[TowerTalk] Computer control antenna switch

[Jim Lux]

At 10:01 AM 3/9/2005, Pat Barthelow wrote:
>Hello Wayne,
>
>I think K1ttt nailed it, but you might find useful a specific boolean search
>on Google that produced a ton of products and DIY information for using the
>Parallel port for monitoring and control.
>In Google:  type in,  exactly so:   "parallel port"      + Control
>
>(Quotes around parallel port, and three spaces before the plus sign, then
>the word Control....
>
>Our meager electronics store in Monterey Bay area has a kit with software
>and hardware to build a control port around the parallel port.  This was by
>one of those companies that has dozens of useful kits for sensors, motion
>detectors, RF switches, etc...
>
>BTW, there is another very useful site for all types of remote control
>switches and systems at very cheap prices.....try  http://www.
>aaaremotes.com
>
>73, DX, de Pat Barthelow  AA6EG   aa6eg at hotmail.com




The real problem with using the printer port these days is that
a) You're pretty well insulated from the hardware using Windows, so you 
have to do all sorts of workarounds to talk to the bits.  You can set up a 
plain printer, and talk to it as if it were a dumb printer, but you don't 
have any control over strobing, timing, or the ability to readback the 
port.  Yes, there are a raft of programs and windows drivers that get 
around this, but since they violate a fundamental part of the design of 
Windows (i.e. insulate user programs from the hardware), they tend to be 
"picky" in their functioning.

b) More and more computers are doing away with the printer port.  It's 
physically a large connector, consumes board space, etc.  and most consumer 
(read inexpensive) printers are USB using the PC as a computational host 
for a lot of the printer function anyway.  When you're selling millions of 
motherboards at $50 each, wholesale, getting rid of a 50 cent connector is 
very attractive.

c) unlike in the good old days, when there were some explicit 8 channel ICs 
driving the printer port, today, those bits are driven by a custom 
peripheral ASIC, which may or may not have the same drive characteristics 
or tolerance to outputs being shorted or tied to 5 volts (or more).  If you 
blow the drivers today, it's buy a new motherboard time, not buy a couple 
chips and solder them into the printer card.


You really, really want some isolation between you and the external 
hardware.  The various USB solutions, like the labjack 
(http://www.labjack.com/) or stuff from DataQ (http://www.dataq.com/) or 
Measurement Computing (http://www.measurementcomputing.com/)

The Labjack U12 is $120 (qty one).  (A year ago, or so, there wasn't any 
Linux support for the Labjack)

The bottom of the line USB Dataq unit is the DI-148, which has 8 analog 
input channels, 6 digital I/O, etc.  Even cheaper is the DI-194RS starter 
kid for $25.. 4 channel 10 bit, and 2 digital I/O with RS232

Measurement Computing's PMD1208LS has 8 (slow) analog inputs, 2 D/A 
outputs, and 16 digital bits with USB interface for $109.  Their PMD1024HLS 
might be more interesting.  It's 24 high current digital io (64mA sink, 15 
mA source) bits for $149 or the -LS version (with lower current drivers) 
for $99.

There's also the MiniLAB1008 for $109, which is clearly competition for the 
LabJack type devices: 8 channels of 12bit analog in, 2 analog outs, 4 
Digitals on screw terminals, plus another 24 bits on a 37 pin connector 
which pins out compatibly with all the usual relay and breakout boards the 
usual sources.


The curse of all USB devices is that Linux support is sometimes dicey.  Not 
that you can't send messages to and from the device with Linux, but the 
manufacturer's drivers are Windows only, so you'd have to write your own 
interface routines.  There ARE various Linux drivers out there for all 
these products (as well as the parallel port), but, no real 
guarantees.  But hey, if you're running Linux with oddball hardware, you're 
already used to kernel version and glibc stuff, right?

(someone just pointed me to: http://www.linux-usb-daq.co.uk/ .. pricey at 
GPB130 (that's >US$250) but at least they outright claim Linux support)


Another approach, if you're a software writing type, is to get an embedded 
controller like one from http://www.zworld.com/.  They have a variety of 
interfaces, program in C, etc.