---
title: "W5500 multiple spi in one Cpu server"
url: "https://maker.wiznet.io/forum/15678"
markdown_url: "https://maker.wiznet.io/forum/15678/md"
type: "Forum topic"
category: "Ethernet Chips"
author: "bahmanroshan"
created: "2025-04-07T17:37:49+09:00"
last_activity: "2025-04-15T11:14:59+09:00"
language: "en"
views: 10
replies: 1
source: "WIZnet Makers forum (https://maker.wiznet.io/forum/)"
---

# W5500 multiple spi in one Cpu server

## Question

Asked by bahmanroshan on 2025-04-07 in Ethernet Chips.

We Have 3 cards 1 . server 1 SET Including Stm32h743iit6 with 2 SEPERATED spi connected to W5500 SPI1,2 Socket 1 Port 5000 Ip :192.168.1.100 Socket 1 Port 5001 Ip :192.168.2.100 ####### 2 .CLIEINT 2 SET client.1 , client.2 EACH ONE HAS STM32F103RCT6 CONNECTED TO SPI w5500 Socket 1Port 5000 Ip 192 168 1 21 Socket 1 Port 5001 Ip 192 168 2 1 When each one of client connected the other one can not connected to the server . For example client 1 connected successful .so now client2 can not connected to the server . I try it to change the board of client 2 to pc by similar ip Port and all of thing . The problem is solved . Without any problem . So i think the program of server is correct What is the problem in client side which change the board to pc the problem would be solved ?

## Replies

### Reply 1 by TheoIm, 2025-04-15

Hi,
the issue you're describing is very likely caused by **shared global variables inside the MCU** that are being used to manage socket state across **multiple W5500 chips**.

### The Core Issue: Shared Socket Variables

In the `iolibrary/Ethernet/socket.c` source file, several static global variables are used to manage the internal state of sockets:

```c
static uint16_t sock_io_mode = 0;
static uint16_t sock_is_sending = 0;
static datasize_t sock_remained_size[_WIZCHIP_SOCK_NUM_] = {0, 0, ...};
static uint8_t  sock_pack_info[_WIZCHIP_SOCK_NUM_] = {0, 0, ...};
```

These variables are indexed by socket number (`Sn`) and are **not chip-aware** — they assume that only **one WIZnet chip is in use**.

### Why It Fails

If both W5500 chips are accessed by the same MCU, and both clients (client.1 and client.2) use **socket 0**, then both will **refer to the same index in these shared variables** (e.g., `sock_remained_size[0]`, `sock_is_sending & (1 << 0)`).

This results in **state corruption or collision**, where the second client cannot properly manage or open its socket because it's unknowingly sharing internal state with the first client.

This explains why:

Replacing client.2 with a PC (which does **not** rely on MCU-side shared memory) eliminates the issue.

The server code remains unaffected, confirming the root cause is on the client firmware side.

**Conclusion**

When two W5500 chips share the same MCU, **reusing socket numbers across chips leads to conflicts due to shared internal state** in the ioLibrary’s global variables.
This prevents the second client from successfully opening or maintaining a socket connection.

To avoid this, you must **separate socket state tracking per chip** — either by modifying the ioLibrary, isolating chip access logic, or enforcing non-overlapping socket numbers with expanded buffer space.

Thanks

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Source: https://maker.wiznet.io/forum/15678