The Ensoniq SQ-R is part of the lower-cost line of SQ products by the company that also included the SQ-1, and SQ-2. The SQ-R is basically a rack mounted version of the SQ-1 keyboard and it does not include keys or a sequencer.  Although it lacks a keyboard, it does support Ensoniq's "Smart Transmit" function, which means you can connect a keyboard to it and it becomes an eight zone controller. Even if you use a MIDI keyboard that lacks multi-channel capabilities on its own, the SQ-R will provide independent control over volume, key range, MIDI channel, program change remapping, transpose, as well as sustain pedal on/off.

For storage the SQ-R uses memory cards, but you can also save your to any MIDI device that has this capability. On the rear panel of the SQ-R you will find the MIDI In, Out, and Thru connections as well as a jack where you can connect the optional Ensoniq Model CVP-1 Control Voltage Foot Pedal. Next to that is the right and left mono output jacks. Everything that you can do with the SQ-R is accessed directly from the front panel. It features a 32 character LCD display as well as numerous buttons. The SQ-R can operate in four different modes, namely Sound Select Mode, Sound Edit Mode, Preset Select Mode, and Track Editor Mode. The parameters for each of these are displayed on the LCD screen and you can use the left or right arrow buttons to navigate screens with multiple parameters.

When it comes to the synthesizer section of the SQ-R, you have access to 100 high fidelity sounds that are stored in Read Only Memory. You can also store up to 80 additional slots in the internal memory of the SQ-R if you want to create and edit your own sounds. Furthermore, the SQ-R supports both RAM and ROM cards that can add an additional 160 sounds. The SQ-R uses three digital oscillators for creating sounds and these make use of more than a hundred sampled acoustic and synthetic waves. In addition, it also supports Ensoniq's Transwaves for some unique sounds that are hard to replicate with other hardware. Finally, the SQ-R has an on-board digital effects processor that can be used to make your sounds even more interesting.

Ensoniq came out with 3 versions of the SQ-R:

SQ-R - original model. It had 21-voices of polyphony.
SQ-R Plus - updated model that still had 21-voices of polyphony but added 16-bit piano waveforms.
SQ-R Plus/32 - last version of the model. It had 32-voices of polyphony and additional 16-bit waveforms.

These rack models closely resembled the specs of their keyboarded counterparts, minus the sequencer feature.

The lineage of these models are as follows:

SQ-1/SQ-R - 21-voice units
SQ-1 Plus/SQ-2/SQ-R Plus - 21-voice units with updated 16-bit piano waves
SQ-1 Plus/32/SQ-2/32, SQ-R Plus/32 - 32-voice units with additional 16-bit waves

The SQ-1 was a 61-key version.
The SQ-2 was a 76-key version.

Some may also put the KS-32 in the mix (no pun intended) because it was basically an SQ-2/32 with a weighted Fatar keybed and a few additional piano programs using existing 16-bit piano waveforms already included in the SQ line.

With regard to the sounds, you could have 80 ROM sounds, 80 RAM sounds and 20 drum kits. Additional sounds could be accessed via RAM/ROM cards that one could purchase separately.

The 16 character/2-line LCD screen was the same for all models.

The SQ-R is far from perfect, but if you are a fan of Ensoniq and don't mind the display, which can be a bit hard to work with, then it can still deliver some interesting sounds