WIZ750SR-TTL looses connection
Serial-to-Ethernet & ConfigTool
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Serial-to-Ethernet & ConfigTool
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RE: WIZ750SR-TTL looses connection
by Grace_Koo WIZnet ·
Hi,
The fact that the problem disappears when you connect a PC with Wireshark is actually a very strong clue — it points to an ARP cache expiration issue rather than a physical layer (auto-negotiation) problem.
Here's what's likely happening:
When only the WIZ750SR and the Safety PLC are connected (with very little traffic between Modbus polls), the ARP entry on the PLC side expires after a certain period. When the next TCP packet needs to be sent, the PLC has to re-resolve the MAC address via ARP. During this re-resolution window, the TCP connection can time out and drop.
When you connect your PC running Wireshark, it generates broadcast traffic (ARP requests, etc.) that keeps the ARP tables on both devices refreshed — so the problem never appears.
To verify and fix this:
1. Check if your Safety PLC supports static ARP entries. If so, manually add the WIZ750SR's IP and MAC address as a static entry. This prevents the ARP cache from expiring.
2. Reduce the keepalive interval on the WIZ750SR to a value shorter than the PLC's ARP timeout (typically 30-60 seconds should work). The keepalive packets will also serve to refresh the ARP cache on both sides.
3. If you're using a managed switch between the two devices, you can also check the switch's ARP/MAC address table aging time and increase it.
I would start with the static ARP entry on the PLC — it's the most reliable fix for this type of issue. Let us know if this resolves your daily disconnects.
RE: RE: WIZ750SR-TTL looses connection
by gsmiderle ·
Thanks for the tips! The PLC does not have something like that. I'm waiting for their response to see if there is something they can do.
The strange thing is that the communication is really fast, I have requests from the Wiznet (it's the master) every 20ms, so the keepalive never starts (right now it is at 20s for the first and then 5 sec).
Does the same thing apply even for the ARP timeout? Aren't all these requests to the PLC enough to sustain the ARP tables? .
UPDATE
I can confirm that with a managed switch, the connection does not drop. The strange thing is that the 2 devices are on a single VLAN, the pc in in a port with port monitoring, so there should be 0 communication between the devices and the pc, yet the connection does not drop. Unfortunately, the connection needs to stay up even with other switches or with none. Any ideas?
RE: WIZ750SR-TTL looses connection
by gsmiderle ·
UPDATE! It seems like the problem is the 750 itself, I tested the communication with an old 7100,A and it works just fine. Do you know if there is any fix for this without changing the cip?
RE: RE: WIZ750SR-TTL looses connection
by Grace_Koo WIZnet ·
(Revised)
Your testing points to a link-layer (Ethernet PHY) interoperability behavior in this specific point-to-point setup — the WIZ750SR and the PLC don't always settle into a stable auto-negotiation when connected directly, whereas a managed switch (which terminates each link independently) resolves it cleanly. The fact that the older 7100 behaves differently here is consistent with a difference in PHY negotiation between the two setups rather than a Modbus or PLC issue. (This also rules out ARP — with 20 ms polling, ARP stays refreshed and keepalive never starts.)
Options without replacing the module:
Recommended order: check firmware version (update to v1.4.5 if behind) → if point-to-point is required, force 10 Mbps fixed and add connection-level auto-recovery in custom firmware → otherwise standardize on a switch with the WIZ port forced.
https://github.com/Wiznet/WIZ750SR
RE: RE: WIZ750SR-TTL looses connection
by Grace_Koo WIZnet ·
One more note: keeping a small managed switch inline isn't just a workaround — since the root cause is at the link/PHY-negotiation layer, the switch genuinely stabilizes it, so it's a valid permanent fix if point-to-point isn't a hard requirement.
And if a firmware update to the latest version doesn't fully resolve the daily drops, please email Grace@wiznet.io with your current firmware version and setup details.