SFF-8472 A2h register map

The diagnostics device at two-wire address A2h holds everything that changes while an SFP-class module runs: alarm and warning thresholds, calibration constants, live monitors, status and control bits, latched flags, user EEPROM and the vendor pages. This page lists the lower 128 bytes byte by byte and the upper-page layout, compiled against SFF-8472 Rev 12.4. Narrative: SFF-8472, Thresholds & alarms, Calibration.

Legend: rsvd = reserved · opt = optional, implemented only if the corresponding A0h option bit is set · all 16-bit values are big-endian (lower byte address = MSB).

Monitor formats (used by thresholds, monitors and constants)

Monitor	Format	LSB	Range
Temperature	signed 16-bit two's complement	1/256 °C	−128 … +128 °C
Vcc	unsigned 16-bit	100 µV	0 … 6.55 V
Tx bias	unsigned 16-bit	2 µA	0 … 131 mA
Tx power / Rx power	unsigned 16-bit	0.1 µW	0 … 6.55 mW (−40 … +8.2 dBm)
Laser temperature (opt)	signed 16-bit	1/256 °C	as temperature
TEC current (opt)	signed 16-bit	0.1 mA	±3.2 A; positive = heating

Alarm and warning thresholds — bytes 0–55

Bytes	Field	Bytes	Field
0–1	Temperature high alarm	2–3	Temperature low alarm
4–5	Temperature high warning	6–7	Temperature low warning
8–9	Vcc high alarm	10–11	Vcc low alarm
12–13	Vcc high warning	14–15	Vcc low warning
16–17	Tx bias high alarm	18–19	Tx bias low alarm
20–21	Tx bias high warning	22–23	Tx bias low warning
24–25	Tx power high alarm	26–27	Tx power low alarm
28–29	Tx power high warning	30–31	Tx power low warning
32–33	Rx power high alarm	34–35	Rx power low alarm
36–37	Rx power high warning	38–39	Rx power low warning
40–41	Laser temp / wavelength high alarm (opt)	42–43	… low alarm (opt)
44–45	… high warning (opt)	46–47	… low warning (opt)
48–49	TEC current high alarm (opt)	50–51	TEC current low alarm (opt)
52–53	TEC current high warning (opt)	54–55	TEC current low warning (opt)

Thresholds use the same format as the monitor they bound; on externally calibrated modules they are in raw units, to be converted with the constants below.

External calibration constants — bytes 56–91

Present and meaningful only when A0h byte 92 bit 4 (externally calibrated) is set.

Bytes	Constant	Format	Used as
56–59	Rx_PWR(4)	IEEE-754 single float	Rx_power = Σ Rx_PWR(i) × raw^i, i = 4…0
60–63	Rx_PWR(3)	float	(typically 0)
64–67	Rx_PWR(2)	float	(typically 0)
68–71	Rx_PWR(1)	float	slope (typically 1.0 for internal-like behaviour)
72–75	Rx_PWR(0)	float	offset
76–77	Tx_I(slope)	unsigned 16-bit, LSB 1/256	Tx_bias = slope × raw + offset
78–79	Tx_I(offset)	signed 16-bit	—
80–81	Tx_PWR(slope)	unsigned 16-bit, LSB 1/256	Tx_power = slope × raw + offset
82–83	Tx_PWR(offset)	signed 16-bit	—
84–85	T(slope)	unsigned 16-bit, LSB 1/256	Temperature = slope × raw + offset
86–87	T(offset)	signed 16-bit	—
88–89	V(slope)	unsigned 16-bit, LSB 1/256	Vcc = slope × raw + offset
90–91	V(offset)	signed 16-bit	—
92–94	rsvd	—	—
95	CC_DMI	(Σ bytes 0–94) mod 256	Checksums

Real-time monitors — bytes 96–109

Bytes	Monitor	Notes
96–97	Temperature	module internal
98–99	Vcc	supply inside the module
100–101	Tx bias	laser bias current
102–103	Tx power	from the monitor photodiode
104–105	Rx power	average (A0h 92.3 = 1) or OMA
106–107	Laser temperature or wavelength (opt)	temperature format, or wavelength in 0.05 nm units for tunable modules per SFF-8690
108–109	TEC current (opt)	signed, 0.1 mA

