ZigBee Sensor Questions
Q: Suppose I put 4-20mA into an intellisense module, the value shoots around the ether on a radio link, then what does it come out as? How do I handle it at the receiver end?
A: The only device we offer today that can accomodate external electrical inputs is our SignalConditionerOne (SC1) wireless signal conditioner. The -1 version (SC1-1) is designed to connect to a low-level mV/V pressure transducer and a Pt100 RTD and so its functionality won't permit connection to a 4-20mA loop. The -3 version (SC1-3) is designed to be connected to a turbine flow meter output.
As a relatively new company however, we listen to what the market wants and make development decisions accordingly. It is possible to offer a version with an integrated current sense resistor on the input make a program change to modify the gain. All-in-all it can be done.
So back to your question. Our products work in a command/reply mthod in which a controller sends a command through the base station to an addressed sensor to initiate a measurement. An input signal is immediately digitized and converted to an engineering unit (mA or V). The measurement is formatted as an IEEE 754 floating point number (4 bytes), and placed in an output buffer to be transmitted as the payload of an RF data packet. The transmitted packet is received by the base station and streamed to the controller. It is up to the application in the controller to decode the payload of the packet and our documentation provides the API description. In your example, the payload would constist of 4 bytes that contain a floating point number. I encourage you to download our product user manual at
http://www.intellisensing.com/downloads/documents/PS12UM.pdf for more details on API usage.
We are working on a new product called the BaseStationTwo (BS2) wireless base station. This device connects to an ethernet network and derives its power from the same cable via Power Over Ethernet (PoE). It will manage the details of the communications tasks with the wireless sensors and provide readings via embedded Web server and embedded Modbus/TCP server.
Q: Suppose I put a contact closure into an intellisense module, the binary status shoots around the ether as a radio link, then what does it come out as? How do I handle it at the receiver end?
A: We could incorporate a pull-up resistor and modify follow the same flow as described above. I would think a more efficient coding scheme would be used however. For example, we could have a payload of 1 byte where each bit represents one contact status. We have the ability to measure 8 single-ended channels or 4 differential channels.
Q: For 8 analogs, what's a reasonable throughput/update rate? 1x/sec? 10x/sec? 100x/sec? 1000x/sec? 10,000x/sec?
A: For analog values, the resolution will determine the effective throughput of the system. We use a 24-bit ADC and normally send analog data as a 32-bit floating point number, however the end user can select a high-speed mode where analog is converted to a 16-bit value. Lets assume 16-bit encoding for now. Our experience has shown we can transmit 50 packets a second, each containing 100 bytes of payload data. That is an effective throughput of:
5,000 bytes/sec
-or-
2,500 analog readings/sec
-or-
40,000 discretes/sec
Throughput Requirements Example #1
1x/sec x 8 readings x 2 bytes/reading = 16 bytes/sec
10x/sec x 8 readings x 2 bytes/reading = 160 bytes/sec
100x/sec x 8 readings x 2 bytes/reading = 1,600 bytes/sec
1,000x/sec x 8 readings x 2 bytes/reading = 16,000 bytes/sec
10,000x/sec x 8 readings x 2 bytes/reading = 160,000 bytes/sec
Key:
Good Bad
Q: For 8 discretes, what's a reasonable throughput/update rate? 1x/sec? 10x/sec? 100x/sec? 1000x/sec? 10,000x/sec?
A: See above
Throughput Requirements Example #2
1x/sec x 8 readings x 1/8 byte/reading = 1 byte/sec
10x/sec x 8 readings x 1/8 byte/reading = 10 bytes/sec
100x/sec x 8 readings x 1/8 byte/reading = 100 bytes/sec
1,000x/sec x 8 readings x 1/8 byte/reading = 1,000 bytes/sec
10,000x/sec x 8 readings x 1/8 byte/reading = 10,000 bytes/sec
Key:
Good Bad
Please let me know if I can explain anything in more detail. I am always available to answer questions via:
Mail
IntelliSensing LLC
Attn: Tom Skwara
4 Centre Drive
Orchard Park, NY 14127
Telephone
V: (716) 972-0075 x212
F: (716) 972-0085
Internet
Email:
[email protected]
Web:
www.intellisensing.com
