— RS-232, RS-422 & RS-485
(Module 2 · Modbus Serial Protocols — The Original Workhorses)
Target Reader Automation engineers, embedded developers, and students who must design or debug real Modbus links—cable in hand, scope on the bench, plant downtime ticking in the background.
Chapter Map
| § | Topic | Why it matters |
|---|---|---|
| 4.1 | Serial Comms Refresher | Bit-frame anatomy, voltage vs. current loops |
| 4.2 | RS-232 | Point-to-point legacy still found inside control panels |
| 4.3 | RS-422 | Full-duplex differential; niche but perfect for motion controls |
| 4.4 | RS-485 | The industrial standard—half/full duplex, multidrop, long runs |
| 4.5 | Engineering RS-485: wiring, termination, biasing, EMC | Where most field faults hide |
| 4.6 | Practical Design Checklist | A single-page audit before you leave site |
(Diagram ID placeholders appear like [Fig-4-x])
4.1 Serial Communication Refresher
4.1.1 Framing Basics
| Element | Default RTU Example @ 9600 bit/s |
|---|---|
| Start | ≥ 3.5 char idle (silence) |
| Data bits | 8 (Normal); 7 for ASCII |
| Parity | Even / Odd / None |
| Stop bits | 1 (RTU), 1 or 2 (ASCII) |
[Fig-4-1] Bit-level oscilloscope capture showing start, 8-E-1 framing, annotated 1.5 & 3.5 char gaps.
4.1.2 Voltage vs. Differential Signalling
- RS-232 = single-ended ± 3–15 V referenced to signal ground → noise magnet.
- RS-422/485 = differential (A/B lines) → immune to ± 6 V common-mode noise.
▶ Field insight A ground loop of only 200 mV can destroy RS-232 data; RS-485 typically tolerates ± 7 V DC and ± 12 V transient.
4.2 RS-232 — The Original Point-to-Point Workhorse
| Attribute | Value |
|---|---|
| Bus topology | One TX, one RX, one GND — strictly 1 : 1 |
| Cable length | 15 m @ 19 200 bit/s typical; 50 ft rule of thumb |
| Voltage window | Logic 1 ≤ – 3 V, Logic 0 ≥ + 3 V |
| Max nodes | 1 pair |
| Typical connectors | DB-9, DB-25, panel screw-terminal on modern PLCs |
4.2.1 When RS-232 Is Still Useful
- Inside a cabinet where noise is minimal and point-to-point is acceptable (e.g., laptop ↔ VFD during commissioning).
- Isolated converter cables (USB-to-RS-232 w/ opto-isolation) for quick diagnostics.
⚠ Pitfall Null-modem vs. straight-through confusion.
– Quick test: jumper pins 2-3 on PLC side; echo visible = wrong cable.
4.3 RS-422 — The Quiet Performer
| Feature | Note |
|---|---|
| Full-duplex | Separate differential pairs A/B (TX) & Y/Z (RX) |
| Node count | 1 driver → 10 receivers (multi-drop listen only) |
| Distance | 1200 m @ 100 kbit/s with Cat-5e or Belden 9841 |
| Noise margin | ± 7 V common-mode |
4.3.1 Ideal Use-Cases
- High-speed deterministic motion controllers needing simultaneous TX & RX.
- Long-haul single-master PLC talking to multiple remote read-only I/O panels.
▶ Tip If you need more than one talker on the bus, jump straight to RS-485.
4.4 RS-485 — King of the Industrial Floor
| Variant | Wires | Duplex | Typical Use |
|---|---|---|---|
| 2-wire (Half-duplex) | A/B | Half | 95 % of Modbus RTU networks |
| 4-wire | TX(A′/B′) + RX(A/B) | Full | Flame scanners, motion axes needing streaming data |
4.4.1 Electrical Cheat-Sheet
| Parameter | Value |
|---|---|
| Driver output | 1.5 V min differential into 54 Ω |
| Receiver threshold | ± 200 mV |
| Bus loading | 32 unit loads (UL) max (classic), 256 for “fractional UL” transceivers |
| Common-mode tolerance | –7 V … +12 V |
[Fig-4-2] Diagram: Multi-drop 2-wire bus with addresses 1…31, master at ID 0 pointing arrows both directions (half-duplex).
4.4.2 Why Half-Duplex Wins
- One twisted pair → cheaper cable, fewer terminations.
