Cal. Code Regs. tit. 21 § 1703.5
The downlink (reader-to-transponder) modulation scheme shall be unipolar ASK of the RF carrier using Manchester encoding. A data bit '1' is transmitted by sending an RF pulse during the first half of the bit period and no signal during the second half, while for a '0' data bit the reverse order is used; i.e., no signal during the first half of the bit period and an RF pulse transmission during the second half.
The data bit rate for reader-to-transponder messages shall be 300 kbps.
The field strength of a reader data message at the transponder shall be greater than 500 mV/m.
The standard portion of a reader data message shall consist of a header and transaction record type code. The subsequent length, data content, and error detection scheme shall then be established by the definition for that transaction record type.
There may be several reader-to-transponder data message formats. The format is determined by the transaction record type code sent by the reader. The following is the reader-to-transponder message format presently specified for AVI electronic toll collection applications:
The polling message (which follows the 100 microsecond delay after the trigger signal) tells the transponder the type of transaction the reader wishes to conduct. For AVI electronic toll collection applications, reader transaction record type 1 (polling message) also would identify the agency or toll authority. For AVI applications, the reader-to-transponder type 1 polling message shall be structured using the following ordered data bit fields:
| Field Definition | No. Bits | Hexadecimal Value | ||
| Header Code | ||||
| Selsyn | 8 | AA | ||
| Flag | 4 | C | ||
| Transaction Record Type Code | 16 | 8000 | ||
| Agency Code | 16 | |||
| Error Detection Code | 16 | |||
| Total: 60 | ||||
A reader-to-transponder acknowledge data message shall be provided to inform specific transponders that they have been successfully processed and to stop responding to further identical reader polling requests. The acknowledge message is used to terminate the transaction, and is only sent if the transaction is successfully completed. Reader transaction record type 2 (acknowledge message) shall consist of the following ordered data bit fields:
| Field Definition | No. Bits | Hexadecimal Value | ||
| Header | ||||
| Selsyn | 8 | AA | ||
| Flag | 4 | C | ||
| Transaction Record Type Code | 16 | C000 | ||
| Transponder ID Number | 32 | |||
| Reader ID Number | 32 | |||
| Transaction Status Code | 16 | |||
| Error Detection Code | 16 | |||
| Total: 124 | ||||
The end-of-message signal for reader-to-transponder data messages shall consist of a minimum of 10 microseconds of no RF carrier signal. Transponder decoders shall have the ability to detect this condition as an invalid Manchester code.
Cal. Code Regs. Tit. 21, § 1703.5
Note: Authority cited: Section 27565, Streets and Highways Code. Reference: Sections 27564 and 27565, Streets and Highways Code.