Data Sheet
Copyright © Parallax Inc. RFID Read/Write Module, Serial & USB (#28440 / 28540) v2.1 2/27/2018 Page 7 of 10
Error Checking
The RFID Read/Write Module responds to each command with a single-byte status/error code in
hexadecimal format followed by any returned data, if applicable. In most standard implementations, the
user will repeatedly call the desired function until the ERR_OK status byte is returned, indicating that the
function successfully completed with no errors.
For scenarios where more error checking is desired, the RFID Read/Write Module provides detailed error
code values:
Value
Name
Description
0x01
ERR_OK
No Errors
0x02
ERR_LIW
Could not find a Listen Window (LIW) from the tag
0x03 ERR_NAK
Received a No Acknowledge (NAK), possible communication error or
invalid command/data
0x04 ERR_NAK_OLDPW
Received a No Acknowledge (NAK) sending the current password during
the RFID_SetPass command, possible incorrect password
0x05 ERR_NAK_NEWPW
Received a No Acknowledge (NAK) sending the new password during the
RFID_SetPass command
0x06 ERR_LIW_NEWPW
Could not find a Listen Window (LIW) from the tag after setting the new
password during the RFID_SetPass command
0x07
ERR_PARITY
Parity error when reading data from the tag
Tag Details
The RFID Read/Write Module works exclusively with the EM Microelectronics-Marin SA EM4x50 1kbit R/W
transponder tags at 125 kHz. A variety of different tag types and styles exist with the most popular made
available from Parallax.
The memory map and corresponding address locations for the EM4x50 tag are shown below. Each
address holds a 32-bit (4 byte) value.
Address (decimal)
Register
Description
0
ADDR_Password
Password (not readable)
1
ADDR_Protect
Protection word
2
ADDR_Control
Control word
3-31
User Data Area
Non-volatile EEPROM storage area for user data (116 bytes)
32
ADDR_Serial
Device serial number (unique, read-only)
33
ADDR_DeviceID
Device identification (used to identify tag family, read-only)
The User Data Area can be optionally secured using a 32-bit password to allow only authorized read and
write operations. This password is stored in
ADDR_Password
and can be changed by the user via the
RFID_SetPass command with knowledge of the current password. For security reasons, it cannot be read.
As such, if your tag is password-protected and you forget the tag’s password, you will be unable to
retrieve the contents of the User Data Area.