STEMMA (classic) i2c connector
The older, larger JST-PH sibling of STEMMA QT. The 4-pin JST-PH carries i2c; the 3-pin version does not.
Pinout
STEMMA (classic) pinout
STEMMA (classic) runs a 4-pin JST-PH at 2 mm pitch, wired SCL → SDA → V+ → GND looking into the socket. The pin order is the part that actually matters: a connector that physically fits but is wired differently will land SDA where SCL should be, and the bus simply goes quiet. Compare it against the others in the pinout visualizer before you trust a cable that merely fits.
Specs
STEMMA (classic) at a glance
| Vendor | Adafruit |
|---|---|
| Connector | JST-PH |
| Pins | 4 |
| Pitch | 2 mm |
| Logic voltage | 3.3 V or 5 V |
| Chainable | yes |
| Interrupt pin | no |
| Official docs | Adafruit's STEMMA (classic) documentation ↗ |
Watch out: A 3-pin STEMMA plug is a single signal line, not i2c — only the 4-pin JST-PH variant is a bus.
Voltage & chaining
Living with STEMMA (classic) on a real bus
STEMMA (classic) is 3.3 V or 5 V. The connector tolerates both rails — but the boards hanging off it may not, so check each part's own range rather than trusting the ecosystem. The level-shifting guide covers the mixed-rail case.
Chaining is the point of STEMMA (classic): two connectors per board, so you hop from one to the next and they all share SDA and SCL. Two things end the party — two boards answering the same address (see address conflicts for the fixes), and the pull-ups. Every board adds its own pull-up resistors in parallel with the rest, so a long chain drives the combined resistance down toward the point where the bus can't pull low cleanly.
Cable length is the other quiet limit — i2c budgets 400 pF of total bus capacitance, and every extra centimetre spends it. Inside an enclosure you'll never notice; if you're reaching across a room, read how far i2c actually goes first. If a chain stops responding, an i2c scan tells you whether it's a wiring or an addressing problem.
Compatibility
Connecting STEMMA (classic) to other standards
Every pairing below is pre-answered — whether it's a direct cable, an adapter, or a level shifter first. Or skip to the compatibility checker and pick both ends.
Questions
STEMMA (classic) FAQ
What connector does STEMMA (classic) use?
A 4-pin JST-PH at 2 mm pitch, wired SCL / SDA / V+ / GND. The pin order matters as much as the plug — a connector that fits but is wired differently will swap SDA and SCL.
What voltage does STEMMA (classic) run at?
3.3 V or 5 V. That tolerance is why it survives mixed benches, but the boards on the chain may not be — check each board's own range before mixing.
Can you daisy-chain STEMMA (classic)?
Yes — STEMMA (classic) boards carry two connectors, so you chain one to the next and every device shares the same SDA/SCL. Two limits bite eventually: each board must have a unique address, and every board adds pull-up resistors in parallel plus cable capacitance.
Does STEMMA (classic) have an interrupt pin?
No. STEMMA (classic) carries only power, ground, SDA and SCL. A sensor with an interrupt output needs that pin wired separately to a spare GPIO — i2c itself has no way to signal "data ready" on the bus.
What should I watch out for with STEMMA (classic)?
A 3-pin STEMMA plug is a single signal line, not i2c — only the 4-pin JST-PH variant is a bus.