why do my 900HP modules get dropped from the network frequently?

I have about 20 XbeePro 900HP modules, (PN XBP9B-DMST-002). They are all operating in AT mode as the transmit modules for some Arduino dataloggers that utilize the serial UART pins of the Xbees. One of the modules is programmed as the coordinator, the rest are end devices. Transmissions are sent every 5 minutes from the end devices at random times, so none of them are on simultaneously. The end devices are powered on and off by controlling the power to the Xbee Vcc pin since my dataloggers can’t support the pin-sleep mode of the Xbees. The end devices are powered up about 5 seconds before a serial string is sent from the datalogger to the UART pin of the Xbee. The Xbee is shut off about 5 seconds later.

My problem is that at least several times a week, one or more of the end modules will “disappear” and will not send any data to the coordinator unless I visit the loggers and physically remove the power to the entire board and restart it. Pressing the Xbee reset button doesn’t help, only removing the board power does it. This seems strange because the Xbee is completely powered down between transmissions by the datalogger, so it should be seen by the coordinator the next time it wakes up. But again, only a complete power cycle of the board will fix the issue. I’ve verified that the Arduino datalogger is still functional and continues to send data out to other ports as well as writing it to onboard memory, only the Xbee seems to be acting up. I have seen other people in the Digi forum mention this same problems with 900HP modules but there was no resolution. Do you have any suggestions? Thanks.

Sounds like you need to look at your sleep functions and make sure that you are using the Asynchronous sleep functions instead of the default synchronous sleep modes.