Copper Ethernet & DAC in SFP/QSFP ports

Not everything in an SFP or QSFP cage is optical. Copper RJ45 modules and direct-attach cables occupy the same ports, carry the same identity memory, and cause a disproportionate share of "why won't it link" tickets.

RJ45 modules

Module	Standard	Reach	Power	Notes
1000BASE-T SFP	802.3ab	100 m Cat5e	~1 W	host talks SGMII or 1000BASE-X to the module's PHY
10GBASE-T SFP+	802.3an	~30 m Cat6a (module-limited)	2.5–4+ W	hot, high latency; many hosts limit ports or reject
2.5G/5G-T SFP	802.3bz	100 m	~1.5–2 W	multi-gig for access

10GBASE-T SFP+ is the notorious one: its PHY needs far more power than the SFP+ class allows, so reach is cut to about 30 m, the module runs hot in dense ports, and operators widely recommend avoiding it in favour of SR optics or DAC (see DAC vs AOC vs transceivers).

A 1000BASE-T SFP has a hidden compatibility axis: the host-side interface it expects (SGMII with auto-negotiation vs plain 1000BASE-X). A module coded for one will not link on a port configured for the other.

Direct-attach copper (DAC)

A DAC is a twinax cable with a module end permanently attached. There is no laser — just the connector, an identity EEPROM and, on active cables, signal-conditioning ICs.

Type	Rate	Typical reach	Power
Passive DAC	10G SFP+	up to 5–7 m	none
Passive DAC	25G SFP28	up to 3–5 m	none
Passive DAC	100G QSFP28	up to 3–5 m	none
Passive DAC	400G QSFP-DD	up to ~2–3 m	none
Active DAC (ACC)	25G–400G	up to ~5–10 m	0.5–1.5 W
AOC	10G–800G	up to 100 m	as a transceiver pair

Passive DACs get shorter as rates rise because the host must drive the whole cable; 25G and above require auto-negotiation and link training (see FEC, auto-negotiation & link training), which is why a DAC often fails where an optic works on the same port.

What the identity memory says about a cable

A DAC's EEPROM follows SFF-8472 / SFF-8636 like any module, with cable-specific fields:

Passive / active cable flags in the transceiver compliance bytes.
Cable length in the length fields (metres of copper rather than km of fibre).
Cable specification compliance (SFF-8431 passive/active limiting) in place of

wavelength — a DAC has no wavelength, and hosts check this byte.

Vendor, part number, serial — subject to the same vendor lock

as optics.

Common problems

Wrong host expectation — SGMII vs 1000BASE-X on copper SFPs.
Unsupported cable — host validates the identity; passive DAC from an unknown

vendor is rejected or held down.

Length beyond the host's drive — a 5 m passive DAC on a port rated for 3 m.
Overheating 10GBASE-T — port errors that come and go with temperature.

In CodingBox

CodingBox reads and codes DAC and copper-module identity like any transceiver: the compliance bytes, length fields, cable-compliance byte and vendor identity are all visible and editable in the EEPROM editor, with checksums recalculated. That is how a DAC is made to present the identity a strict port expects.