Why do all fragment registers read zero after provisioning?

Many DER devices clear or mask the fragment payload registers after a successful certificate commit. A read of zero on frg1 through frg80 after a completed write sequence is normal. Confirm the operation succeeded by checking certuid, role, and seq rather than the raw fragment data.

The gateway connects but certuid always returns zero. What now?

The device may not yet have Model 9 active in its Modbus register map. Confirm the Secure SunSpec Modbus option is enabled in the device communication settings. Also verify the Modbus unit ID is correct; a mismatch causes all registers to return zero or no response.

What happens if I write fragment registers in the wrong sequence?

Writing fragment registers out of order or with an incorrect seq value will typically cause the device to reject the entire certificate transfer and may leave the certificate store unchanged. Always write seq before the fragment registers and set totln and frgln accurately. Re-start the transfer from fragment 1 if an error occurs.

Is it safe to write the RW registers on a live production device?

Writing to certuid, certrole, fmt, typ, totln, frgln, the fragment registers, uid, role, or alg pushes a new certificate to the DER device's secure store. An incorrect certificate or role value can lock out legitimate operator access or invalidate the device's authorization state. Always test certificate provisioning on a non-production device first and keep a backup of the previous certificate.

Free Modbus template: SunSpec Model 9 — Set Operator Security Certificate

Name: SunSpec Model 9 — Set Operator Security Certificate
Brand: SunSpec
SKU: c0480786-7752-417e-9a51-854bbda4ef20

SunSpec Model 9 is a protocol-level model defined by the SunSpec Alliance for provisioning operator security certificates over Modbus TCP. It is used by DER devices, such as inverters and energy storage systems, that implement the Secure SunSpec Modbus specification. The template exposes the certificate identifier, role, format, type, total and fragment lengths, 80 fragment payload registers, a write timestamp, sequence number, operator user ID, role, cryptographic algorithm, and fragment count. It allows a Modbus client to deliver X.509v3 certificates in-band, enabling role-based access control on the DER device.

Registers (93)

