For those of you using TwinSAFE and the EL2904 output terminals, how are you making use of the (+) and (-) connections for each channel? I was trying to determine whether these connections are both failsafe to achieve a redundant circuit. For example, I'm seeing more motor starters and drives these days that have dual channel STO where one channel is 24V+ and the other expects DC common. Can the (+) and (-) outputs be run directly to these STO inputs and be considered two-channel safety?

The block diagram of the EL2904 in the manual is a bit confusing.

What is the purpose of the high- and low-side switches (circled in blue by me)? Since they aren't in the red box they apparently feed all four channels. So under what conditions would these switches open? They'd have to be closed if at least one of the channels was activated, so it would seem that the (-) connection does not necessarily drop when a channel is deactivated. But this appears to contradict another picture elsewhere in the manual which shows identical logic on both the (+) and (-).

Any thoughts? I sent this question to Beckhoff support and didn't get a very clear answer.