Servo - February 2011

Control Your Steel Army With the RPM3

by Fred Eady

RF is magic and I believe that all RF engineers graduated from the University of 666. Despite my prejudice, I still love doing embedded RF projects. The embedded radio we’re going to talk about this month can be considered the FEDEX of UHF radios. If a packet absolutely has to get there, one of the RPM3 series of UHF radio packet modems is driving the big white truck.

Have Packet, Will Travel

The RPM3 (Radio Packet Modem 3) in our possession is technically identified as an RPM3-914-17 UHF radio packet modem. We’ll call it RPM3 for short. As you will see as we move along, the RPM3 is not your average embedded radio. However, we can use all of the embedded microcontroller platforms we’ve built over time to host an RPM3. Before we look at what we will stand behind an RPM3, let’s examine the RPM3 in detail.

The RPM3 is an intelligent radio packet modem that can communicate in point-to-point, point-to-multipoint and broadcast modes. The RPM3’s brains are obvious to the most casual observer in Photo 1. I’d say that the microcontrollers we captured thinking in Photo 1 are responsible for the RPM3’s ability to address a data packet and guarantee its delivery. Guaranteed packet delivery entails the process of checking for errors and acknowledging the reception of a good packet. If the packet is damaged in transit, the RPM3s that are involved directly in the communications link can retransmit the crippled packet. The RF business end of our RPM3 is captured in Photo 2.

Basically, if the embedded host can tap out and recognize signals that look like UART RS-232 data, that host can work with an RPM3 to form one portion of a wireless serial link. Any number of RPM3-equipped devices can be interconnected. All of the low level packet assembly, error checking, and RF control are done for us thanks to the pair of PIC microcontrollers we photographed in Photo 1. The larger PIC16LF876A’s maximum clock speed is 10 MHz with a supply voltage