# What makes signal lines hot swappable?

In MAX14780E datasheet, it's explicitly featured as "hot swappable", however in AM26C31 datasheet this isn't mentioned. Moreover, I suspect that I burnt one of the AM26C31 ICs while hot swapping.

I'm aware that hot swapping is also related with the connector (GND and power should make the first contact) but electromechanical part of story is out of interest in this question.

Are the driver chips burnt only because of the rising voltage caused by dI/dt during the disconnection? If so, is placing a TVS between RS422 driver (and receiver?) and connector enough to prevent from such effects?

• Most likely cause is voltage difference between two devices which is mainly caused by what kind of power supplies they have and how these power supplies are connected. Even manuals of two-pronged home equipment like TV, players and amplifiers say to disconnect mains plug when connecting between other equipment. – Justme Jun 16 at 12:16
• I just read Maxim MAX485 datasheet. I found no mention of hot-pluggable. Can you link to a datasheet that says so? – Justme Jun 16 at 15:04
• @Justme My mistake, it was MAX14780E. I added the corresponding link. – ceremcem Jun 16 at 15:22

The hot swap feature of MAX14780E does not mean the RS-485/RS-422 bus side, as most RS-485/RS-422 transceivers are obviously hot-swappable on the bus side by definition.

For this chip, the hot-swappability means the logic level side.

It simply means that when you supply power to the chip, the RE/DE inputs have safety mechanisms that prevent the chip from accidentally driving the bus while the powers are still rising and stabilizing and the RE/DE inputs can also be at indeterminate voltages while the power is still turning on.

If you do that to a standard chip that has no hot-swap feature, it is not guaranteed and it may drive the data bus momentarily until power has stabilized, so this may corrupt ongoing transactions.

Devices aren't really hot swappable. Boards however can be. (Obviously this IC is designed to make designing a hot swappable board easier.)

The main issue with hot swapping is that you can't control the order in which the pins mate. So you have to consider that whatever pins are on the connector might come up in any old order. Also, depending on what the system is, some of those pins might be driven with signals or power busses.

Typical measures taken to make a design OK for hot swap:

1. protective diodes to stop (for instance) a signal line getting to a much higher potential than the power pins on a device (some linear regulators for instance can latch up if pins connect in the wrong sequence.)
2. selecting devices that handle this kind of situation well
3. possibly some sort of timing logic to manage the way the board powers up, or the sequence in which various circuits on it start.

Another issue might be (if you have digital busses) a newly introduced device causing corruption of comms between other devices. So that needs to be handled somehow.

Yet another problem might be that the board being plugged in as at a significantly different static potential to the system, if the operator is careless. So it might well be necessary to have protection against this.

There are likely other possible issues that others will discuss, but this a partial list.

• But you can choose the order how pins mate, by selecting/defining a connector. Many interfaces do this, USB, HDMI, DMX-512 just to name a few. – Justme Jun 16 at 14:42
• that's true if the physical design allows for it. Sometimes you don't have the possibility. – danmcb Jun 16 at 14:55
• I had to accept one answer and the other one is shedding a light on the roots of the issue although this answer is more focused on my actual intentions. Sorry for not accepting. – ceremcem Jun 17 at 13:01
• absolutely no problem! thanks for your courtesy! – danmcb Jun 17 at 13:38