Siemens Micromaster 420/440 digital input max voltage rating?

I've been trying to find the max voltage rating of the 24VDC digital inputs on the Micromaster 420/440 drives from Siemens. I have found the min/max for the On/Off levels in the manual but not the max allowed connected voltage.

I found a drive in service (for 4+ years) that had the 0VDC of the control voltage connected to the +24VDC of the drive's isolated power supply. Then the +24VDC of the control voltage was used for controlling the Digital Inputs. This put almost 50VDC on the digital inputs and it ran like this for years before the drive eventually failed. I believe the failure was due to this bad wiring, but am curious what the ratings were to allow it to run this way for over four years.

You are right, it doesnt say in the manual for MM440.
I checked the same manual for the control unit on G120, which is the new equivalent of MM440, so should be approximately the same.
It says:

Low < 5 V, high > 11 V, maximum input voltage 30 V, current consumption 5.5 mA

Click to expand...

So max 30V. That it works for some time above 30V is not so surprising.

edit: Thinking again...
If the 24V from the drive was connected to your common 0V, and common 0V is tied to ground (as is normal) then I dont think that the input would see 24V + 24V = 48V. I think it would see 24V, while the drives internal 24V power supply would be delivering max current, which in turn would cause overheating, and in turn it would fail after running for a long time under this condition.

Thanks.

It appears that someone in the past (before we received the machine) replaced a 420 with this 440, and did a one to one terminal transfer. Terminal number 9 on the 420 is 0VDC, but on the 440 Terminal number 9 is 24VDC.

They also put terminals 10/11 of the 420 (a relay output) onto terminals 10/11 of the 440 (an analog input).

I'm still kind of surprised nothing bad happened connecting up the wiring wrong like they did, but at least it's all fixed now.

Edit:
The digital inputs were getting 50VDC, that's what I measured (0VDC of the drive to DIN1 and DIN2 when activated were right around 51VDC). It's what made me initially question the existing wiring.

Since the drive's voltage supply was isolated, grounding the 24V by connecting it to the grounded 0V of the control voltage didn't really affect it. Until you used 24V from the control voltage on the Digital Inputs and put 50VDC potential across the input circuitry