I wish to convert a legacy PLC system to current.

I have a product made by CSI that interfaces to a custom
PLC.  This device has 6 keys, 3-7seg displays, and a thermistor
circuit.  The micro is a 6805.

The thermistor is connected to a single data line through an
inductor.  The other side of the thermistor connects to another
inductor that goes to ground.  I have no idea how the system
reads this, as no rise/fall timing seems to appear on the scope.

The PLC appears to send three packets of data every second. The
data spans 0-5V. Which is acknowledged by the slave with a partial
loading of the data line at 2.5V for ~68ms.  The packets are each
spaced with a ~2ms 5V level.

The slave device appears to pull-down the data line to 0V at
certain bit positions of the data stream depending on what is
selected by keypress.  The minimum bit time is about 40us.  There
appears to be sync pulses throughout that are sharp and significantly
less than 40us.

Any ideas what this is?  If it is a nasty proprietary thing, I'm
going to have to replace the PLC right away too, increasing the
development risk.

I wish to replace it with a 16F84, so I can save setpoints etc.
The current unit loses it power down.

Craig