Electronics Questions & Answers ← browse Q&As

I have this circuit which I have to study but i dont know how does the DAC circuit works combined with the other componentsI understand the functioning of the DAC but not that of the other components.What is the function of P5 component,what does it do? https://i.stack.imgur.com/cX2Pr.jpg

It looks like a fuse. Simple answer would be that it creates an open circuit when the temperature gets too high. The mechanism for this used to be that the fuse would self-destruct but nowadays they often trip off intact and can be reset.

Description P5 is a field-configurable "switch". Default manufacturing state is closed, as in your circuit. The switch may optionally be opened during installation of the circuit board on site.

There are two common implementations of this type of "switch": link and jumper. The link is a short wire soldered into the pcb which can be cut to open the switch. This very robust method is preferred for rugged industrial installations.

The jumper is a tiny plug and socket (also commonly found on an HDD). Remove the shorting socket to open. Suitable in benign environments with low vibration and shock.

In practice, be prepared for both names to be used interchangeably!

Function P5, with P4 and P6. This is much more complicated because it involves other systems not shown on your circuit. U4, connector J2 and the components in between form a half-duplex node on an RS-485 bus. Typically, there are multiple other nodes on the same bus.

Simplified statements of an RS-485 bus system "rules" are -

• only two termination resistors R12 in the entire bus, one each at the physical extremities;

• only one each of the pull-up (R11) and pull-down (R13) resistors in the entire bus.

Typically, links P4-P6 on each node will be configured at the time of RS-485 bus build and test.

The standard industrial current loop (through connector J1), the isolators U5-U7 and the separate GND points (eg U5 pins 4,5) all say that this board will be used in an environment with high levels of electrical noise. The DAC will be relatively slow due to its serial input (external source through U6 pins 2,7).