When a gigabit Ethernet port on your own board only links at 100 Mbps, the PHY has either been configured for 100 Mbps or could not bring up 1000BASE-T and fell back. 1000BASE-T uses all four twisted pairs, while 100BASE-TX needs only two, so a broken path on pair C or D still allows a 100 Mbps link. Rule out the cable and link partner with known-good gigabit equipment, then check four things on the board: the magnetics are a four-channel gigabit part, not a two-channel 10/100 part; pairs C and D are continuous from PHY to RJ45; center taps and terminations follow the PHY and magnetics datasheets; and the PHY is not strapped or configured to advertise 100 Mbps only.
Packet loss, EMC failures and a hot port follow the same path: MDI traces, LAN transformer, RJ45 and, with PoE, the DC current through the center taps. VOOHU supplies gigabit LAN transformers and RJ45 jacks with integrated magnetics whose datasheets state each test condition.
#Why is my gigabit Ethernet only connecting at 100 Mbps?
Many gigabit PHYs fall back to 100 Mbps on purpose. In the TI DP83867 datasheet (section 7.4.6.5) this is called speed optimization, or link downshift: after several consecutive failed gigabit attempts, four by default, the PHY falls back to 100 Mbps. TI notes this happens when only two twisted pairs are connected, and that in its enhanced mode the PHY can fall back after one failed attempt if it detects no energy on the C and D channels. If your PHY reports a downshift status, read it: it tells you the board tried gigabit and failed, rather than being configured for 100 Mbps.
On the board, check these four causes of a gigabit port that settles at 100 Mbps:
- 1.A 10/100 part in a gigabit footprint. A two-channel 10/100 transformer, or a 10/100 RJ45 jack with integrated magnetics, carries only two pairs, so the port cannot run above 100 Mbps. Check the datasheet title and schematic of the part actually fitted.
- 2.Pair C or D broken. An open solder joint, a pin-mapping error between the PHY, magnetics and RJ45 footprints, or swapped pairs leaves pairs A and B working. PHY cable diagnostics help here: TI's DP83867 lists opens, shorts, bad connectors, termination mismatches and cross faults among the faults its TDR can identify.
- 3.Center tap or termination wrong. A center tap that is left open, tied to the wrong supply, or a missing termination network unbalances the pairs. Wire them as the PHY and magnetics datasheets show; see how to connect a LAN transformer to a PHY.
- 4.Configuration. Strap pins or advertisement registers set to 10/100, or a MAC interface that only supports 100 Mbps, cap the link without any hardware fault.
#Gigabit Ethernet not working as expected: what should you check?
#Which LAN transformer parameters matter on a gigabit port?
Insertion loss, return loss, common-mode rejection (CMR) and crosstalk set how much of the channel's margin the magnetics use. Each limit in a magnetics datasheet comes with a frequency range and test conditions, so read the whole line. For example, the WHSG24301JM gigabit transformer datasheet gives, at 25 °C:
- Insertion loss −1.1 dB max at 1–100 MHz
- Return loss −18 dB min at 1–30 MHz, stepping to −14.4 dB at 40 MHz, −13.1 dB at 50 MHz, −12 dB at 60–80 MHz and −10 dB min at 100 MHz
- CMR −30 dB min and crosstalk −30 dB min at 1–100 MHz
- Hi-Pot 1500 VAC, 1 mA, 60 s
Compare like with like: a limit over 1–100 MHz is not a single-frequency value, and a stepped return-loss limit is read band by band. Our guide to reading a LAN transformer datasheet walks through each line.
Open-circuit inductance (OCL) is often specified with a DC bias current, and the bias differs from part to part. WHSG24702PG lists 350 µH min at 100 kHz, 0.1 V with 14 mA DC bias; WHSG24Z01C0 lists 180 µH min at 100 kHz, 0.1 V with 35 mA DC bias. Values measured at different bias currents are not directly comparable, so compare OCL only at the same test condition.
Isolation is also written in different ways: 1500 VAC, 1 mA, 60 s on WHSG24301JM; 5000 VAC, 1 mA, 2 s on WHSG24701D1; and 2250 V DC, 6 s, 1 mA primary to secondary on the SYT111Q066AA2A1D RJ45 jack. These are different tests, so do not convert one into another. For isolation failures, see why an Ethernet port fails the Hi-Pot test.
#Why does a gigabit port drop packets or fail EMC?
Physical-layer packet loss points to the channel between the PHY and the RJ45. TI's Ethernet PHY PCB Design Layout Checklist (SNLA387) gives these layout points for the MDI side:
- Route MDI pairs at 100 Ω differential, ±10%; an impedance mismatch reduces throughput and can stop communication.
