Motor Driver board Question

[vargoal]

I was working on fixing one of my two Dagu motor controller boards. I found this schematic (http://www.robotgear.com.au/cache/Files/Files/159_4ChannelDCMotorControllerSchematic.pdf). I figured out that what was broken on the board was the 74HC08 chips, I think I just plugged in a 12 V to VCC by accident Anyway when I was looking at the schematic I was curious as to how the P-Channel Mosfet gate is set to 12V by a 5v logic circuit. I think it's because of the pull up resistors to the 12v battery but I don't see anywhere in the datasheet for the 74HC08 that says that it only pull's current. Would the 12v rail  back drive the 74HC08, and if so does that damage it? Also forgive any faults in my statements as I am just beginning with electronics 

[SeanB]

74HC03 is what is used, which is an open drain CMOS device. Thus it can be connected to a higher voltage than the supply, as indicated in the data sheet, though it is limited to 7V max.

http://www.nxp.com/documents/data_sheet/74HC_HCT03.pdf

The IC might survive 12V applied to the output for a while, but some devices will not survive that. Running the motor at 12V will require something which is rated for 16V on the outputs.

[vargoal]

Thanks SeanB. Based on what you said I looked up more on CMOS and went back to the datasheet. It says "The input and output voltage ratings may be exceeded if the input and output current ratings are observed." It was in small print below the electrical characteristics section. I did the calculation and it does stay within that limit so it now makes sense that it works.