FREQUENTLY ASKED QUESTIONS
Q:What is the range of the SignalFire Remote Sensing System?
A:Radio range depends on what is between or near the two radios.
If there is a clear, unobstructed path between two SignalFire 300 mW modules, they can be separated by two to three miles and achieve robust communications. At the other end of the spectrum, communications through a dense forest could be limited to about 500 yards.
Because data moves from module to module, range can be substantially extended by placing intermediate modules in the system. When moving data by this method, effective ranges of well over ten miles are practical.
Q:How many modules will the SFRSS support?
A:The SFRSS Gateway can support up to 240 Modbus® addresses, and therefore 240 SFRSS modules.
In other configurations, such as a streaming system, there is no limit to the number of modules that can be supported. The maximum size of the system is determined by the amount of traffic at the Gateway. A very conservative rule of thumb is one message per second, or 80,000 messages per day. For a system that sends data in once every five minutes, a network size of about 300 modules is an effective maximum.
For most applications, the availability of outside communications (e.g., Cellular, Satellite, RS485, Ethernet) dictates the Gateway locations and drives the actual number of modules supported. As a practical matter, most of our customer’s installations run between 3 and 75 modules.
Q:How much of my link budget can I safely use?
A:The SFRSS-A2, MB, and GW modules use SignalFire’s 300 mW radio with low-noise amplifier. This system has a total link budget of about 135 dB.
This is a combination of radio sensitivity, transmit amplifier power, low-noise receiver amplifier, and high-gain antenna. As a general rule, the minimum reliability link between radios should be 5 dB more than the noise floor.
Each transmission in the SFRSS reports its Receive Signal Strength Index (RSSI). For the SFRSS-A2, MB, and GW, the limit of communications is reported as approximately -95 dB (this uses the full 135 dB of link budget), so the limit of effective communications would be reached at an RSSI of -90 dB.
Q:Can I really run a 4-20 mA Analog Sensor for years on a single battery?
A:Yes, the SFRSS system powers the sensor for only a few seconds every time it takes a reading. The sensor is on long enough only to generate a stable reading and is then turned off. Battery life is determined by the sensor’s power requirements and how often it sends data in.
Q:If the system’s modules are sleeping most of the time, how can I poll them with an RTU?
A:The gateway is powered continuously and stores the most recent data that is sent in from the modules in its network. When the RTU sends a Modbus poll request to the gateway, the gateway responds with the most recent data stored for that module.
The data for a particular module will “time out,” if it is not replaced by new data in a timely manner, and that Modbus address will be dropped, indicating that the module is down. In this case, the RTU will time out on the data request for this module’s slave ID.
Q:How difficult is it to add a module to the system?
A:It’s easy! Turn a rotary switch to match the network number of the gateway. Turn a rotary switch to set the check-in time. Assign a Modbus address with a dip switch or simple PC utility, and you’re done. The module will automatically configure itself on the network and start sending data to the Gateway.
Q:Do I have to tell each module how to communicate with the Gateway?
A:No! The SFRSS is self configuring and self healing. The message will get through as long as there is a communications path.
Q:Can I send image or video over the SignalFire System?
A:Probably not. It would be practical to send an infrequent low-resolution image over the network, but it is designed to support applications where a module generates tens of bytes of data on a per-event or on a regularly scheduled basis (a minute to a day).
Q:If I connect a SFRSS-Modbus module to a Modbus unit, how many registers can I read?
A:Up to 32.
Q:When should I choose the A1 (short-range) over the A2 (long-range) sensor module?
A:The A1 is an excellent choice if you want to connect a single analog sensor to the system and the distance between the sensor and a gateway or repeating node is less than ¼ mile.