Why do the address string registers return all nulls?

SunSpec strings are NULL-terminated and NULL-padded. A register block filled with 0x0000 means the field is unimplemented or unconfigured on this device. Confirm the device actually supports IPv6 and that cfgst is not 0 (NOT_CONFIGURED) before reading addr, gw, or dns registers.

The gateway shows a connection but cfgst stays at 0. What is wrong?

A cfgst value of 0 means NOT_CONFIGURED: the device has no valid IPv6 address assigned. Check whether the cfg register is set to DHCP (1) and whether the cap bitfield confirms DHCP capability. If cfg is STATIC (0), a valid addr and cidr must be written first, followed by a ctl apply command.

Writing addr or gw drops the Modbus TCP connection immediately. Is this expected?

Yes. Writing network configuration registers such as addr, gw, cfg, or ctl can cause the device to reconfigure its network interface and temporarily drop all TCP connections, including the active Modbus session. Wait several seconds and reconnect. Do not write these registers during active monitoring cycles without a reconnection strategy in your client.

Is it safe to write cfg, ctl, or addr registers at any time?

Write registers in this model directly affect the device's live network stack. An incorrect addr or cidr value can make the device unreachable on the network. Always verify the intended values before writing, test on one device before rolling out to a site, and ensure a local fallback access method exists in case connectivity is lost.

Free Modbus template: SunSpec Model 13 — IPv6

Name: SunSpec Model 13 — IPv6
Brand: SunSpec
SKU: b47a90c4-b199-499b-b5ca-161baf509f91

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Free Modbus template: SunSpec Model 13 — IPv6 | ModbusCloud

Registers (17)

17 read / 12 write

Name	Address	Function	Type	Access
id SunSpec model identifier, fixed value 13 for IPv6.	•••	•••	•••	R
l Model block length in registers.	•••	•••	•••	R
nam User-assigned interface name, writable string.	•••	•••	•••	R/W
cfgst Configuration status: not configured, valid setting, or valid hardware.	•••	•••	•••	R
chgst Change status bitfield; PENDING bit set when a write is awaiting apply.	•••	•••	•••	R
cap Capability bitfield: DHCP, DNS, NTP, static config, and reset flags.	•••	•••	•••	R
cfg Address configuration mode: static, DHCP, BOOTP, or ZEROCONF.	•••	•••	•••	R/W
ctl Control register for applying or resetting interface configuration changes.	•••	•••	•••	R/W
addr IPv6 address of the interface, writable string.	•••	•••	•••	R/W
cidr IPv6 subnet prefix length in CIDR notation, writable string.	•••	•••	•••	R/W
gw IPv6 default gateway address, writable string.	•••	•••	•••	R/W
dns1 Primary DNS server IPv6 address, writable string.	•••	•••	•••	R/W
dns2 Secondary DNS server IPv6 address, writable string.	•••	•••	•••	R/W
ntp1 Primary NTP server address or hostname, writable string.	•••	•••	•••	R/W
ntp2 Secondary NTP server address or hostname, writable string.	•••	•••	•••	R/W
domnam DNS domain name assigned to this interface, writable string.	•••	•••	•••	R/W
hostnam Hostname assigned to the device on this interface, writable string.	•••	•••	•••	R/W

Loading registers…

SunSpec Model 13 — IPv6

Overview

Use cases

Installation

Things to watch for

Registers (17)

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