93 read / 91 write

Name	Address	Function	Type	Access
id SunSpec model identifier, fixed value 9 for this model.	0	FC03	uint16	R
l Length of the model block in 16-bit registers.	1	FC03	uint16	R
certuid Unique identifier of the certificate being transferred.	2	FC03	uint16	R/W
certrole Role encoded in the certificate, maps to an access role.	3	FC03	uint16	R/W
fmt Certificate encoding format, e.g. DER or PEM.	4	FC03	uint16	R/W
typ Certificate type, e.g. X.509v3 client or CA certificate.	5	FC03	uint16	R/W
totln Total byte length of the complete certificate payload.	6	FC03	uint16	R/W
frgln Byte length of the current certificate fragment being written.	7	FC03	uint16	R/W
frg1 Certificate payload fragment 1, one 16-bit word.	8	FC03	uint16	R/W
frg2 Certificate payload fragment 2, one 16-bit word.	9	FC03	uint16	R/W
frg3 Certificate payload fragment 3, one 16-bit word.	10	FC03	uint16	R/W
frg4 Certificate payload fragment 4, one 16-bit word.	11	FC03	uint16	R/W
frg5 Certificate payload fragment 5, one 16-bit word.	12	FC03	uint16	R/W
frg6 Certificate payload fragment 6, one 16-bit word.	13	FC03	uint16	R/W
frg7 Certificate payload fragment 7, one 16-bit word.	14	FC03	uint16	R/W
frg8 Certificate payload fragment 8, one 16-bit word.	15	FC03	uint16	R/W
frg9 Certificate payload fragment 9, one 16-bit word.	16	FC03	uint16	R/W
frg10 Certificate payload fragment 10, one 16-bit word.	17	FC03	uint16	R/W
frg11 Certificate payload fragment 11, one 16-bit word.	18	FC03	uint16	R/W
frg12 Certificate payload fragment 12, one 16-bit word.	19	FC03	uint16	R/W
frg13 Certificate payload fragment 13, one 16-bit word.	20	FC03	uint16	R/W
frg14 Certificate payload fragment 14, one 16-bit word.	21	FC03	uint16	R/W
frg15 Certificate payload fragment 15, one 16-bit word.	22	FC03	uint16	R/W
frg16 Certificate payload fragment 16, one 16-bit word.	23	FC03	uint16	R/W
frg17 Certificate payload fragment 17, one 16-bit word.	24	FC03	uint16	R/W
frg18 Certificate payload fragment 18, one 16-bit word.	25	FC03	uint16	R/W
frg19 Certificate payload fragment 19, one 16-bit word.	26	FC03	uint16	R/W
frg20 Certificate payload fragment 20, one 16-bit word.	27	FC03	uint16	R/W
frg21 Certificate payload fragment 21, one 16-bit word.	28	FC03	uint16	R/W
frg22 Certificate payload fragment 22, one 16-bit word.	29	FC03	uint16	R/W
frg23 Certificate payload fragment 23, one 16-bit word.	30	FC03	uint16	R/W
frg24 Certificate payload fragment 24, one 16-bit word.	31	FC03	uint16	R/W
frg25 Certificate payload fragment 25, one 16-bit word.	32	FC03	uint16	R/W
frg26 Certificate payload fragment 26, one 16-bit word.	33	FC03	uint16	R/W
frg27 Certificate payload fragment 27, one 16-bit word.	34	FC03	uint16	R/W
frg28 Certificate payload fragment 28, one 16-bit word.	35	FC03	uint16	R/W
frg29 Certificate payload fragment 29, one 16-bit word.	36	FC03	uint16	R/W
frg30 Certificate payload fragment 30, one 16-bit word.	37	FC03	uint16	R/W
frg31 Certificate payload fragment 31, one 16-bit word.	38	FC03	uint16	R/W
frg32 Certificate payload fragment 32, one 16-bit word.	39	FC03	uint16	R/W
frg33 Certificate payload fragment 33, one 16-bit word.	40	FC03	uint16	R/W
frg34 Certificate payload fragment 34, one 16-bit word.	41	FC03	uint16	R/W
frg35 Certificate payload fragment 35, one 16-bit word.	42	FC03	uint16	R/W
frg36 Certificate payload fragment 36, one 16-bit word.	43	FC03	uint16	R/W
frg37 Certificate payload fragment 37, one 16-bit word.	44	FC03	uint16	R/W
frg38 Certificate payload fragment 38, one 16-bit word.	45	FC03	uint16	R/W
frg39 Certificate payload fragment 39, one 16-bit word.	46	FC03	uint16	R/W
frg40 Certificate payload fragment 40, one 16-bit word.	47	FC03	uint16	R/W
frg41 Certificate payload fragment 41, one 16-bit word.	48	FC03	uint16	R/W
frg42 Certificate payload fragment 42, one 16-bit word.	49	FC03	uint16	R/W
frg43 Certificate payload fragment 43, one 16-bit word.	50	FC03	uint16	R/W
frg44 Certificate payload fragment 44, one 16-bit word.	51	FC03	uint16	R/W
frg45 Certificate payload fragment 45, one 16-bit word.	52	FC03	uint16	R/W
frg46 Certificate payload fragment 46, one 16-bit word.	53	FC03	uint16	R/W
frg47 Certificate payload fragment 47, one 16-bit word.	54	FC03	uint16	R/W
frg48 Certificate payload fragment 48, one 16-bit word.	55	FC03	uint16	R/W
frg49 Certificate payload fragment 49, one 16-bit word.	56	FC03	uint16	R/W
frg50 Certificate payload fragment 50, one 16-bit word.	57	FC03	uint16	R/W
frg51 Certificate payload fragment 51, one 16-bit word.	58	FC03	uint16	R/W
frg52 Certificate payload fragment 52, one 16-bit word.	59	FC03	uint16	R/W
frg53 Certificate payload fragment 53, one 16-bit word.	60	FC03	uint16	R/W
frg54 Certificate payload fragment 54, one 16-bit word.	61	FC03	uint16	R/W
frg55 Certificate payload fragment 55, one 16-bit word.	62	FC03	uint16	R/W
frg56 Certificate payload fragment 56, one 16-bit word.	63	FC03	uint16	R/W
frg57 Certificate payload fragment 57, one 16-bit word.	64	FC03	uint16	R/W
frg58 Certificate payload fragment 58, one 16-bit word.	65	FC03	uint16	R/W
frg59 Certificate payload fragment 59, one 16-bit word.	66	FC03	uint16	R/W
frg60 Certificate payload fragment 60, one 16-bit word.	67	FC03	uint16	R/W
frg61 Certificate payload fragment 61, one 16-bit word.	68	FC03	uint16	R/W
frg62 Certificate payload fragment 62, one 16-bit word.	69	FC03	uint16	R/W
frg63 Certificate payload fragment 63, one 16-bit word.	70	FC03	uint16	R/W
frg64 Certificate payload fragment 64, one 16-bit word.	71	FC03	uint16	R/W
frg65 Certificate payload fragment 65, one 16-bit word.	72	FC03	uint16	R/W
frg66 Certificate payload fragment 66, one 16-bit word.	73	FC03	uint16	R/W
frg67 Certificate payload fragment 67, one 16-bit word.	74	FC03	uint16	R/W
frg68 Certificate payload fragment 68, one 16-bit word.	75	FC03	uint16	R/W
frg69 Certificate payload fragment 69, one 16-bit word.	76	FC03	uint16	R/W
frg70 Certificate payload fragment 70, one 16-bit word.	77	FC03	uint16	R/W
frg71 Certificate payload fragment 71, one 16-bit word.	78	FC03	uint16	R/W
frg72 Certificate payload fragment 72, one 16-bit word.	79	FC03	uint16	R/W
frg73 Certificate payload fragment 73, one 16-bit word.	80	FC03	uint16	R/W
frg74 Certificate payload fragment 74, one 16-bit word.	81	FC03	uint16	R/W
frg75 Certificate payload fragment 75, one 16-bit word.	82	FC03	uint16	R/W
frg78 Certificate payload fragment 78, one 16-bit word.	83	FC03	uint16	R/W
frg79 Certificate payload fragment 79, one 16-bit word.	84	FC03	uint16	R/W
frg80 Certificate payload fragment 80, one 16-bit word.	85	FC03	uint16	R/W
ts Unix timestamp of the certificate write operation, seconds.	86	FC03	uint32	R/W
ms Millisecond sub-second component of the write timestamp.	88	FC03	uint16	R/W
seq Sequence number for fragment ordering during multi-write transfers.	89	FC03	uint16	R/W
uid Operator user identifier associated with this certificate operation.	90	FC03	uint16	R/W
role Operator role being granted or updated by this certificate.	91	FC03	uint16	R/W
alg Cryptographic algorithm used to sign or encode the certificate.	92	FC03	uint16	R/W
n Number of certificate fragments in the complete transfer sequence.	93	FC03	uint16	R/W