> Was someone working on a data logger design?  My brother wants me to throw
> something together as part of a larger part of his system (personal
> use...non commercial) and rather than reinvent the wheel...thought I'd
> just borrow what was done unless it was commercial of course.  He just
> needs two channels of analog, but in about 10 places and then all wired
> together.
>

I did something like this to monitor batteries in an electric car. The
module monitors battery voltage, temperature, and, by sensing the voltage
across the negative lead to the battery below it in the series string,
current. The module also PWMs a FET with a series resistor to load the
battery if its voltage gets too high. This provides a bypass current
during charging.

The trick is the fact that each module is floating at a different voltage
(referenced to the negative side of a different 12V battery in the
string). I did a VERY SIMPLE optically isolated "Aloha" network. Each unit
randomly transmits a packet that includes a header, an identifier, the
data, and a checksum. The PIC drives the LED of an opto coupler where the
detect side is an open collector transistor. All the boards have these
open collector transistors wired in parallel (forming a simple two wire
network). The central device has a pull-up, then goes into the UART on the
PIC. When a device is not transmitting, the LED is out and the transistor
is off, so the pull-up pulls the line high. Any device can transmit at any
time. Packets are short and time between transmissions is long (a second
or so), so collisions are rare. When they occur, the packet is thrown out
due to the bad checksum. I actually used a 16 bit error checking value
that's a combination of shifts and sums of the data. With an 8 bit
checksum, there's about a 0.5% chance of bad data getting through. A 16
bit check reduces this substantially.

Anyway, that's a data collection network I did...

Harold



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