and tdi of ic 400 respectively in step 4c asm 138a is effectively disabled by causing a logic one level at output do at decoder 507 of ic400 since output do is not connected to any asw of asm 138a in step 4d the point a asw of asm 136a is set to connect point a to tdi of ic600 through asw 136a e in step 4e point b asw of asm 138b is set to connect point b to ground in step 4f point a asw of asm 134a is set to connect point a to tdo of ic 400 through asw 134 e when tck is at a logic one level in step 4g the point b asw of asm 134b is set to connect point b to tdo of ic 402 through asw 134b e when tck is at a logic zero level as stated previously the tdo pins of ic400 and ic402 are connected by virtue of asw 128b having been set to the connect state and no asw of asm 134a having been set to connect when tck 0 nor is any asw of asm 134b set to connect when tck 1 when other ics utilizing the present invention are connected to form extensions of the tdi to tdo path of ics 400 and 402.