Hi all,

I'm currently designing a system that will have two RS-232 serial ports, two CAN ports (low speed), and a programming port. The system itself just converts from serial to CAN and vice-versa.

The 4 layer board will be in a small enclosure (probably aluminum), with a circular connector to it. Outside the box, the serial & CAN wires are going to individual DB9 connectors on 6 inch pigtails. Inside the box, all of the grounds will be connected together at a star point then connected to the enclosure.

All pins will need to have some sort of ESD protection on them. This is also going in an automotive environment.

I have been searching this site and google for the last couple days with regards to ESD surges protectors, and have only come up with more questions. Most posts/sites I've found say to just 'use them', without explicitly saying how to, and without answering the questions I have below.

On the last page of this datasheet for a TVS, it shows the bidirectional TVS connected from the IO line to ground. http://www.avxcorp.com/docs/Catalogs/mguard.pdf
-Since this TVS is bidirectional, I presume it will guard against both positive and negative ESD events? Or, will I need one connected to ground, and one connected to Vcc like I saw in some circuits? (I don't think I'll need two per IO, since, for a positive surge, one diode is reverse biased, the other is forward biased, and the surge is captured. Same thing with a negative surge, just the other way around).

-Do I need the ferrite bead shown in the application circuit on the right? If I understand correctly, this is only to help protect the IO pin from the clamping voltage of the TVS? ie, the TVS clamps the voltage at, say, 18v. Obviously, this is too high for a standard IO pin to handle. So, the bead helps reduce this current/voltage spike, while the TVS handles the large spike? If that's correct, I will plan on using one. If that's not correct, I'd like to know what it is there for.

Next, I am planning on using this RS-232 driver: http://focus.ti.com/lit/ds/symlink/sn65c3222.pdf
-Since it has built in +-15kV protection, I presume I don't need any additional protection on the TX and RX lines? (I don't mind adding additional protection if necessary).
-If I do need additional external protection, how do I choose a proper working voltage if the line is negative? Would I pick a 14v working voltage part for the -10v RS-232?

My CAN transciever has built in 2000v ESD protection I believe. So, I'll plan on using TVSs on its two lines as well.

-Do I need to protect the ground wires in any other way than connecting them to the enclosure ground?

-What do they mean by 10/1000us or 8/20us surges? (Everyone quotes them, but no one defines them?)

Thanks a lot in advance,
-Dave