Did I fry my FTDI cable by hooking up +/-5V RS232 up to it?

[rgm]

I have a USB FTDI TTL-232 cable - TTL-232R 3.3V  (from Adafruit) and also a device that outputs RS-232 signals using +5V and -5V level voltages.  I understand (now!) that TTL means 0-VCC, whereas RS232 means -10volts to +10volts plus-or-minus 7 volts (ish), where logic 1 is negative voltage.

I plugged my RS232 device directly into my FTDI cable without using an invert / scale circuit.  Junk came out despite the right baud/parity settings.   Do you think my cable is dead now that I've exposed its RX pin to -5V?   Reading of the datasheet doesn't seem to mention negative voltages.

The cable only outputs gibberish from my MaxSonar EZ1 sensor's 0-3.3V TTL output.   I'll try it with an Arduino that I know can output good Serial tonight, but I thought I'd see if any of y'all had experience blowing up these FTDI chips.

I've got a MAX3232CPE chip on order to build a level converter so that I can interface with my Tandy 102 properly.

[Martin Bogomolni]

Yep, ya'll fried it most likely.

I've done something similar a few times.

-M

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[Jessica Ross]

Need to borrow one? I've got a spare of the same model if you promise to apply what you've learned. ;)

[Danny Miller]

I kind of doubt it. The input should be protected by a diode to GND and
Vcc that is normally reversed-biased.
When you exceed the input range, it shunts current away. The problem is
primarily if you apply an excess positive with a significant current
source, the device's Vcc may rise beyond an acceptable range and cause
latchup (which can cause a lot of heat and eventually melt the chip).

But the thing is, RS232 signals usually aren't driven very strongly. If
I were to hazard a guess, it'd be that it's still ok.

I've moved away from FTDI, the SiLabs CP2102 part is very cheap,
capable, and good Windows/Linux drivers. Tons of those on eBay for like $3.

Danny

[rgm]

No thank you -- my replacement cable should arrive in a few days, and I can't really do anything until my parts for building an RS232 level converter arrive anyway.

[rgm]

The FTDI cable is not fried after all (yay!).

The issue with the MaxSonar EZ0 and EZ1 sensors I have is that, although serial output on these is from 0 to Vcc, it's not TTL logic, it's scaled RS232.  So 0 volts means "logic 1" and Vcc (5 volts in my case) means "logic 0."  The datasheet for these things could be a little more clear on this point, but at least they mention it on their website FAQ "Troubleshooting Serial".

I built a NOT gate using an MPS2222N transistor (and a couple 1kOhm resistors) to invert the signal.  I could have also done it in software using XOR but that's yucky.