- Master polls → half-duplex still yields deterministic latency.
4.5 Engineering RS-485 Links
4.5.1 Cabling & Topology
| Rule | Rationale |
|---|---|
| Daisy-chain only (no star tees) | Avoid signal reflections & stub capacitance |
| Balanced twisted pair, 120 Ω characteristic | Matches standard termination |
| Shield grounded at one end (cabinet) | Breaks ground loops but drains noise |
⚠ Never connect shield to both ends and to instrument GND—induces hum loops.
4.5.2 Termination Math
- End-to-end termination = one 120 Ω resistor at each physical end.
- For 4-wire full-duplex: two pairs ⇒ four terminators.
▶ Quick-calc Max line impedance mismatch reflection coefficientΓ=ZL−Z0ZL+Z0\Gamma = \frac{Z_L – Z_0}{Z_L + Z_0}
Keeping ZLZ_L (terminator) ± 10 % of Z0Z_0 (cable) keeps ∣Γ∣<0.05|\Gamma| < 0.05.
4.5.3 Fail-Safe Biasing
Without traffic the line floats; noise can mimic start-bit → phantom frames.
| Component | Typical Value (24 V bus) | Purpose |
|---|---|---|
| Pull-up (RPU) on B | 680 Ω – 1 kΩ | Forces Mark (Logic 1) |
| Pull-down (RPD) on A | 680 Ω – 1 kΩ | – ditto – |
Bias voltage target: +200 mV ≤ VABV_{AB} ≤ +400 mV idle.
[Fig-4-3] Schematic: RS-485 transceiver with termination + bias resistors explicitly labelled, showing DC idle differential.
▶ Tip Only one bias network per segment. Duplicate bias = excess load, weak swing.
4.5.4 Grounding & Isolation
- Provide a 0 V reference conductor (COM).
- Isolate at least one end (opto or transformer) when devices sit > 250 m apart or on separate earth grids.
- Surge arrestors (TVS diodes) at panel entry for outdoor runs.
4.5.5 EMC Proven Tricks
- Route RS-485 away from VFD output cables; cross at 90 °.
- Tie shield to drive chassis near noisy equipment, still ground at cabinet.
- Use ferrite clamp cores on long leads entering control cabinet.
4.6 Design & Commissioning Checklist
| ✔︎ | Task | Pass Criteria |
|---|---|---|
| ☐ | Cable = twisted pair, 120 Ω | Verified with TDR or vendor datasheet |
| ☐ | Daisy-chain topology | No unterminated stubs > 10 cm |
| ☐ | Termination resistors | 120 Ω ± 5 % at both ends (2-wire) |
| ☐ | Single bias network | Idle differential > +200 mV |
| ☐ | Node count ≤ 32 UL (or spec) | Datasheets confirm “1/8 UL” transceivers if > 32 |
| ☐ | Ground reference conductor | < 1 Ω between far ends; isolation if > 250 m |
| ☐ | Shield ground scheme | One-end only unless spec requires both |
| ☐ | Scope verification | Eye-diagram clear; no reflections > 15 % |
| ☐ | Configuration parity/baud | Same on every slave; master tested |
Print this table, laminate it, and tape it inside your toolbox lid.
Key Takeaways
- RS-232 = simplest, shortest, point-to-point → laptop service cable.
- RS-422 = differential full-duplex; specialised for high-speed one-to-many read-only.
- RS-485 = differential half-duplex multi-drop; de facto Modbus workhorse.
- Robust RS-485 requires correct termination and single bias; 80 % of “Modbus not working” calls stem from violating those two rules.
- Use the nine-point checklist before declaring a link “commissioned.”
Visual Assets to Create
| ID | Description | Format |
|---|---|---|
| Fig-4-1 | Scope capture annotated with T1.5 / T3.5 timing | PNG |
| Fig-4-2 | Multi-drop RS-485 bus with master, slaves, termination | SVG |
| Fig-4-3 | RS-485 transceiver schematic inc. bias & surge | SVG |
| Fig-4-4 | Cable cross-section showing twisted pair + shield drain | PNG |
Up Next
With the physical layer mastered, Chapter 5: Modbus RTU — Efficient Binary Communication will drill into the frame byte order, CRC math, and timing tweaks that let RS-485 networks hum along at 115 kbit/s across a 1-kilometre factory. Keep the scope handy.