Before their merger with Voyetra, Turtle Beach was an early player in the PC audio card business. The 56K was an ISA card that came with an optional interface box with AES/SBU connections for your DAT. This ad is from 1992.

Rob Warmowski: Do not get me started about full-length ISA cards. I never used the 56K (well, I sure used a 56K modem, but that's a different kettle of fish.) I did however have experience with installing older full-length ISA-card based systems, (which were designed for the full-sized horizontal fat pizza-box case) into newer vertical minitower cases. Shouldn't have been a problem -- a PC is a PC, right? Oy. The headache. What do you do when the card is too long for the case? Yes, it could and did happen with mini-towers and full-length cards. The segment of the card leading to the back of the machine could exceed the total length available once you seated the card. The solution? A big tin snips / pincers to chew away the bi-metal skin of the back, to make room for the part of the card that poked out. And since no screw mounting was therefore available (the part I snipped away contained the screw holes by necessity) to relieve the gravity stress I trussed up the card using small cable ties and handy holes in the breadboard. Oh, Session 8, you were so much "fun."