I am building a OBD-II reader and wonder if I should tie these two GND together. What is the pros and cons of doing that? My device is powered by car battery and it reads OBD-II protocols through OBD-II ports. Thanks.
The two grounds come from different places. The signal ground comes from the PCM and the Chassis ground come from the body ground.
If there are grounding issues in the car such as poor grounds there may be a voltage across them. Jumping then together would then induce a current loop. Best case scenario nothing happens. Worst case scenario wires get melted.
Don't tie them together. Only use the chassis ground.
Depending on the ground configuration of the car you might not want to tie them together. On the other hand, if you don't, and you need both, then you need to consider what else to do with them. Grounding schemes always depend on the exact requirements for the system, which you have not listed.
The ground pins extend further than the signal pins, giving protection against ESD by mating first, so you should connect to at least one.
If you just need to use CAN for example, then include only the signal ground. The potential will likely be lower between signal pins and signal ground pin, compared to chassis ground pin.
If you need the battery voltage, then you're going to have to connect to the chassis ground.
Exact system specifications would help give more exact advice.
In any case, you should make sure your device does not connect to chassis ground through other paths than the connector, such as having a metal enclosure lying on the chassis.
To answer the pros and cons question specifically: Pro: Same ground reference across the board. Con: Risk of ground loop.
Edit: Since you do need both grounds, then you must tie then together somehow. Either with a solid connection right at the connector, so they become one ground om your PCB, and so that any potentional difference evens out close to the connector and not through a convoluted path on your PCB. The other option would be to isolate the communication lines and control the impendace between the grounds, but that seems like overkill for this project. Since you have noted that equipment manufacturers tie the grounds together, I would say that this is both the best and simplest way to go.
If one is curious as to the voltage difference on the these two lines, you can measure them with reference to a chassis ground like a bare metal screw or bolt. Connect one lead of voltmeter to bare metal point, and the other (+) first to signal ground on the connector, and then to chassis ground on the connector. If this is large it will cause some ground- loop current inside the Car circuitry. Tie them together on the PCB it will keep the PCB free of loop currents.