- Keep each MDI trace under 2 inches, length-match within 20 mils for 1G, and keep at least 30 mils between different pairs.
- Keep a continuous ground under the MDI traces and void it only under components on the traces, such as the magnetics, chokes, AC coupling capacitors and ESD diodes; a two-layer void is recommended for standard applications.
- Keep metal out from under discrete magnetics on every layer; metal under an RJ45 jack with integrated magnetics is allowed.
The same checklist notes that the Ethernet-specific guidance places ESD protection on the PHY side of the magnetics, because the connector side can see high common-mode voltage swings. Choose ESD parts whose capacitance and working voltage suit the PHY; see how to choose an ESD protection diode for an Ethernet port.
When radiated emissions are marginal, a signal-line common mode choke adds common-mode suppression. WHLC-2012A-102T0 is rated 1000 Ω typical at 100 MHz and 300 mA max, so use it on lines that do not carry PoE current. Where a choke belongs relative to the transformer is covered in common mode choke placement around a LAN transformer.
#Why does the Ethernet port get hot under PoE?
With PoE, DC current flows through the cable-side center taps of the transformer. That current heats the windings through their DC resistance. For a hot port, check the PoE or DC balance current rating, the DCR, and whether the operating temperature range includes self-heating, then match the rating to the current your port actually carries.
The datasheets of three VOOHU gigabit PoE transformers state the rating this way:
- WHSG24Z01C0: each channel supports 1.5 A DC balance current, 3.0 A per port; operating −40 to +85 °C including temperature rise.
- WHSG24702PG: PoE 60 W, 600 mA DC balance current per channel and 1200 mA for all channels.
- WHSG24701D1: IEEE 802.3af 15 W, 350 mA DC balance current per channel.
For an RJ45 jack with integrated magnetics, the PoE rating is part of the jack. The SYT111Q334AB2A2DP057 datasheet states IEEE 802.3at for PSE or PD applications, while SYT111Q066AA2A1D is a non-PoE gigabit jack.
#Discrete LAN transformer or RJ45 jack with integrated magnetics for a gigabit port?
A discrete transformer with a separate RJ45 lets you choose and place each part; an RJ45 jack with integrated magnetics, often called a MagJack, saves board area and removes the transformer-to-jack routing. These four VOOHU parts cover the common gigabit cases:
To compare more parts, filter 10/100/1000BASE-T LAN transformers, browse gigabit LAN transformers, or filter RJ45 jacks with integrated magnetics. VOOHU also distributes JL2101-series gigabit Ethernet PHYs.
#FAQ
Why is my Ethernet link speed capped at 100 Mbps?
On a PC or switch, check for a damaged pair in the cable, a 100 Mbps device at the other end, or a speed setting. On your own board, also check for 10/100 magnetics, a broken pair C or D, and PHY strap or register settings. See our general Ethernet troubleshooting guide for faults beyond link speed.
Can a 10/100 LAN transformer be used on a gigabit Ethernet port?
Not for gigabit operation. A 10/100 transformer has two channels for the two pairs 100BASE-TX uses, while 1000BASE-T needs all four pairs, so the port will link at 100 Mbps at best. Use a four-channel gigabit transformer or a gigabit RJ45 jack with integrated magnetics.
Does a gigabit LAN transformer need to be PoE-rated?
Only if PoE current flows through its center taps. A PoE port needs magnetics with a stated DC balance current that covers the port's current, for example WHSG24Z01C0 at 1.5 A per channel or WHSG24702PG at 600 mA per channel. A port without PoE can use a non-PoE gigabit transformer such as WHSG24301JM.
Is the problem the LAN transformer or the PHY?
Isolate it. Run the PHY's cable diagnostics if it has them, check continuity of all four pairs through the magnetics with power off, and swap in a known-good magnetics part or test a second board. If the port died after a surge, follow our surge diagnosis steps instead.
Want a second check on your gigabit port?
Send us your gigabit port schematic, the PHY you use and whether the port carries PoE through our sample and support form. Our FAE team can suggest suitable parts and help check the design, from the magnetics channel count and center-tap wiring to the PoE current rating. Samples are available for evaluation, and selected parts are free of charge. Our FAE team will be glad to confirm the options for your project. VOOHU supplies RJ45 and SFP connectors, LAN and PoE transformers and common mode chokes from Suzhou, China, to more than 1,500 customers.
Part of the series: Ethernet port magnetics from 10/100 to 10G
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