EM4100 vs T5577

The silicon — what is actually on each chip

Both chips are 125 kHz LF passive devices using ASK / Manchester encoded data, and both work with the same reader hardware. The difference is purely in the memory model and the modulator configuration.

• EM4100 (also sold as EM4102, EM4200 in subsequent revisions) — 64-bit total memory. 9 bits of header, 40 bits of unique ID (10 hex characters, often presented as a 5- or 8-digit decimal Wiegand-26 / Wiegand-34 value), 14 row-and-column parity bits, 1 stop bit. The UID is laser-trimmed at the factory and immutable. No challenge-response, no cryptography, no secondary memory blocks.
• T5577 — 330-bit user EEPROM organised as 7 × 32-bit blocks plus a configuration block. Block 0 carries the modulator configuration register (which protocol to emulate, bit rate, encoding scheme). Block 7 is reserved for the password (if password protection is enabled). Blocks 1–6 carry the actual ID data, padded out to whatever the emulated protocol requires (40 bits for EM4100, 26/35/37 bits for HID Prox, 27 bits for Indala, etc.).
• Read range and reader compatibility are identical — both are 125 kHz ASK Manchester at the air interface, both work with all standard LF readers including HID ProxPoint, ProxPro, Indala ASR, EM-Marin readers, ESP8266 / Arduino DIY readers, Proxmark3 and the entire $20–$50 'card duplicator' commodity tier.
• Cost differential at MOQ — EM4100 wafers are simpler silicon (smaller die, no EEPROM) so they cost less. The cost ladder at 50k card MOQ FOB Shenzhen is roughly EM4100 $0.08–0.15, EM4200 (re-revision) $0.10–0.16, T5577 $0.12–0.22, depending on chip-availability cycles and antenna geometry.

Cloning mechanics — how a $25 duplicator copies an EM4100 onto a T5577

The reason T5577 exists commercially is because it is the cheapest way to put an arbitrary 125 kHz credential into a re-writable form factor. The cloning tool reads the source EM4100, decodes the 40-bit UID, configures the T5577 modulator block to emulate EM4100 at the same bit rate, writes the UID into the data blocks and the resulting T5577 card is indistinguishable from the original to any standard reader.

• Step 1 — Tool reads the EM4100 card by energising its antenna with a 125 kHz field and capturing the ASK-modulated Manchester response. Decode time is under 50 ms.
• Step 2 — Tool extracts the 40-bit UID from the response, validates parity.
• Step 3 — Tool writes the T5577 configuration block to emulate EM4100 (modulator = ASK, bit rate = RF/64, encoding = Manchester, data = blocks 1–2).
• Step 4 — Tool writes the 40-bit UID into blocks 1–2 of the T5577 EEPROM.
• Step 5 — Tool reads the cloned T5577 to verify. Total operation time: 2–6 seconds depending on the tool.
• Common commercial tools — Keysy ($35), Handheld Card Copier ($25 on Amazon), Proxmark3 RDV4 ($350 for the pro tier with HID Prox / Indala support too).
• Defensive countermeasure on the reader side — Some HID Prox readers (ICLASS SE, multiCLASS) reject T5577-emulated credentials by detecting subtle timing differences in the ASK response. This is brittle and not present in commodity readers.

Side-by-side specification table

The quick-reference table for the first shortlist conversation.

When EM4100 is the right choice

Read-only is a feature, not a limitation, in several deployment patterns.

• Mass-volume single-use credentials — Animal-ID ear tags, hotel-amenity tokens, gym lockers, parking-lot day passes. The chip is consumed once and never re-issued. The 50–80% chip-cost saving compounds at volumes above 250k units per year.
• Legacy reader fleets where re-encoding never happens — A 1990s-era HID ProxPoint reader fleet on a single-tenant building has no operational need for in-field credential re-write. EM4100 (or HID Prox on a non-EM4100 chip) suffices.
• Anti-counterfeit assistance — When the deployment relies on a factory-issued UID being audit-trailed to a specific batch / customer, EM4100's immutability is a feature.
• Animal ID compliance — ISO 11784/11785 Full Duplex/Half Duplex 134.2 kHz is a separate chip family (HDX/FDX-B), but for the older ICAR-registered 125 kHz EM4100 animal-ID applications, the chip's immutability is part of the regulatory chain.
• Lowest-cost simple proximity — When the only requirement is 'reader sees a unique number', EM4100 hits the spec at minimum cost.

When T5577 is the right choice

Rewritable becomes valuable as soon as the deployment crosses any of these patterns.

• Hotel + multi-tenant retrofit — Where the existing locks are HID Prox or Indala but the customer wants to procure a single chip family for ongoing replenishment. T5577 lets one SKU emulate the existing format on day one and migrate to a different format later if the lock platform is upgraded.
• Pre-encoded credential issuance — The customer wants to ship cards with the credential ID already programmed (e.g., employee badges with payroll-system ID). T5577 supports this on a Proud Tek encoder line; EM4100 cannot because the UID is factory-set.
• In-field credential rotation — Security-conscious deployments that rotate credential IDs every 90 days. T5577 allows the re-write; EM4100 forces a full card re-issue.
• Single-SKU procurement across multiple reader formats — A building portfolio with HID Prox in some sites, EM4100 in others, Indala in a third — T5577 cards can be configured per site without separate SKUs.
• Test / engineering / penetration-test fleets — T5577 is the de facto standard chip for red-team exercises, capture-the-flag exercises and lab credentials, because any 125 kHz credential format can be staged on a single SKU.

Migration patterns and the read-only-to-rewritable upgrade path

Buildings that started on EM4100 and now want T5577 do not have to flip the entire credential fleet in a single weekend.

• Hybrid issuance — Issue new credentials on T5577 (emulating EM4100) while existing EM4100 credentials remain in circulation. Both work on the existing reader fleet. Over 12–18 months the EM4100 population attrits out (lost, broken, retired employees).
• Reader-side hardening — The harder upgrade is on the reader side. Replacing 125 kHz proximity readers with HF (13.56 MHz, MIFARE Plus / DESFire) readers is the only path to actual cloning resistance. T5577 only changes the credential medium, not the fundamental 125 kHz cloning surface.
• Dual-frequency cards — For buildings starting the HF migration in parallel, dual-frequency cards carry both an LF (EM4100 or T5577) chip and an HF (MIFARE Classic / Plus / DESFire) chip in the same body. Both readers see the same card during the migration window. Proud Tek's dual-frequency RFID card SKU covers this pattern.
• Audit logging — Whatever the credential medium, the meaningful security control is the access-control software logging the reader response and flagging unusual patterns (e.g., the same credential appearing at two readers 200 m apart in the same minute). This is independent of EM4100 vs T5577.

FAQ