define(`TITLE', `Another (better) MBus-Master, Part 6')
define(`DATE', `2014-04-07')
define(`CONTENT', `
Finally, over current protection has been added.

<a href="http://a385e-5.de/wp-content/uploads/2014/04/main.pdf"><img src="http://a385e-5.de/wp-content/uploads/2014/04/main-300x211.png" alt="main" width="300" height="211" class="alignnone size-medium wp-image-447" /></a>

The over current protection is separated in a sensing part and a shutdown part. The comparator IC6B gets a threshold voltage using the potentiometer R18 and a signal voltage through the low pass filter of R19 and C3 (the values are not yet fine tuned). R21 and D7 are used to limit the output signal of the comparator, which is 35V power to a logic level. Through an inverter it goes to an interrupt line of the controller. An interrupt service routine in the controller firmware controls the transistor Q3 and pull down the positive input of the OP IC3A, which is part of the current source to shutdown the bus current.

A receiver shutdown transistor (Q4) has been added. During tests with a dedicated poewr supply serious cross talk from the sender path in to the receiver path for a bus current of about 150mA has been discovered. This has not been investigated enough, just the receiver path has been shutdown during a send cycle. This is certainly only the second best solution, but it was a quick one.

')