* SPICE netlist
XU2 N003 N001 vcc 0 RX1 level2 Avol=1Meg GBW=10Meg Slew=10Meg Ilimit=25m Rail=0 Vos=0 En=0 Enk=0 In=0 Ink=0 Rin=500Meg ;pnba In+)In-)V+)V-)OUT
B1 TX1 0 V=(sgn(rand(1e3*time)-0.5)+1)/2
B2 TX2 0 V=(sgn(rand(1e3*time+10000)-0.5)+1)/2
R1 vc TX1 50
R2 vc TX2 50
V1 vcc 0 1
R3 TX1 N001 20k
R4 vc N003 10k
R5 N001 RX1 20k
R6 0 N001 20k
R7 0 N003 20k
XU1 N004 N002 vcc 0 RX2 level2 Avol=1Meg GBW=10Meg Slew=10Meg Ilimit=25m Rail=0 Vos=0 En=0 Enk=0 In=0 Ink=0 Rin=500Meg ;pnba In+)In-)V+)V-)OUT
R8 TX2 N002 20k
R9 vc N004 10k
R10 N002 RX2 20k
R11 0 N002 20k
R12 0 N004 20k
.tran 0 0.1 0 10u

* LTspice schematic (.asc)
Version 4
SHEET 1 1520 448
WIRE -664 -208 -680 -208
WIRE -472 -208 -584 -208
WIRE 584 -208 472 -208
WIRE 680 -208 664 -208
WIRE -440 -64 -456 -64
WIRE -312 -64 -360 -64
WIRE -264 -64 -312 -64
WIRE 296 -64 -264 -64
WIRE 360 -64 296 -64
WIRE 456 -64 440 -64
WIRE -616 -48 -616 -112
WIRE 616 -48 616 -112
WIRE -472 -32 -472 -208
WIRE -472 -32 -584 -32
WIRE -264 -32 -264 -64
WIRE 296 -32 296 -64
WIRE 472 -32 472 -208
WIRE 584 -32 472 -32
WIRE -680 -16 -680 -208
WIRE -680 -16 -760 -16
WIRE -648 -16 -680 -16
WIRE 680 -16 680 -208
WIRE 680 -16 648 -16
WIRE 760 -16 680 -16
WIRE -568 0 -584 0
WIRE -456 0 -456 -64
WIRE -456 0 -568 0
WIRE 456 0 456 -64
WIRE 568 0 456 0
WIRE 584 0 568 0
WIRE -616 32 -616 16
WIRE 616 32 616 16
WIRE -472 64 -472 -32
WIRE -440 64 -472 64
WIRE -264 64 -264 48
WIRE -264 64 -360 64
WIRE 296 64 296 48
WIRE 360 64 296 64
WIRE 472 64 472 -32
WIRE 472 64 440 64
WIRE -264 80 -264 64
WIRE -264 80 -296 80
WIRE 296 80 296 64
WIRE 296 80 264 80
WIRE -568 96 -568 0
WIRE -472 96 -472 64
WIRE -264 96 -264 80
WIRE 296 96 296 80
WIRE 472 96 472 64
WIRE 568 96 568 0
WIRE -744 112 -744 80
WIRE -744 224 -744 192
WIRE -568 224 -568 176
WIRE -472 224 -472 176
WIRE -264 224 -264 176
WIRE 296 224 296 176
WIRE 472 224 472 176
WIRE 568 224 568 176
FLAG -264 224 0
FLAG -616 32 0
FLAG 296 224 0
FLAG -296 80 TX1
FLAG 264 80 TX2
FLAG -616 -112 vcc
FLAG -744 80 vcc
FLAG -744 224 0
FLAG -472 224 0
FLAG -312 -64 vc
FLAG -760 -16 RX1
FLAG -568 224 0
FLAG 616 32 0
FLAG 616 -112 vcc
FLAG 472 224 0
FLAG 760 -16 RX2
FLAG 568 224 0
SYMBOL OpAmps\\UniversalOpAmp2 -616 -16 M0
SYMATTR InstName U2
SYMBOL bv -264 80 R0
SYMATTR InstName B1
SYMATTR Value V=(sgn(rand(1e3*time)-0.5)+1)/2
SYMBOL bv 296 80 R0
SYMATTR InstName B2
SYMATTR Value V=(sgn(rand(1e3*time+10000)-0.5)+1)/2
SYMBOL res -280 -48 R0
SYMATTR InstName R1
SYMATTR Value 50
SYMBOL res 280 -48 R0
SYMATTR InstName R2
SYMATTR Value 50
SYMBOL voltage -744 96 R0
SYMATTR InstName V1
SYMATTR Value 1
SYMBOL res -344 48 R90
SYMATTR InstName R3
SYMATTR Value 20k
SYMBOL res -344 -80 R90
SYMATTR InstName R4
SYMATTR Value 10k
SYMBOL res -568 -224 R90
SYMATTR InstName R5
SYMATTR Value 20k
SYMBOL res -456 192 R180
SYMATTR InstName R6
SYMATTR Value 20k
SYMBOL res -552 192 R180
SYMATTR InstName R7
SYMATTR Value 20k
SYMBOL OpAmps\\UniversalOpAmp2 616 -16 R0
SYMATTR InstName U1
SYMBOL res 344 48 M90
SYMATTR InstName R8
SYMATTR Value 20k
SYMBOL res 344 -80 M90
SYMATTR InstName R9
SYMATTR Value 10k
SYMBOL res 568 -224 M90
SYMATTR InstName R10
SYMATTR Value 20k
SYMBOL res 456 192 M180
SYMATTR InstName R11
SYMATTR Value 20k
SYMBOL res 552 192 M180
SYMATTR InstName R12
SYMATTR Value 20k