EM4100 vs EM4305 vs T5577: Which 125 kHz Chip Do You Need?

What is the difference between EM4100, EM4305, and T5577?

All three are 125 kHz low-frequency RFID chips, but they sit at three different points on a read-only to fully-programmable scale. EM4100 (and the near-identical EM4102) is a read-only chip: it ships with a permanent 40-bit identifier set in at the factory and transmits the same number every time, forever. It cannot be rewritten, which makes it inexpensive, simple, and ideal as a fixed credential.

EM4305 is read/write. It can store and re-store data, present the same output as other EM-family read-only chips, and is the standard transponder for the ISO 11784/11785 FDX-B animal-identification format used in pet and livestock microchips. T5577 (also sold as ATA5577) is the broadest programmable blank: it is read/write, configurable across multiple modulation and data-rate options, and can be encoded to present EM4100, EM4102, EM4200, HID Prox, Indala, AWID and many other open 125 kHz formats. Think of EM4100 as a fixed ID, EM4305 as a writable EM/animal tag, and T5577 as the universal LF programmable blank.

• EM4100 / EM4102 — read-only, fixed 40-bit ID, lowest cost
• EM4200 — read-only like EM4100, larger memory and a newer process
• EM4305 — read/write, EM-family encoding, animal-ID (FDX-B) capable
• T5577 / ATA5577 — read/write, multi-format programmable encoding blank
• Q5 / T5555 — the older programmable chip T5577 superseded

Read-only vs read/write: which do you actually need?

Read-only chips are the right answer when the ID is set once and never has to change. EM4100/EM4102 cards and fobs are perfect for a building where each holder gets one permanent number that the access controller already knows. Because the ID is locked at the factory, a read-only credential cannot be silently re-encoded in the field, and the per-unit cost is the lowest of the group, which is why so many simple proximity systems and some hotel locks use EM4100.

Read/write chips earn their place whenever the number needs to be written, changed, or matched to an existing card. If you need a replacement for a credential you already use on an open format, you need a writable blank, because the same ID has to be encoded onto the new chip so your readers accept it identically. If you are encoding fresh credentials in-house, you want a writable chip so your encoder can assign each number. And if you need a blank that presents a different chip family's output, only a read/write chip configurable to that protocol will do. EM4305 covers writable EM and animal-ID work; T5577 covers the widest range of formats and is the default choice for general 125 kHz encoding.

Can you supply a compatible 125 kHz credential, and onto which blank?

Yes, for open 125 kHz formats. An EM4100/EM4102 ID, and most HID Prox, Indala and AWID credentials, carry their data in the clear with no encryption, so a programmable blank can be encoded to present the exact data your readers already accept, and the reader then treats it as identical to the original. The standard target for that job is a T5577, because it can be configured to reproduce the exact modulation, bit rate and data layout the original chip used. A Q5/T5555 can do the same on older formats and was widely used before T5577 became the default.

Open-format encoding has hard limits. Low-frequency chips that use a proprietary rolling or crypto scheme, such as the Hitag family used in some Cotag, Nedap and Paxton systems, are not plain read-only IDs and cannot be reproduced by simply encoding a number onto a T5577. And nothing at 125 kHz applies to a secured 13.56 MHz smart credential such as MIFARE DESFire with AES, HID Seos, or iCLASS SE/Elite — those are a different frequency and a different security model entirely. For those secured tiers, the correct approach is to supply a compatible blank of the right chip type that the customer's own system enrols with its own keys, exactly as it would credentials ordered through the OEM channel.

Where does EM4305 and animal ID fit in?

EM4305 is the chip to reach for when you need a writable EM-family tag rather than a universal programmable blank. It can present EM4100/EM4102 read-only output, store rewritable data for inventory or asset tags, and, importantly, it implements the ISO 11784/11785 FDX-B protocol used by animal-identification microchips operating around 134.2 kHz. That makes EM4305 (and its EM4205 sibling) the common building block for pet and livestock tags and for readers that expect the FDX-B standard.

For pure access-control encoding, T5577 is usually the more flexible blank because of its broader format support, but EM4305 is preferable when the target system specifically expects EM4305 behavior or when animal-ID/FDX-B encoding is required. If you are unsure which chip your reader or encoder is built around, identify the original credential first and match the chip family, rather than assuming every 125 kHz job is a T5577 job.

How do I choose a blank for a replacement, fresh encoding, or bulk supply?

Match the chip to the task. For a replacement on an open format, order T5577 (or Q5/T5555 for older formats) so the original ID can be encoded onto the blank and read identically by your existing readers. For encoding fresh credentials in-house, order writable chips your encoder supports — typically T5577 for general LF work or EM4305 where EM/animal-ID behavior is needed. For bulk supply of a fixed, unchanging credential, EM4100/EM4102 read-only cards are the most economical because no rewriting is ever required.

When you order compatible cards or fobs from us, tell us the source format, whether you need them blank or pre-encoded, and the ID or facility-code and card-number range to write. We supply compatible 125 kHz blanks and pre-encoded credentials as a cost-effective alternative to OEM credentials; request a quote with your card details. We are an independent manufacturer and supplier of compatible products and are not affiliated with, authorized by, or endorsed by EM Microelectronic, NXP, HID, or any other manufacturer.

• Replacement on an open format — T5577 / ATA5577 (or Q5/T5555 for older formats)
• Fresh in-house encoding — T5577 for general LF, EM4305 for EM/animal-ID
• Bulk fixed credentials — EM4100 / EM4102 read-only, lowest cost
• Animal ID / FDX-B — EM4305 (ISO 11784/11785)