a value proportional to the last process variable current of the 4 to 20 ma type signal when digital communications are completed the capacitor 34 is switched back into the circuit by the switch 32 and the process variable pv current transmission is restored with minimal settling time of the system the following is a detailed list of the circuit components used in a preferred construction of the illustrated example of the present invention as shown in fig5 and 7 ______________________________________cpu 10 22 rca type 1802uart 12 23 rca type 1854r sub 1 30k ohmsr sub 2 r sub 5 r sub 6 10k ohmsr sub 3 20k ohmsr sub 4 124 ohmsr sub 7 312 ohmsr sub 8 1k ohmsr sub 9 r sub 11 250 ohmsr sub 10 750 ohmsr sub 12 100 ohmsr sub 12 100 ohmsr sub 13 1 m ohmsd sub 1 d sub 2 1n4004c sub 1 01 u03bcfc sub 2 47 u03bcfc sub 3 0047 u03bcfc sub 34 2 u03bcmfamp 16 18 icl 7641 intersilnand 14 20 type 4011 rcaexclusive or 28 type 4030 rca v 5v in series with 62 ohmsfet 1 vn98 intersil______________________________________ as shown in fig8 the communications device 8 may include a keyboard 42 for supplying digital signals to the cpu 10 such digital signals and other digital data including stored programs may be stored in memory.