There is possibly a far better way to solve the address interrogation problem than by the means trying to be discussed within this thread so far.

1 - Let us consider that for any reasonable installation covered by the distance that the RF field works is of finite size.

2 - It is only feasible to fit a reasonable number of the "slave devices" into the RF field region.

3 - Consider that some human action is needed to install, setup, or deploy the various slave devices.

4 - Keep in mind that it sounds like from AP's original posting that the slave devices are an original custom design just for this project/product line.

OK here is the basic idea that relies upon the above four basic concepts for its feasibility.

I would equip each slave with some type of wide address source that is setup to be unique for all conceivable types and quantity of each type ever to be made. Burn the code in at manufacturing time, use an off the shelf ID code chip, or get some serial EEPROM pre-programmed from Arrow or Avnet or RS Electronics that has a non-repeating 48 or 64 bit ID code embedded in it. Use such as the slave address.

Design it so that the slave device MCU can read the ID code and use that as his/her address.

Provide for a local interface at the slave wherein the slave MCU can spit out its ID to some type of interface. This could be done via a local RS232 port, IR Transmitter LED, I2C header under the battery door, SD card interface slot, USB Stick interface, local RF transmit mode where slave and address reader are put inside a metal setup box, RFID tag system, magnetic coupled coils, serially coded blinked LED.... i.e whatever seems to fit the scheme of things but make it a local interface that can be used to gather the address of slaves.

Each slave needs a very simple way to put it into its "spit-out-your-address" mode. This could be a button, magnet that closes a reed switch, an access door switch, looped back contact in a battery box connector.... i.e find something suitable that is cheap and works.

Make a slave address collection tool that can be used to gather all the slave addresses in an installation. This could be a datalogger device, an RFID reader, an RS232 connection, a laptop, a carry around version of the master unit. Use your imagination here on a way that suits the product deployment scenario, the type of interface used, the amount of development time you have for this. Depending upon the nature of the product suite that this is all part of it may be even possible to make this be a really cool hand held controller that becomes another revenue item for the product line. Or it could be a re-deploy thing where the connection is done with a programmable PDA or handheld type device.

Fixup your master mode product with a means to accept a download of the gathered slave unit addresses and save them to internal non-volatile storage.

Turn on the RF and let the communications begin.

Michael Karas