Hi Jeff,

Edited by amal - 12 July 2010 at 6:45pm

The links have been fixed :)

The answer to your question depends on the tag, its transmit strength, its transmit frequency (how often a data packet is sent), and its battery capacity. Battery life specs can be found for Ananiah tags on each tag's respective page. Here is one example, the RF8315RT "8 meter" tag that can be powered for 5000 hours (approximately 7 months) using a CR2025, or 7000 hours (approximately 9 months) with a CR2032.

With that reader there is no RSSI value reported, but you can attenuate both reader antenna and tag antenna to get the kind of range you're looking for by shortening the length of the antenna. Focus on adjusting the tag antennas because each tag will be tuned slightly differently and/or have slightly different transmit power/characteristics. Once you have the reader antenna trimmed down to reduce range, fine tune the ranges of each tag to suit by adjusting their each of antennas accordingly.

Sort of. The RSSI value will tell you signal strength, but there are several factors that affect RSSI values that have nothing to do with distance. Everything from weather conditions to interference between tag and reader, to even antenna orientation will drastically affect RSSI. What you can typically do with RSSI however is create a kind of rough "squelch".

The truth is, RF is wild stuff. It bounces off certain things, is absorbed by other things, and generally is hard to predict unless you've mapped everything out and done about a trillion calculations... and then never introduced new variables. Adjusting antenna lengths and/or using RSSI will only result in very rough tuning. You won't be able to guarantee much fine-tuned accuracy or be able to know for certain that the tag will always be picked up by the reader at 10 feet and not 11 feet. Some days it might be 11 feet, or 12, or 15... other days it might be 9.

It is possible to create radio range-finding systems, however they require a lot of trickery in order to get that kind of accuracy. This is not something you will find in an active RFID system in this price range.

So, as long as you understand those issues, you could use the antenna tuning idea or RSSI to roughly tune in an effective range.

Hi Jeff,

You're going to need to figure out a way to get the TTL level serial data out of the RF9315R reader. The RS232 specification is not compatible with the STAMP (or just about any microprocessor), which is expecting TTL serial communication.

From http://answers.yahoo.com/question/index?qid=20090211134205AApzAvY
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TTL is typically used within a single system, such as within a computer, and generally does not leave the system enclosure. When one device within the box needs to talk with another device (e.g. a CPU needs to talk to a hard drive) TTL would be appropriate. It uses voltages in the 0V to +5V range. TTL can be very, very fast, but can only travel a short distance.

RS232 (I believe it means Recommend Specification #232) is a spec for communicating between different systems. It is specifically designed to leave the system enclosure (e.g. a computer needs to talk to an external modem). It is far slower than TTL, but can also go long distances, up to 300 feet and beyond I believe. Its voltage is range +/- 15V.
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But you do have some options;

1) Find the TTL level data as it exists on the reader board before it gets converted to RS232 levels and tap the TX/RX lines over to your STAMP.

2) Purchase or build an RS232 to TTL converter. The Parallax site has a whole section on various converters you can build fairly cheaply, including one designed for use with serial RFID readers.

3) Use the RS232 port on the STAMP. This can be done, but it is not "ideal" and is not normally supported. The RS232 port on the STAMP is there for one reason - programming your STAMP with a PC. Unfortunately I can't remember the code tricks you need to use to access the RS232 serial port during runtime operation.