From left to right: DC power input 9 volts standard Boss barrel connector negative tip, DC power output for daisychaining, Modulation Bus polarity switch and phone jack (input/output), Effect Remote on/off footswitch, Delay audio signal output on phone jack and RCA phono, Mixed audio signal ouptut on phone jack and RCA phono, Level (Unigain) switch, Input audio signal on phone jack and RCA phono.
The Boss Micro Studio system of half-rack sized modules from the mid-eighties used the same 9 volt, negative tip, barrel connector as the Boss effect pedals, and here they conveniently gave you a daisychain power output on each unit, with the current draw of that particular unit printed below, so a sufficient supply/current capability could be calculated. Maximum total draw is recommended not to exceed 200 mA.
The Mod Bus allowed a 0 - 5 volt CV to control the VCO that provided the master clock for the main controller chip (the custom Roland/Boss "long chip" used in so many of their effects around this time). Several other units in the series sported a Mod Bus connector - the idea was you could link two units (perhaps more?) and have one as the "master" (Mod depth turned up) and the other as the slave (Mod depth turned down), changing the polarity as necessary, for stereo effects.
The Effect Remote jack expects to see a typical Roland type footswitch, normally closed, press-to-break type switch. While this is handy, I wanted an easy way to implement a "dub delay" function, where the unit is used as a send for special effects, rather than say, corrective or fixed delay uses. The simple but surprisingly effective Tone control in the feedback circuit makes this unit great for this sort of thing. I disconnected this jack from it's stock purpose (by de-soldering R63), and ran a wire from the tip to the audio input. Thus, when a footswitch is plugged in here, the audio input is grounded. Pressing the switch breaks the contact allowing you to briefly drop in sounds at a touch, for those classic dub effects.