PURPOSE: To execute easily connection and maintenance which follow a fall- back by providing a fall-back control line between each terminal control device, and reducing to half the number of control lines and input/output circuits in an MC (input/output channel device).

CONSTITUTION: In order to execute a fall-back control, a control line of a crossing connection provided on MCs 4a, 4b and TCs (terminal control device) 50aaW 50bm and input/output circuits provided on the MCs 4a, 4b are eliminated, a fall-back control line is provided between the corresponding TCs 50aaW50am and TCs 50baW50bm, and the number (m) of TCs is set to, for instance, "10", a data line is set to parallel 16-bit and also a control line is set to "2", by which the total number of control lines eliminated by the old crossing connection becomes 10×(16+2)=180 pieces except a ground line, and this control line goes to 10×8, and it is possible to realize a terminal equipment controlling system in a data processing system which is capable of executing easily connection and maintenance following the fall-back control.