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DRAGON™ D151
4
SIMPLE SERIAL INTERFACE
This chapter describes the system requirements of the Simple Serial Interface, which
provides a communications link between readers and a serial host. SSI allows the
host to control the reader.
4.1
COMMUNICATIONS
All communications between the reader and host occur over the hardware interface
lines using the SSI protocol. See
The host and the reader exchange messages in packets. (A packet is a collection of
bytes framed by the proper SSI protocol formatting bytes.) The maximum number of
bytes per packet allowed by the SSI protocol for any transaction is 257 (255 bytes +
2 byte checksum).
Decode data may be sent as ASCII data (unpacketed), or as part of a larger
message (packeted), depending on the reader configuration.
SSI performs the following functions for the host device:
•
Maintains a bi-directional interface with the reader
•
Allows the host to send commands which can control the reader
•
Passes data from the reader to a host device in the formatted SSI packet
format or straight decode message.
The SSI environment consists of a reader, a serial cable that attaches to the host
device, and in some instances, a power supply.
All commands sent between the reader and host must use the format described in
SSI Message
Formats.
messages in specific cases.
The
SSI Commands Table
the SSI partner allowed to send a message of each type. The host transmits type H
opcodes, the reader transmits type D opcodes, and either partner can transmit
Host/Reader (H/D) types.
58
Serial Interface Specification
SSI Transactions
lists the SSI opcodes supported by the reader. It identifies
in Appendix A.
describes the required sequence of
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