Extending the Project
The biggest limitations of this project are that it doesn’t handle very high data rates — if you were going to remote a modem, for example, you’d want it to run at 56 Kbps, not 2.4 Kbps — and it only supports one active full duplex link.
Different radio modules are the key to addressing these issues.We found a variety of other possibilities, some suited to building a few prototypes, and others targeted at production runs.
Radiotronix — Their business is building radio modules, including products for rates as high as 152.34 Kbps. Their Wi.232DTS module runs in a low-power mode up to 38.4 Kbps, or in a high-power mode at up to 152.34 Kbps. Either way, the modules handle multiple links in the same spectrum and are suitable for battery-driven applications. The Wi.232DTS-EVM evaluation module — $80 in relatively small quantities — would simplify building a few prototypes.
Microhard Systems — This company is in the business of complete serial wireless modules, too.Their MHX-910 OEM Wireless Modem Development Kit is intended to support product development prior to production runs using their MHX-910 modules, but everything’s there you’d need to build a link for your own use. The module itself runs in the 900 MHz band (there’s an equivalent one for the 2.4 GHz band), uses a single 5-volt supply, and supports rates to 115.2 Kbps. The kit adds RS-232 transceivers, connectors, power supplies, and status indicators. Cost could be a factor, as the kit is $200 in quantity.
Pegasus Technologies — Addressing much the same market as the units from Radiotronix and Microhard, the PTSS-2003 Developer’s Kit includes transceiver modules, demo boards (including RS-232 interfaces, connectors, and batteries), antennas, cables, software, and other bits and pieces. The Developer’s Kit (including enough for a full wireless link) is about $400. The modules themselves run at rates up to 115.2 Kbps.
Texas Instruments — The TI evaluation board is as expensive as the one from Pegasus Technologies, at $200 apiece, but only supports rates to 38.4 Kbps. The radio hardware is more capable than that; the data rate limitation is elsewhere on the board. The key advantage of the TI product is that it’s highly configurable for a variety of error codes, frequencies, and other characteristics, giving you a platform for more advanced experimentation.
RF Monolithics — This is a high-volume manufacturer of radio modules; they expect you to work out the details of a complete system. They do offer development kits, such as the DR1200-DK, able to run at up to 19.2 Kbps using firmware in an associated 8051 microprocessor. The development kit runs about $350, and includes both ends of the link.
Somewhat harder is to build a wireless USB data link. Inserting a radio link transparently into a USB connection means you’ll need to run at data rates of at least 1.5 Mbps for low speed USB or 12 Mbps for full speed. You must supply power at the remote (non-PC) end too, because USB devices expect to get power from the USB connection.We found development kits from Eleven Engineering good for 1.5 and 3.0 Mbps, and Micro Linear.
The truth be told, we did find a complete USB RF modem by MaxStream. It’s available from Sealevel Systems for $250 each. Of course, where’s the fun in just plugging it in?