DB9 Simple Signal Splitter

[as400jockey]

I have several IBM AS400 servers (now called IBMi).  I have several APC 1200VX UPSs.  Yes, they're decades old but they are workhorses, as are the AS400s.  I use the APC DB9 to AS400 simple signal cable to talk to the AS400's UPS shutdown features.  Works great, as long as the UPS is only talking to a single AS400 for shutdown/restart.

I want to use a single 1200VX for TWO AS400s at the same time.  I tried a standard serial DB9 splitter cable (one male DB9 to two female DB9), and it ALMOST works.  It works great, mostly.. both machines will power down during a power failure and come back up after the power is restored.  So for power fail purposes it seems to be OK.  BUT... if I power down one of the AS400s using the standard 'power down' command, that machine must be sending some signal up the serial cable that's confusing the other AS400, and the other AS400 decides to shut down.  Or in the case I observed, to shut down and restart.

So... my question is... is there a way to make some sort of multi-port splitter for the simple signalling on a 1200VX so that I won't see this odd behavior? I assume the db9 splitter cables I bought are simply connecting all 9 pins. Perhaps disconnect a pin?  Or a diode on the right pin in the right polarity?

[Teken]

You need a switch . . . 

[as400jockey]

Baffling...

I can't recreate the problem.  But it happened twice, right before my eyes.  And now it works fine.  Both AS400s shut down correctly.  Both restart after power is restored.  I can shut down either with no effect on the other.

Go figure....