The INTERSPEC is a BUS Expansion module, developed and produced by DCP Microdevelopments in 1983*
I bought mine in 1984, primarily to control display Meccano and other models.
Having recently cleared the loft, I found and restored my Spectrum and then rediscovered the INTERSPEC. Sadly I could not fine the original instruction manual and there did not appear to be any available on the Internet. I found a reference to one at the South West Retro-Computing Archive: https://retro-computing.org/?page=item&id=256 I contacted them and they kindly sent me scanned copy. I OCR'd the document into WORD. Some editing, corrections and re-drawing of the graphics resulted in my version which you can download a PDF version of here: https://cil-electronics.co.uk/retro-computing/dcp-interspec
In this video, I demonstrate the Inputs and Outputs of the Interspec with the hardware and software examples shown in the manual. I could not test the DCP Bus as I have no other DCP units to connect it to but will experiment with the guidance shown in the manual.