Chapter 6 – Modbus ASCII

— Human-Readable Communication for Noisy & Low-Bandwidth Links

(Module 2 · Modbus Serial Protocols — The Original Workhorses)

Reading roadmap

§	Topic	Bread-and-Butter	Deep-Dive Nuggets
6.1	Why Modbus ASCII was invented	Dial-up modems, radio, teletype	How 1970-era acoustic couplers shaped the frame
6.2	Frame anatomy	Start “:”, ASCII hex, CR LF, LRC	Byte-by-byte with ASCII→binary mapping
6.3	Longitudinal Redundancy Check (LRC)	8-bit two’s-complement checksum	Manual example + table-less C & Python code
6.4	Timing & throughput	1-second inter-frame vs RTU’s 3.5-char	Bandwidth model & optimisation levers
6.5	ASCII vs RTU head-to-head	Pros, cons, mixed networks	When to tunnel RTU vs convert in gateway
6.6	Worked examples	Read & write frames with live decode	Wireshark TCP-over-serial capture, RealTerm log
6.7	Implementation patterns	UART config, buffers, error handling	Converting ASCII↔RTU inside microcontroller
6.8	Troubleshooting cookbook	Classic field failures & fixes	“Ghost colon”, CR LF polarity, HEX typo traps

(diagram & listing placeholders appear as [Fig-6-x] / Listing x)

6.1 Why Modbus ASCII Exists

Human readability
Every data byte is transmitted as two high-ASCII characters (0‥9 A‥F). A technician armed with only a dumb serial terminal in 1984 could read frames and type commands by hand.
Character-oriented framing
Start sentinel : and terminator CR LF remove dependence on inter-byte silence. Perfect for:
- acoustic-coupler modems (300 Bd)
- half-duplex UHF/VHF radios with unknown turn-around delay
- RS-232 links that insert variable gaps (UART FIFOs, SCADA pollers)
Noise resilience on mutilated channels
ASCII hex doubles the byte length but halves the probability that a single flipped bit converts one legal byte into another legal byte.

6.2 Frame Anatomy — Byte-for-Byte Breakdown

Mnemonic “S A F D … D LRC E”
Start : Address Function Data … Data Checksum End(CR LF)

Order	Element	Length (ASCII chars)	Purpose
①	Start	`:` (3A h)	Marks absolute frame start
②	Slave Addr	2	Hex of 0–247 (00–F7)
③	Function Code	2	Hex of FC (01–06, 0F, 10…)
④	Data	N×2	Parameters or payload
⑤	LRC	2	8-bit checksum (two’s complement)
⑥	End	`0D 0A` (`CR LF`)	Frame terminator

[Fig-6-1] Colour-coded ASCII frame -> binary field map.

6.2.1 ASCII → Binary Mapping

Every ASCII pair (3F) == one binary byte (0x3F).
Total binary data per register = 2 (bytes) × 2 = 4 ASCII characters.

Binary	ASCII High	ASCII Low	Comment
`0x11`	`'1'` `'1'`	Slave address 17
`0x03`	`'0'` `'3'`	Function 03

6.3 Longitudinal Redundancy Check (LRC)

6.3.1 Definition

LRC = Two’s-complement (negated sum) of every binary byte from Slave Addr → Data inclusive. LRC=[−∑i=1nbytei]8 bit\text{LRC} = \left[ -\sum_{i=1}^{n} \text{byte}_i \right]_{8\,\text{bit}}

6.3.2 Hand Calculation Walk-Through

Assume request :110300000006 (Addr 17, FC03, Start 0000, Qty 0006).

Byte	Hex	Running sum (8-bit)
Addr	11	0x11
FC	03	0x14
Hi	00	0x14
Lo	00	0x14
QtyH	00	0x14
QtyL	06	0x1A

Negate: -0x1A = 0xE6 (two’s complement).
ASCII E6 appended before CR LF ⇒ full frame:

:110300000006E6␍␊

6.3.3 Table-less Code (C) – Listing 1

uint8_t calc_lrc(const uint8_t *buf, size_t len) {
    uint8_t lrc = 0;
    while (len--) lrc += *buf++;   // 8-bit add (auto wraps)
    return (uint8_t)(-lrc);        // two's complement
}

6.4 Timing & Throughput

6.4.1 Framing Rules

Parameter	Recommended	Notes
Inter-char timeout	≤ 1 s	Longer aborts frame
Inter-frame gap	≥ 1 s	Gives slave parse window
Baud rate range	300 Bd … 38 400 Bd	ASCII decoding overhead becomes dominant above 19 200 Bd

Unlike RTU, ASCII framing does not depend on character time; start/stop chars govern the parser.

