Network Bandwidth Meter

Posted on Feb 6, 2014

The meter came from eBay, and has a 1 mA full scale movement. It was already marked 0 to 100. I simply removed the original scale, scanned it, and added the `% NETWORK` markings over some of the original ones with an image editor. I then printed the new scale and glued it over the old one. The circuit is a simple design using a Maxim MX7224 8-bit

Network Bandwidth Meter
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D/A converter, with the digital inputs driven directly by the parallel port. Power and reference voltage come from a disk drive connector. The schematic is available here. I built the circuit on a piece of perfboard and affixed it to the back of the meter. There`s nothing critical about the circuit, but be particularly careful to avoid shorts. PC power supplies can source 30 amps or more. This will destroy a meter movement in an instant, and smoke the insulation right off thin wire without the supply even recognizing a fault. The MX7224 is a CMOS chip, so use some precautions to avoid static discharge while handling it, and use a grounded soldering iron if you don`t socket the chip. Possible refinements would include using the MX7224`s latch lines to allow driving multiple meters off one port, or to allow a printer pass-through. An antique wood-cased meter with this circuit built inside would make a lovely external version; the MX7224 can source up to 5 mA, and an emitter follower could be used to boost the current for meters that need more. The software includes a simple C program to do the actual bit-twiddling, and a Perl program to calcuate the bandwidth usage from netstat`s output. These are both pretty FreeBSD-specific. It should, in theory, be possible to do everything from Perl, but using non-standard ioctls from Perl isn`t easy.

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