---
title: "Can't write to Sn_DHAR-Register"
url: "https://maker.wiznet.io/forum/11399"
markdown_url: "https://maker.wiznet.io/forum/11399/md"
type: "Forum topic"
category: "Ethernet Chips"
author: "JoergH"
created: "2015-10-21T14:33:29+09:00"
last_activity: "2017-05-04T08:37:16+09:00"
language: "en"
views: 703
replies: 1
source: "WIZnet Makers forum (https://maker.wiznet.io/forum/)"
---

# Can't write to Sn_DHAR-Register

## Question

Asked by JoergH on 2015-10-21 in Ethernet Chips.

Hello,
I try to send Data with the W5500-Chip, but according to use SEND_MAC-Command I have to write the Receiver(PC)-MAC to the Sn_DHAR-Register, but this doesn’t work. I always read out the default-value of 0xFF…FF. But: If the PC sends some Date to the W5500, the Sn_DHAR-Register is filled with the MAC of the PC. I wonder, if my cip is broken, or if I have to write a special sequence to another Register so I can write to the Sn_DHAR-Register.
(According to Datasheet Vers. 1.0 from August 2013 Sn_DHAR is read/writeable)
Greetings
Joerg

## Replies

### Reply 1 by lawrence, 2015-10-23

Hello, JoergH

To quote W5500 datasheet, SEND_MAC used with only UDP mode.

If you use W5500 with TCP, it is right to get PC MAC address.

Could you check if it is UDP or TCP ?

Thank you,

Lawrence

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Source: https://maker.wiznet.io/forum/11399