Linx Technologies Modules, Boards, and Modifications

I have a set of the Linx Technologies 900 MHz band modules intended for Part 15 use, and the evaluation boards on which to test them. In fact, I think you can't buy just one or two of the modules by themselves, since the mfr is aiming at mass sales, and wants to restrict sales to folks who might operate them illegally. These are very useful modules, and work great.

For Part 15 use, removable antennas have to use a connector not generally available, so the Linx eval modules use reverse polarity SMA connectors (that is, the male thread is the male connector). Personally, I don't think that an SMA connector is generally any more available than the funky RP one, since firms like Pasternak sell the RP SMAs, as well as the RP TNCs and other connectors that the Part15 supply world is fond of.

Anyway, I modified one of the Linx boards to use standard polarity SMAs. This involves sacrificing the RP SMAs currently soldered in (I couldn't get all the solder off all 5 pins at the same time) by cutting the connector up with a pair of big cutters, then removing the pieces one by one. Now I can connect the modules to another antenna, like the 900 MHz helices.

I modified the boards further to run the RTS line from the RS232 interface chip (MAX232) to the Tx power down (PwrDn*) control, allowing the host device to control the half duplex comms. It turns out (of course) that the sense of the signal is wrong, so an NPN transistor serves as an inverter between the MAX232 and the Tx module.

For use in the ham bands, you have to identify your transmitter periodically. It's not real clear what the minimum requirement is, especially for a data link, using some homegrown comm protocol. One reference says you can ID in ASCII, but I decided to take the easy way, and ID with Morse Code by just keying the Tx on and off using the power down line, nominally connected to the RTS line. (It's also how I ID the video carrier...)

W6RMK = di-dah-dah,dah-di-di-di-dit ,di-dah-dit, dah-dah, dah-di-dah

= 1,1, 3,1, 3, 3,
3,1, 1,1, 1,1, 1,1, 1,3,
1,1, 3,1, 1,3,
3,1, 3,3,
3,1, 1,1, 3,3

900 MHz Band Plan

If one is using the Linx modules as an amateur, particularly if you are running more power or a gain antenna, you should really be aware of the nominal 33 cm Band Plan. The following diagram shows the ARRL approved Band Plan (which doesn't have the force of law or regulation, but still is a good thing to follow) compared with the Linx module channels. It looks like Channel 0 and Channel 5 are appropriate for use.

radio/linx.htm - 19 December 2001 - back to Radio home - Jim's Home - mail to Jim Lux