A finite state machine (sometiems shortend to a 'state machine') is a way of designing and building sequential logic circuits -- that is circuits where the outputs(s) depend not only on the inputs now, but also on previous sets of inputs. The basic idea is tha tthe circuit stores internally a 'state', normally as an n-bit binary number in n D-tyoe flip-flopes (that's the 74LS174 in your circuit, a hex D-type flip-flop). All flip-flops are clocked at the same time to change to a new state, Andthen there is a block of combinatorial logic (a cricuit where the outputs depend only o nthe inouts now, not what they were) which has as inputs the external inptus ot the state machine and the outputs of the D type flip-flops and which produces the inputs to the D-type flip-flops to give the new state. Sievn a ROM is a gnneral combinatorial circuit, you can make a stte machine from a ROM and a set of D-type flip-flops, aas here. The 9216 was a very common data separator about 25 eyars ago. It was used on a lot of PC/XT cloen FDC cards (not on the original IBM one though) and in other machines too. I am pretty sure it was made by several manufacturers. The 2979 is essential a 1797 with an intenral data separator. It works well, but IIRC there are 3 analogue adjustments associated with it (a couple of puse widths and the free-running frequency of a VCO). It's not hard to set these up, but you do need a 'socpe or logic anlyser to do it. The 9216 data separator IIRC needs no external adjustments. -tony