Akai S1000 / S1100 samplers

I'm about to install the Digital IO in my S1100.
I just noticed however that there are 2 slots available
(SCSI and SMPTE not included).  Which one to use?

Also, what is the fourth option slot on the S1100 for?
I can account for 3 slots: SCSI, SMPTE, and Digital IO.

But the fourth one?? what more goes in this machine?

--- In akaiS1000S1100Samplers@yahoogroups.com, "tbiggz" 
<infarmah@h...> wrote:
> 
> I'm about to install the Digital IO in my S1100.
> I just noticed however that there are 2 slots available
> (SCSI and SMPTE not included).  Which one to use?
> 
> Also, what is the fourth option slot on the S1100 for?
> I can account for 3 slots: SCSI, SMPTE, and Digital IO.
> 
> But the fourh one?? what more goes in this machine?

I don't see anything else listed online either: 
http://translate.google.com/translate?
hl=en&sl=de&u=http://www.synrise.de/docs/types/a/akai/akai_ib-
serie.htm&prev=/search%3Fq%3DIB-104%2BIB-105%2Bakai%26hl%3Den%26lr%
3D%26sa%3DN