Not only did Opcode produce excellent MIDI software for the Macintosh but they also made the some of the best MIDI interfaces for the Mac to ever exist. It all started with the Opcode MIDIMAC back in 1986 and went on from there.
Opcode Studio 3 MIDI Interface
My first MIDI interface was the rack mount Opcode Studio 3 purchased back in 1991. The Studio 3 was a single rack space unit with two 8 pin mini-DIN serial ports (standard Mac serial port type at the time), one for the Mac printer port and one for the Mac modem port, along with serial pass through ports so you could still have your printer and modem hooked up to the Mac. You could use both printer and modem ports simultaneously, but it was generally accepted that the modem port was a bit better at handling the rush of MIDI data, so if given the option that was your choice. Rumor is that Opcode almost built a modem directly into the Studio 3 but later ditched the idea due to cost concerns.
The Studio 3 had eight MIDI ports on the rear of the unit as well as various inputs for SMPTE time code and tape tracks.
Opcode Studio 4 MIDI Interface
Next came the Studio 4 which could support 128 MIDI channels. Still in a single rack space form factor, the Studio 4 came equipped with eight independent MIDI inputs and ten MIDI outputs and support much more sophisticated MIDI routing than the Studio 3. It was actually possible to network four separate Studio 4 interfaces together to allow for addressing 512 MIDI channels simultaneously. One improvement the Studio 4 had over the Studio 3 was the addition of a few MIDI ports on the front panel. The Studio 4 actually had two MIDI inputs on two MIDI outputs on the front panel. That made for some quick temporary patch-ins without having to dig around in the maze of MIDI cables on the back of the unit.
One major pet peeve: The Studio 3 naturally came with a built-in, quality power supply. The Studio 4 on the other hand, which was much more expensive than the Studio 3, came with a cheap wall-wort external AC power supply.
Opcode Studio 5 MIDI Interface
Later on down the road the Studio 5 came along and was immediately the big dog on the block. It came in a two rack space box and sported 15 MIDI inputs and 15 MIDI outputs and was able to address 240 MIDI channels. If you were so equipped, you were able to network several of these together to create one MIDI interface beast. I never had the chance to get my hands on one, they remained a MIDI industry standard for many years.
You can download the user manuals for the both the Studio 4 and the Studio 5 in the Opcode manuals section. At this time i have not yet found a manual for the Studio 3 (lost mine many years ago in hurricane Andrew) but i am still searching.