---
title: "Issue with the WIZ105SR on RXD from Serial port"
url: "https://maker.wiznet.io/forum/12626"
markdown_url: "https://maker.wiznet.io/forum/12626/md"
type: "Forum topic"
category: "Serial-to-Ethernet & ConfigTool"
author: "jmora"
created: "2017-12-19T06:48:54+09:00"
last_activity: "2017-12-20T08:00:22+09:00"
language: "en"
views: 749
replies: 1
source: "WIZnet Makers forum (https://maker.wiznet.io/forum/)"
---

# Issue with the WIZ105SR on RXD from Serial port

## Question

Asked by jmora on 2017-12-19 in Serial-to-Ethernet & ConfigTool.

We are facing a strange behavior on a WIZ105SR module when transmitting data on the serial port.

We succeed when connecting the WIZ105SR to the PC and transmitting data using a virtual COM on the PC. However, when the MCU in our target board responds, we observe that the voltage at pin 3 of J1 (RXD) is always high (3.3V):

[![system_interface](https://maker.wiznet.io/forum_uploads/wiznet/original/2X/5/558c84ddb9de97e7f03bb79d396682e1780ff682.jpg) system_interface1135×275 35.4 KB](https://maker.wiznet.io/forum_uploads/wiznet/original/2X/5/558c84ddb9de97e7f03bb79d396682e1780ff682.jpg)

Seems like the RXD pin on the GC89L591A0-LQ44I of the WIZ105SR is set as an output and keeps always high level.

[![scope_capture](https://maker.wiznet.io/forum_uploads/wiznet/original/2X/6/6538fd386d0711fec54de2eb94673bb6c844b4c3.jpg) scope_capture807×620 100 KB](https://maker.wiznet.io/forum_uploads/wiznet/original/2X/6/6538fd386d0711fec54de2eb94673bb6c844b4c3.jpg)

Could you please support us on this and check what we are doing wrong on the device?

Thanks in advance.

Regards, Julio

## Replies

### Reply 1 by scott, 2017-12-20

Hi jmora

I suspect transistor circuits.

A typical level shift circuit is shown below.

![image](https://maker.wiznet.io/forum_uploads/wiznet/original/2X/1/12960fa091f1437e4efb7a8a32c2196107ce5982.jpg)

[![image](https://maker.wiznet.io/forum_uploads/wiznet/original/2X/9/92aa986b597f8fdb0fae61d35a2b1c395be22fd7.png) image816×718 8.82 KB](https://maker.wiznet.io/forum_uploads/wiznet/original/2X/9/92aa986b597f8fdb0fae61d35a2b1c395be22fd7.png)

Remove WIZ105SR and check the waveform of CHANNEL 3.

Thank you.

---

Source: https://maker.wiznet.io/forum/12626