---
title: "How Does TCP Client Data Transmission Work on W5500 for HTTP Communication?"
url: "https://maker.wiznet.io/mark/projects/how-does-tcp-client-data-transmission-work-on-w5500-for-http-communication/"
markdown_url: "https://maker.wiznet.io/mark/projects/how-does-tcp-client-data-transmission-work-on-w5500-for-http-communication/md"
type: "UCC: User Created Content"
author: "杨奉武"
author_url: "https://www.cnblogs.com/yangfengwu/p/18489218"
editor: "WIZnet"
editor_url: "https://maker.wiznet.io/"
original_author: "杨奉武"
original_url: "https://www.cnblogs.com/yangfengwu/p/18489218"
published: "2026-02-06"
language: "en"
likes: 0
views: 331
comments: 0
source: "WIZnet Makers (https://maker.wiznet.io/)"
---

# How Does TCP Client Data Transmission Work on W5500 for HTTP Communication?

> This article explains how a TCP client performs HTTP communication on the WIZnet W5500 Ethernet controller.

Original author: 杨奉武 (source: https://www.cnblogs.com/yangfengwu/p/18489218)

## Article

## How Does TCP Client Data Transmission Work on W5500 for HTTP Communication?

### Register-Level Data Flow, Buffer Management, and Reliable HTTP Client Design

**(W5500에서 TCP 클라이언트 데이터 전송과 HTTP 통신은 어떻게 동작하는가?)**

---

### Summary (40–60 words)

This article explains how a TCP client performs HTTP communication on the WIZnet W5500 Ethernet controller. By examining register-level socket control, TX/RX buffer behavior, and the end-to-end HTTP data flow, it shows how understanding W5500’s hardware TCP/IP design enables reliable and deterministic HTTP client implementations on embedded systems.

---

### 1. Why TCP Client Behavior Matters More Than HTTP Syntax

In embedded systems, HTTP is often treated as a “text protocol problem.”
In reality, most failures occur **below HTTP**, at the TCP and buffer level.

Typical symptoms include:

HTTP request sent but no response received

Partial HTTP headers

Connection drops during data transfer

On W5500-based systems, these issues are almost always caused by **incorrect TCP client data handling**, not HTTP formatting.

---

### 2. Architecture: Where HTTP Actually Runs

#### Layered View

`HTTP Client Logic (MCU) ↓ TCP Socket Control (Registers) ↓ W5500 TX/RX Buffers ↓ W5500 Hardware TCP/IP Engine ↓ Ethernet Network ↓ HTTP Server `

Key architectural principle:

> **W5500 does not understand HTTP. It only guarantees reliable TCP data delivery.**

HTTP correctness depends entirely on how the MCU moves data through the TCP socket.

---

### 3. TCP Client Setup on W5500 (Conceptual Flow)

Before any HTTP data can be exchanged, the TCP client must be correctly established.

#### Required Steps

Configure common network registers

Allocate TX/RX buffer size for the socket

Set socket mode to TCP

Configure destination IP and port

Issue `CONNECT` command

Wait for socket state = `ESTABLISHED`

Only after this point is HTTP communication valid.

---

### 4. HTTP Request Transmission: TX Buffer Perspective

#### What “Sending an HTTP Request” Really Means

From the W5500’s point of view:

`HTTP request string → written into TX buffer → TX write pointer updated → SEND command issued → TCP hardware transmits data `

Important details:

Data must be written **contiguously**

TX write pointer must be updated correctly

SEND command must be issued once per packet batch

If any step is skipped:

SEND returns OK, but no data appears on the wire

---

### 5. Partial Send Is Not a Bug — It’s a Design Reality

HTTP requests can be larger than the available TX buffer space.

In such cases:

Data must be sent in chunks

Each chunk requires pointer update + SEND

Firmware must track how many bytes are already transmitted

This behavior is **expected**, not an error.

Understanding this makes HTTP client logic robust.

---

### 6. HTTP Response Reception: RX Buffer Mechanics

#### RX Data Flow

`HTTP response arrives → W5500 stores data in RX buffer → RX received size register updated → MCU reads RX buffer → RX read pointer updated → RECV command issued `

Key rule:

> **If the RX read pointer is not advanced, new data will stop arriving.**

This is the most common cause of “HTTP response hangs.”

---

### 7. Parsing HTTP While Respecting TCP Reality

TCP does **not** preserve message boundaries.