Status and control — bytes 110–127

Byte	Field	Bits
110	Status/control	7 TX_DISABLE state (pin or soft) · 6 soft TX_DISABLE (R/W, if A0h 93.6) · 5 RS1 state · 4 RS0 / rate-select state · 3 soft RS0 select (R/W, if 93.3) · 2 TX_FAULT state · 1 RX_LOS state · 0 data not ready (1 while monitors initialise)
111	rsvd	reserved for SFF-8079
112	Alarm flags	7 temp high · 6 temp low · 5 Vcc high · 4 Vcc low · 3 Tx bias high · 2 Tx bias low · 1 Tx power high · 0 Tx power low
113	Alarm flags	7 Rx power high · 6 Rx power low · 5 laser temp/wavelength high (opt) · 4 … low (opt) · 3 TEC current high (opt) · 2 TEC current low (opt) · 1–0 rsvd
114	Tx input EQ / Rx output emphasis control (opt)	bits 7–4 Tx input equalisation level, 3–0 Rx output emphasis level (active cables, SFF-8431); else rsvd
115	rsvd	—
116	Warning flags	same layout as byte 112 (temp, Vcc, Tx bias, Tx power high/low)
117	Warning flags	same layout as byte 113 (Rx power, laser temp, TEC current)
118	Extended status/control	7–4 rsvd · 3 soft RS1 select (R/W) · 2 power level operation state (1 = level 2/3 active) · 1 power level select (write 1 to request level 2) · 0 power level 3 select rev 12.3
119–122	Vendor specific	often password change area on controller-based modules
123–126	Vendor specific	often password entry area (e.g. `00 00 10 11` defaults on some controllers) — Write-protection types
127	Page select	selects the upper page (128–255) when A0h byte 64 bit 4 is set; 00h default

Flags in 112–117 are latched: they stay set until read, and the host clears them by reading. Alarm and warning flags are implemented only when A0h byte 93 bit 7 is set.

Upper pages — bytes 128–255 via byte 127

Page	Bytes	Contents
00h / 01h	128–247	User EEPROM — 120 bytes free for the host or vendor (page 01h is an alias of 00h in most implementations)
00h / 01h	248–255	Vendor control — vendor-specific control functions
02h	128–255	Control functions — SFF-8690 tunable registers: 128–129 dither control · 130–131 tuning capability flags · 132–133 first frequency THz · 134–135 first frequency GHz ×0.1 · 136–137 last frequency THz · 138–139 … GHz · 140–141 grid spacing GHz ×0.1 · 144–145 channel number set · 146–147 wavelength set ×0.05 nm · 152 Tx dither/wavelength control · 168–169 frequency error GHz ×0.1 · 170–171 wavelength error nm ×0.005 · 172 tuning status (bits: Tx tuning in progress, wavelength unlocked, tuning error) · 173–175 latched status/flags
03h–7Fh	—	reserved
80h–FFh	—	vendor specific

Decoding example

A2h bytes 96–105 read 1A 80 80 E8 1B 58 0F A0 03 E8 on an internally calibrated module: temperature 0x1A80 / 256 = 26.5 °C; Vcc 0x80E8 × 100 µV = 3.300 V; bias 0x1B58 × 2 µA = 14.0 mA; Tx 0x0FA0 × 0.1 µW = 400 µW = −3.98 dBm; Rx 0x03E8 × 0.1 µW = 100 µW = −10.0 dBm. Bytes 110 = 00 → Tx enabled, no fault, no LOS, data ready; 112–117 all 00 → no flags. Step-by-step with tools: Reading DDM with tools.

SFF-8472 A0h map — identity block.
SFF-8636 lower page 00h — the QSFP equivalent (monitors, flags, controls).
CMIS lower page — the CMIS equivalent.