unit. If the data is addressed to this unit it responds, then removes from this dual-verified data any information it needs. Finally this data is used to update the state of the outputs, to faithfully reflect any output states sent to it from the first unit. Any ‘single bit’ error in the whole process will result in an unverified data packet, and that data will be completely ignored. If ‘correct and double verified’ data does not arrive at a receiver within a (user programmable) time delay the unit will extinguish the ‘COMMS OK’ status LED, set the ‘system ok’ relay output to inactive, and set all analog and digital outs to the default, off state. After good data has been received at one unit it will repeat the whole process by transmitting its own input states to the other units in a similar manner. Units will complete the whole cycle (input/encrypt, transmit data, receive data, decrypt, update outputs) several times a second to insure that the output data is valid. If any unit fails for any reason then all outputs change on all units to the default (off) state within a few seconds, the default setting is 10 seconds but it is user programmable for each unit.
Functional Integrity and Verification We want to replace some competitors units that have poor range and marginal performance. Also, since we now realize the competitors units we own are just ‘one way’ devices we have actually been using two complete sets of them for each signal monitored, as we need to verify that they are actually functioning - data integrity is very important for this application. With the RFScada units is there a way to tell if the remote unit is receiving my signal correctly? Yes, there is full data verification. Unlike competitors ‘one way’ or ‘report on an event’ type these maintain continuous communication and are fully bi-directional so there are several ways to verify full and correct data transfer. Since all units are identical with built in transmitters and receivers it is easy to verify reception of data remotely. First of all, by the very nature of the device if the two on board transmit and receive status LED’s are quickly flashing (normal operation) the devices are exchanging good, verified data. Secondly the ‘system status’ LED and relay will be active all the time that all of the devices are maintaining a verified data exchange. When communication is interrupted at any unit and for any reason the ‘system status’ LED and relay will become inactive within seconds of the interruption (this time may be changed by the user). If this unit is communicating correctly the LED will slowly flash and the relay will toggle slowly, indicating a problem on another unit. It is therefore possible to tell, from any unit, the status of every unit in the system. Finally, if required it is possible to manually verify complete operation of any unit. Take an unused output of the remote unit and directly connect it to an unused input on the remote unit. At the ‘local’ unit close (short together) the input. This will cause the second output of the remote unit to close, which is now directly wired to another input on the remote unit. This state of input two will now be transmitted back to the ‘local’ unit by the remote. Therefore, providing both units are functioning and communicating changing the state of the ‘local’ unit input will cause the monitored output at the local unit to also change, all within a second. This verifies, in order, full local data input, encryption, local RF transmission, remote RF reception, remote decryption, remote output, remote input, remote encryption, remote RF transmission, local RF reception, local decryption, local verification and local output all within a few seconds.
AC Power Loss and Generator Operation Our remote unit is powered from a generator that automatically starts and runs for a couple of hours then stops for a few minutes. The remote unit is successfully monitoring the tank level, and continues to operate correctly when the generator is stopped, powered by the generators battery. Is it possible to also monitor when the generator is running? Yes. Simply connect a 115 Volts AC relay coil across the generators 115 Volts AC output; connect the relays normally open contacts to any unused digital input to continually indicate the running status of the generator. If you have a computer connected via Modbus to the local RFScada device it is also possible to monitor the generators DC battery Voltage.
Interference between Multiple Units We have a pair of RFScada units that have been operating flawlessly for some time. We now need another pair but they will be located within a couple of miles from the first. Will two pairs of RFScada interfere with each other? Each system of up to 32 units are matched together for transmission and reception. There are seven sets of identity that the units may have, so up to seven pairs of units may all operate in close proximity so long as they all have different id’s. When supplied the standard units are configured for set ‘0’, but they can be configured at the factory for ‘1’ to ‘6’. A record is kept of the units supplied for each customer, so when ordering additional units please advise if you need a replacement for a damaged one or a unit to operate on a different frequency set. There are other solutions offered for applications where more than seven pairs are required to operate, or systems where there is a single ‘base’ master unit and multiple slave units, please consult the factory for further details.
Rain Attenuation Here in the jungle we often experience heavy rain. Does it affect operational range? Heavy rain does have an affect on the range, but it is generally negligible. It attenuates the signal by approximately 0.2dB per mile for a torrential storm, which means a range difference of less than a few feet per mile.
Medical Applications We have some medical equipment that may be able to utilize RFScada units. Are they suitable for this for application? No, they are not. The RFScada units are not authorized nor intended for life support or medical applications.
FCC License Our old SCADA system radio modem & RTU combination has failed again and we would like to replace it with two RFScada units. Do we need to convert our present FCC license to use the RFScada devices?. It depends. No need if ISM spread spectrum devices are purchased, as no license is required to own or operate the RFScada devices within the USA, and the FCC already approves them. Save the renewal fees and throw away your old FCC license along with the failed SCADA system. If the replacement RFScada units are VHF or UHF models then they can be programmed for the existing frequencies.
Solar Panel Operation Can I just connect a 12 volt solar panel directly to the RFScada and operate the unit without AC power? No, at least not at night. However, the RFScada may easily be operated from a solar panel if suitable components are used. First of all a panel must be selected that can supply several times the energy required to operate the RFScada (varies with the model and options, approximately one to two watts is typical), since the panel must both supply the unit and be capable of charging the battery in the shortest daylight available, even on a cloudy day. A system using a panel having a nominal 10 or 20 watts output should have an ample safety margin. Secondly a battery or batteries must be selected that will be able to hold enough energy to operate the RFScada for a period of time, plus the battery must be able to withstand the sometimes fairly heavy charging and discharge cycle. Small auto or marine ‘deep cycle’ type batteries are most commonly used, as they are universally available at a very reasonable cost. Finally, as mentioned earlier it is possible to damage batteries by overcharging them, so some type of charge controller is normally required. Some solar panels have the charge controller already built in, if that type of panel is used another controller is not needed. The components required will vary depending on the location, for example Alaska in winter has very little daylight available, so a solar system there would need much greater capacity than one located in Arizona. It is usually possible to connect the RFScada directly to an existing 12 or 24 Volt solar power system that is being used to power other equipment such as RTU’s or transducers, simply connect the external source to the DC input.
Cabinet Construction The brochure states that the RFScada has a corrosion resistant NEMA 4X rated cabinet but the antenna is located inside. Doesn’t the stainless steel cabinet severely restrict the unit’s range? No, the corrosion resistant NEMA 4X rated cabinet is not stainless steel but made from fiberglass with stainless steel fittings (hinge, locking latch etc.) so it has very little effect on the signal.
Mounting Inside Steel Enclosure If the unit is mounted inside a steel electrical switchboard will the range be affected? Yes, the range will be reduced if it is mounted inside a steel enclosure and uses the internal antenna. Whether the attenuation will be enough to require an external antenna will depend on the cabinet, distance to the other unit and all the other factors that affect range.
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Data
Delivery Devices LLC 3996 Wright Road Bartlesville OK
74006