125kHz RFID Chips Explained: EM4200 vs T5577 vs TK4100

If you’ve been sourcing 125kHz RFID cards, you’ve probably seen EM4200, TK4100, T5577, and HID Prox all listed under the same “125kHz” heading and wondered what actually separates them. The short answer: two of these chips are permanently locked to one ID, one can be rewritten thousands of times, and only one of the four is a proprietary format the others aren’t fully compatible with. Here’s what each chip actually does and which one fits your project.

What “125kHz” Actually Means

125kHz is a low-frequency (LF) passive RFID band. Cards in this band don’t carry a battery — a nearby reader’s field powers the chip just long enough to transmit its ID. Typical read range is 2–10cm depending on the reader, transmission speed is a modest 106 kbit/s, and none of the chips below support anti-collision (only one card can be read at a time). That’s a feature, not a limitation, for the use cases this band dominates: door access badges, time-and-attendance clocking, membership cards, and simple asset ID tags where a slow, short-range, tamper-resistant read is exactly what you want.

EM4200 vs TK4100 vs T5577: At a Glance

The numbers above come from the current spec sheets for ZealTag’s EM4200, TK4100, and T5577 cards — all three ship in CR80 credit-card format, roughly 0.84mm thick, in PVC/PET/ABS with the usual glossy/matte/frosted finishes and custom printing.

EM4200 and TK4100: The Read-Only Options

EM4200 and TK4100 solve the same problem the same way: a 64-bit UID is burned in at the factory, the card can never be rewritten, and any EM4100-format reader will accept it. From a buyer’s standpoint there’s no functional difference between them — both are read-only, both speak the EM4100 protocol, and both hold ≥10 years of data retention with no encryption or anti-collision. The distinction is really about sourcing: EM4200 is the original EM Microelectronic chip, TK4100 is a widely used second-source equivalent, and manufacturers stock whichever is more cost-effective at the time.

That makes either one the right call when you just need a permanent, tamper-proof ID and nothing more — staff badges, membership cards, or asset tags where you issue once and never touch the data again. There’s no security benefit to picking one over the other; if your priority is resistance to casual copying, read-only status matters far less than moving to an encrypted 13.56MHz chip like MIFARE, since 125kHz UIDs transmit in the clear and a $15 duplicator can capture them either way.

T5577: The Rewritable Option

T5577 is the odd one out. Instead of a fixed UID, it carries 330 bits of EEPROM you can write and rewrite up to 100,000 times, with optional password protection to lock data against casual overwrites. That flexibility is what makes it the standard blank for two very different jobs: system integrators who need one card SKU that can be encoded to emulate EM4100 or TK4100 data on demand, and anyone replacing a lost or damaged 125kHz card who needs a blank that will take whatever format the original used. It’s also the chip referenced as the standard 125kHz blank in our guide to copying an RFID card, for exactly this reason — a budget handheld duplicator reads the original card’s static data stream and writes it straight onto a T5577.

The tradeoff is that a rewritable chip costs more per unit than a locked one, and if your deployment never needs to change a card’s data after issuance, that flexibility is paying for a feature you won’t use.

Where Does HID Prox Fit In?

HID Prox (HID ProxCard/ProxKey) is a separate proprietary 125kHz format from HID Global, not an open standard like EM4100. It shows up in the same searches as EM4200 and T5577 because it operates on the same frequency and readers can sometimes be configured to accept more than one format — but “same band” doesn’t mean “interchangeable.” T5577’s spec lists HID compatibility as partial: it can be encoded to work with some HID-format readers, but whether a specific reader accepts it depends on that reader’s configuration and which HID variant it’s checking for. EM4200 and TK4100 don’t claim HID compatibility at all — they’re EM-format only.

If you’re replacing or supplementing an existing HID Prox deployment, don’t order in bulk on the assumption that any 125kHz card will work. Test a small T5577 sample against your actual readers first, or confirm the format with whoever manages your access control system.

How to Choose the Right 125kHz Chip

• New system, fixed IDs, lowest cost per card: EM4200 or TK4100 — pick whichever your supplier has in stock, since they’re functionally identical.
• Replacing lost cards or standardizing one blank across multiple legacy formats: T5577, so the same SKU covers EM4100, TK4100, and (with testing) some HID-format readers.
• Migrating away from an existing HID Prox system: confirm reader-level compatibility with a small T5577 sample before committing to a bulk order — don’t assume compatibility from the datasheet alone.
• Security is the actual concern, not chip choice: none of these three resist cloning — that requires moving to an encrypted 13.56MHz chip such as MIFARE DESFire, not a different 125kHz option.

All three chips are available as blank or custom-printed cards, fobs, and tags from ZealTag’s 125kHz RFID card range. Send us your format, quantity, and reader details and we’ll confirm the right chip before you order.

125kHz RFID Chip FAQs

Is EM4200 the same as TK4100?

Functionally, yes. Both are read-only 64-bit UID chips compatible with the EM4100 protocol, with the same read range and no encryption. The difference is chip origin (EM4200 is the original EM Microelectronic part, TK4100 a common second-source equivalent), not behavior at the reader.

Can a T5577 card replace an EM4200 card?

Yes — T5577 supports the EM4100 format, so it can be encoded to present the same data an EM4200 card would. It costs more per unit, but one T5577 blank can stand in for an EM4200, a TK4100, or be reprogrammed later if the format needs to change.

Does T5577 work with HID Prox readers?

Partially, and it depends on the specific reader configuration — T5577’s own spec lists HID compatibility as partial, not universal. Test a sample against your actual reader before ordering in bulk. EM4200 and TK4100 don’t support HID formats at all.

Are 125kHz cards secure against cloning?

No. EM4200, TK4100, and T5577 all transmit their data with no encryption, so a budget handheld duplicator can read and copy any of them in under two minutes. If cloning resistance matters, the fix is switching frequency and chip family — to an encrypted 13.56MHz chip like MIFARE DESFire — not choosing a different 125kHz option.