---
title: "Problem with rec_size (Sn_RX_RD) in tcp server mode"
url: "https://maker.wiznet.io/forum/15029"
markdown_url: "https://maker.wiznet.io/forum/15029/md"
type: "Forum topic"
category: "Ethernet Chips"
author: "Sergey"
created: "2023-09-22T12:35:39+09:00"
last_activity: "2023-09-26T13:04:17+09:00"
language: "en"
views: 263
replies: 4
source: "WIZnet Makers forum (https://maker.wiznet.io/forum/)"
---

# Problem with rec_size (Sn_RX_RD) in tcp server mode

## Question

Asked by Sergey on 2023-09-22 in Ethernet Chips.

I’ve organized a tcp server to receive (not to transmit) tcp packets from tcp client based on wiz610MJ module. Only Socket0 is employed in my application, RX buf size is set to 2 kbytes. Server works, but a strange error occurs from time to time: size of received data is bigger than 2048 bytes. After 1 week of searching a bug I find out that the problem relates to S0_RX_RD value when it close to a limit (65536). So I send to W6100, for example, S0_RX_RD = 65440 at the end of RECEIVING_DATA section of code, then I get S0_RX_RSR > 20000 and new value of S0_RX_RD = 41976! However, W6100 itself mustn’t change S0_RX_RD value. This problem doesn’t occur every time when S0_RX_RD is close to 65536, just from time to time.

I’ve checked data and wr pulses on parallel bus to make sure that I send correct value of S0_RX_RD to W6100, also I’ve checked the value of S0_RX_RSR on the parallel bus.
The problem is confirmed by Wireshark utility: sometimes server sends to client packets where window length is bigger than 2048 bytes.

## Replies

### Reply 1 by Eugeny, 2023-09-24

Read RX_RD, RX_WR and RX_RSR every time you:

- write RX_RD,

- is about to perform RECV

- just after RECV

- before getting data out of the buffer.

This info may help in troubleshooting.

### Reply 2 by Sergey, 2023-09-25

Now I see that the problem is with reading S0_RX_RD value. I write it correctly to W6100.
But the problem sometimes occurs with S0_RX_RD reading from W6100. And this is very specific error: second byte of S0_RX_RD value becomes high byte, and low byte becomes 0x00. So I read 5 S0_RX_RD 5 times in a row (address = 0x0228, BS = 0x08) and, for example, get 65464 (= 0xFFB8), 65464, 47104 (=0xB800), 47104, 47104. It means that sometimes I get correct value and sometimes I get shifted value (<<8).
I read S0_RX_RD values before adding rec_size to it.

I check reading process and see that W6100 gives wrong values (in case of correct value
0xFFB8 data[5] gets ‘1’ and ‘0’ values, in case of 0xB800 data[5] gets ‘0’ and ‘0’).

### Reply 3 by Eugeny, 2023-09-25 (in reply to reply 2)

How do you control parallel bus - in particular can you please show **exact** read/write cycle timing involving all control signals and data latch/set timings?

### Reply 4 by Sergey, 2023-09-26

During meticulous checking of read transaction I noticed that interference sometimes influences RD jump-wire (I use jump-wires to connect breadboard with installed wiz610mj and evaluation board with host processor).

So, it seems that on a single board everything must be OK.

Eugeny, thanks for assistance.

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Source: https://maker.wiznet.io/forum/15029