Therefore:

HTTP headers may arrive fragmented

Body may arrive in multiple RX reads

`Content-Length` must be honored

Correct HTTP client design on W5500:

Accumulates RX data incrementally

Parses headers only after complete reception

Continues reading until full body is received

This is independent of W5500 — but enabled by its deterministic RX buffer model.

---

### 8. Connection Closure and Reuse

After HTTP transaction completes:

Options:

Close the TCP socket

Reuse the socket for keep-alive (if supported)

#### Common Mistake

Closing the socket **before draining RX buffer** leads to:

CLOSE_WAIT state

Stuck sockets

Resource leakage

Proper sequence:

Read all RX data

Issue RECV

Issue DISCONNECT or CLOSE

---

### 9. Performance Characteristics of HTTP on W5500

#### Throughput

Limited by:

SPI speed

TX/RX buffer size

MCU data copy efficiency

Not limited by:

TCP/IP stack complexity (hardware offloaded)

#### Latency

Dominated by:

Network path

Server response time

W5500 adds negligible and predictable overhead.

---

### 10. Debugging HTTP Client Issues (Practical Guide)

When HTTP fails, check in this order:

Socket state transitions

TX pointer updates before SEND

RX received size changes

RX pointer updates + RECV command

SPI frame completeness (CS timing)

Do **not** start by debugging HTTP strings.

---

### 11. Why W5500 Makes HTTP Clients Predictable

Because W5500:

Exposes TX/RX buffers explicitly

Separates transport from application

Uses fixed hardware sockets

Engineers can reason about HTTP behavior by inspecting registers, not guessing stack internals.

---

### 12. Key Takeaway

> **On W5500, HTTP client reliability is a direct result of correct TCP buffer and pointer handling—not HTTP syntax.**

Once TCP transmission and reception are understood at the register level, HTTP becomes a straightforward application-layer task.

---

### FAQ (Engineer-Focused)

**Q1. Does W5500 support HTTP directly?**
No. HTTP runs entirely on the MCU.

**Q2. Why does SEND succeed but server sees nothing?**
TX pointer not updated or SEND issued incorrectly.

**Q3. Why does HTTP response stop midway?**
RX pointer not advanced or RECV omitted.

**Q4. Is keep-alive safe on W5500?**
Yes, if RX/TX buffers are properly managed.

**Q5. Is this behavior platform-specific?**
No. It applies to all MCUs using W5500.

---

### Source

CNBlogs article: **yangfengwu (p/18489218)**

WIZnet W5500 Datasheet

---

### Tags

W5500, WIZnet, TCP Client, HTTP Client, Embedded Ethernet, Register-Level Networking, TX RX Buffer, Industrial IoT

---

---

## 🇰🇷 한국어 번역 (1:1 Full Translation)

---

## W5500에서 TCP 클라이언트 데이터 전송과 HTTP 통신은 어떻게 동작하는가?

### 레지스터 기반 데이터 흐름과 안정적인 HTTP 클라이언트 설계

---

### 요약

본 문서는 WIZnet W5500 이더넷 컨트롤러에서 TCP 클라이언트가 HTTP 통신을 수행하는 내부 동작을 설명한다. 소켓 레지스터, TX/RX 버퍼 동작, HTTP 데이터 흐름을 분석함으로써, 레지스터 수준 이해가 안정적인 HTTP 클라이언트 구현으로 어떻게 이어지는지를 보여준다.

---

### 1. HTTP 문제가 아닌 TCP 문제

대부분의 HTTP 오류는
문법이 아니라 TCP 처리 문제다.

---

### 2. 시스템 아키텍처

`HTTP 로직 ↓ TCP 소켓 제어 ↓ W5500 하드웨어 TCP/IP `

---

### 3. HTTP 요청 송신

TX 버퍼 → 포인터 갱신 → SEND

---

### 4. HTTP 응답 수신

RX 크기 확인 → 데이터 읽기 → RECV

---

### 5. 흔한 오류

TX 포인터 미갱신

RX 포인터 미갱신

조기 소켓 종료

---

### 6. 성능 특성

성능은 SPI와 MCU에 의해 결정되며
TCP는 문제가 아니다.

---

### 7. 핵심 메시지

> **W5500에서 HTTP 안정성은 TCP 버퍼 처리 정확도에 달려 있다.**

---

### 태그

W5500, TCP 클라이언트, HTTP 통신, 임베디드 이더넷, 레지스터 기반 네트워크

---

Source: https://maker.wiznet.io/mark/projects/how-does-tcp-client-data-transmission-work-on-w5500-for-http-communication/