We are using a cFP 2220 as multipurpose controllers, and in some scenarios, we implement additional safety circuitry to protect the I/O ports against incorrect connections; typically power connected to signal. Up until now, we have been using the RS485 ports on cFP2220, and safety circuitry is trivial with TVS's and TBU's
However, on a recent project, the spec's are slightly different and we need to use the RS232 port.
Page four of TIA-232 interface http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=slla037&fileType=pdf states that:
TIA/EIA-232-F Electrical Specification
All 232 circuits carry voltage signals, with the voltage at the connector pins not
To exceed ±25 V. All pins must be able to withstand a short circuit to any other pin
Without sustaining permanent damage.
(I need to dig deeper on google to find the complete standard, but the above should illustrate the issue)
I guess it's possible to interpret the standard in different ways; but since any pin can have upto +/-25V, then my understanding of the standard is that any pin should be able to withstand +/-25V. As 24VDC is our supply voltage, it would seem like we don't need to protect a RS232 line.
But: The RS232 IC driver (farnell:2496636) clearly states that the maximum input voltage limits for the transmitter is -0,3 / +6V . There are no additional safety components in the cFP 2220.
In most cases internal current limitations probably ensures that the conditions outlined by the standard are met, but that makes the protection of the cFP dependent on the properties of the device connected to it. It is not independently capable of protecting itself.
And it definitely blows up when a 24VDC power supply is applied to pin 3 and GND. So; We need to implement a protective circuit. Our "go-to" method of TBU's and TVS's doesn't really work here, as we need to use 15V TVS diodes, and the remaining ~15V is still enough to blow up the driver. Under these conditions, the TBU's also doesn't trigger correctly. There are loads of alternative solutions, I could simply add a 2.2kOhm between the driver and TVS, but this degrades the driver output to ~4V, which is not thrilling. Other options are a "crowbar" or lower voltage diodes, but both of these solutions are complicated or have other unfortunate implications.
So; my questions:
1) Is my interpretation of the standard correct, and that the cPF is not in complete compliance with TIA-232?
2) What is the NI recommendation for continuous overvoltage protection of a rs232 line?