6.4.2 Efficiency Equation

η=NdataNascii=(1+D+2)(1+D)×2+5\eta = \frac{N_\text{data}}{N_\text{ascii}} = \frac{(1 + D + 2)}{(1 + D) \times 2 + 5}

(where 1=Addr, 2=LRC, D=Data bytes, 5 = Start+CRLF)

Example read of 60 regs (binary 125 B data):
ASCII size = (1+1+4+2) × 2 + 3 = 232 chars (1.28× bigger than RTU).
At 9600 Bd = 232 × 11 / 9600 ≈ 266 ms vs 91 ms for RTU.

▶ Optimisation tip If you must stick with ASCII, crank baud to 19 200 or 38 400 to regain cycle time.

6.5 ASCII vs RTU — Choosing Wisely

Feature	ASCII	RTU
Human readable	✅	❌
Works through ASCII-only media (GPRS SMS, e-mail tunnelling)	✅	❌
Bandwidth efficiency	❌ (≈½)	✅
Parser complexity	✅ (no timing)	⚠ Gap-aware
Interoperability with hobby tools (PuTTY)	✅	❌
Field popularity (2020s)	5 % of new builds	95 %

Decision matrix

Choose ASCII if:

Communication traverses text-only channels (ASY-000 AT-modem, radio telemetry, SMS).
A human will type configuration frames (legacy drive commissioning).
Debugging takes place via simple terminal without protocol plug-ins.

Choose RTU otherwise; wrap RTU inside an ASCII-capable modem or tunnel over TCP if needed.

6.6 Worked Examples

6.6.1 Request — Write Single Coil (ON) to Slave 07

:0705 0013 FF00  79␍␊

Spaces added for clarity.

Field	ASCII	Bin	Meaning
Start	`:`	—	—
Addr	`07`	0x07	Slave 7
FC	`05`	0x05	Write Single Coil
Addr	`00 13`	19	Coil 19
Value	`FF 00`	On	—
LRC	`79`	—	Checksum

Slave response echoes same PDU → :07050013FF0079\r\n

6.6.2 Terminal Capture [Fig-6-2]

Screenshot of RealTerm log, highlighting Start → LRC, colourising TX vs RX.

6.7 Implementation Patterns & Gateways

6.7.1 UART Configuration

Setting	Recommended
Data bits	7 (ASCII) or 8 (if parity used for 7E1)
Parity	Even (`7E1`) — optional
Stop bits	1

Why 7E1? Each ASCII char is 7 bits; parity gives simple per-char error check.

6.7.2 Buffer Strategy

Scan for :; when found, switch to collect mode.
Store until LF; verify LRC before parsing.
Reject frames > 513 chars (spec safety guard).

6.7.3 ASCII ↔ RTU Conversion

Many modern gateways (e.g., Moxa, Anybus) transparently convert:

Strips : and CR LF.
Converts ASCII pairs → binary bytes.
Re-computes CRC-16 to send onto RS-485 RTU slaves.

Latency cost ≈ 1 ms + (ASCII length/baud). Ensure gateway FIFO can buffer full ASCII frame.

6.7.4 In-Microcontroller Conversion (bare metal)

Listing 2 Minimal C routine (STM32) to accept ASCII on UART 1, forward RTU on UART 2; includes interrupt state machine.

(full listing trimmed for brevity but complete in downloadable repo)

6.8 Troubleshooting Cookbook

Symptom	Possible Cause	Fix
Slave never responds	Start char missing (`:`)	Verify terminal sends 0x3A; some GUIs strip
“LRC error” only on long frames	Admin typed wrong HEX pair	Echo input, visual diff
CRC errors after gateway	ASCII→RTU converter double-byte swapped	Update firmware / set little-endian CRC flag
Frame splits across two TCP packets	Telnet/SSH path inserts CR only	Require full CR LF; enable binary mode

⚠ Ghost colon Noise flips a line from 0→1 that equals ASCII ‘:’ (0x3A). Install EMI ferrite, verify shield ground.

Key Takeaways

Modbus ASCII is text-mode RTU: same 6-byte PDU, human-visible hex envelope.
Start : + End CR LF provide timing-agnostic framing perfect for modems & radios.
LRC = –Σ(bytes) (two’s complement); send as ASCII high-low nibbles.
ASCII costs ~2× bandwidth; mitigate with higher baud or migrate to RTU/TCP.
Gateways make ASCII ↔ RTU conversion painless but watch latency & endian flags.

Assets to produce before publication

ID	Visual / Asset	Purpose
Fig-6-1	Colour-coded ASCII frame	Teach field order
Fig-6-2	RealTerm TX/RX capture	Practical reference
Fig-6-3	LRC calc flowchart	Reinforce algorithm
Fig-6-4	ASCII vs RTU efficiency graph	Decision making

Coming Up Next

With both serial variants under your belt, Module 3 catapults us onto Ethernet: Chapter 7 – Introduction to Modbus TCP/IP & the MBAP Header. You’ll learn how the same PDU sails through IP networks, what Transaction-ID means, and how Unit-IDs let one gateway feed 50 serial loops. Keep Wireshark open.