* Optocoupler vs K-Line: CTR grade sweep at 510 ohm + R2=100
* Tests all PC817 grades with aggressive R2 at K-line impedance
* PC817 pin order: 1=Anode, 2=Cathode, 3=Collector, 4=Emitter

.param CTRgain 1m

V_BAT V12 0 12
V_MCU VCC 0 3.3

* === TX Test Signal at 10400 baud ===
V_TX TX 0 PWL(
+ 0u 0
+ 99u 0
+ 100u 3.3
+ 292u 3.3
+ 292.5u 0
+ 580u 0
+ 580.5u 3.3
+ 772u 3.3
+ 772.5u 0
+ 1500u 0)

* === K-Line: 510 ohm pull-up ===
R_PULL V12 KLINE 510
C_BUS KLINE 0 100p

* === TX Driver ===
R5 TX R5_R3 470
R3 R5_R3 Q1B 10k
Q1 Q1C Q1B 0 BC547B

* === TX Optocoupler (R2=100 for maximum LED current) ===
R2 VCC U2_A 100
XU2 U2_A Q1C KLINE 0 PC817 Igain={CTRgain}

* === RX Optocoupler on same K-Line ===
R1_RX KLINE U1_A 2k
XU1 U1_A 0 VCC RX PC817 Igain={CTRgain}
R4_RX RX 0 1k

* === PC817 Subcircuit ===
.subckt PC817 1 2 3 4
R1 N003 2 2
D1 1 N003 LD
G1 3 N004 N003 2 {Igain}
C1 1 2 18p
Q1 3 N004 4 [4] NP
.model LD D(Is=1e-20 Cjo=18p)
.model NP NPN(Bf=1200 Vaf=140 Ikf=100m Rc=1 Cjc=19p Cje=7p Cjs=7p C2=3e-15)
.ends PC817

* === BC547B Model ===
.model BC547B NPN(IS=2.39E-14 NF=1.008 ISE=3.545E-15 NE=1.541 BF=294.3 IKF=0.1357 VAF=63.2 NR=1.004 ISC=6.272E-14 NC=1.243 BR=7.946 IKR=0.1144 VAR=25.9 RB=1 IRB=1.00E-06 RBM=1 RE=0.4683 RC=0.85 XTB=0 EG=1.11 XTI=3 CJE=1.358E-11 VJE=0.65 MJE=0.3279 TF=4.391E-10 XTF=120 VTF=2.643 ITF=0.7495 PTF=0 CJC=3.728E-12 VJC=0.3997 MJC=0.2955 XCJC=0.6193 TR=1.00E-32 CJS=0 VJS=0.75 MJS=0.333 FC=0.9579 Vceo=45 Icrating=100m mfg=NXP)

.step param CTRgain list 0.5m 1.0m 1.5m 2.3m 3.4m
.tran 0 1500u 0 0.1u

.meas tran KLINE_HIGH MAX V(KLINE)
.meas tran KLINE_LOW MIN V(KLINE)
.meas tran LED_CURRENT MAX I(R2)
.meas tran RX_HIGH MAX V(RX)
.meas tran RX_LOW MIN V(RX)
.backanno
.end