From e6fc5ad4e9efd64787e06b206c1375013f6b0791 Mon Sep 17 00:00:00 2001
From: Ryan Malloy <ryan@supported.systems>
Date: Fri, 13 Feb 2026 02:18:20 -0700
Subject: [PATCH] K-line compatibility analysis: optocoupler vs 510 ohm pull-up

R2 sweep (47-470 ohm) and CTR grade sweep at ISO 9141 minimum
bus impedance. R2<=100 ohm works; BMW's R2=220 does not.
Confirms transistor design is better fit for OBD-II K-line.
---
 reference/opto_vs_kline_510.cir | 70 +++++++++++++++++++++++++++++++++
 reference/opto_vs_kline_ctr.cir | 64 ++++++++++++++++++++++++++++++
 2 files changed, 134 insertions(+)
 create mode 100644 reference/opto_vs_kline_510.cir
 create mode 100644 reference/opto_vs_kline_ctr.cir

diff --git a/reference/opto_vs_kline_510.cir b/reference/opto_vs_kline_510.cir
new file mode 100644
index 0000000..d48b964
--- /dev/null
+++ b/reference/opto_vs_kline_510.cir
@@ -0,0 +1,70 @@
+* Optocoupler (PC817) vs K-Line: Can it handle 510 ohm pull-up?
+* Tests BMW opto design against OBD-II K-line worst-case impedance
+* R2 sweep to find if any value works at 510 ohm + 10400 baud
+* PC817 pin order: 1=Anode, 2=Cathode, 3=Collector, 4=Emitter
+
+.param R2val 220
+
+V_BAT V12 0 12
+V_MCU VCC 0 3.3
+
+* === TX Test Signal at 10400 baud (96.15us bit period) ===
+* Two pulses to show both edges
+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 Bus Model: 510 ohm pull-up (ISO 9141 minimum spec) ===
+R_PULL V12 KLINE 510
+C_BUS KLINE 0 100p
+
+* === TX Driver: Q1 (BC547B) ===
+R5 TX R5_R3 470
+R3 R5_R3 Q1B 10k
+Q1 Q1C Q1B 0 BC547B
+
+* === TX Optocoupler (U2) ===
+R2 VCC U2_A {R2val}
+XU2 U2_A Q1C KLINE 0 PC817 Igain=1m
+
+* === RX Optocoupler (U1) on same K-Line wire ===
+R1_RX KLINE U1_A 2k
+XU1 U1_A 0 VCC RX PC817 Igain=1m
+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)
+
+* === Sweep R2 to find what works at 510 ohm bus ===
+.step param R2val list 47 68 100 150 220 330 470
+
+.tran 0 1500u 0 0.1u
+
+* === Measurements ===
+.meas tran KLINE_HIGH MAX V(KLINE)
+.meas tran KLINE_LOW MIN V(KLINE)
+.meas tran KLINE_SWING PP 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
diff --git a/reference/opto_vs_kline_ctr.cir b/reference/opto_vs_kline_ctr.cir
new file mode 100644
index 0000000..74e5c66
--- /dev/null
+++ b/reference/opto_vs_kline_ctr.cir
@@ -0,0 +1,64 @@
+* 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