Symbol SE 923, Integration Manual

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6-1
Chapter 6
Simple Serial Interface
Introduction
This chapter describes the system requirements of the Simple Serial Interface, which provides
a communications link between Symbol Technologies decoders (e.g., SE-923 scan engine, slot
scanners, hand-held scanners, two-dimensional scanners, hands free scanners, and RF base
stations) and a serial host. It provides the means for the host to control the decoder.
Communications
All communications between the decoder and host are done over the hardware interface lines
using the Simple Serial Interface (SSI) protocol. The
Serial Interface Specification (SIF) is
described in Appendix A .
The host and the decoder 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 decoder configuration.
The Simple Serial Interface performs the following major functions for the host device:
•
Maintains a bi-directional interface with the decoder
•
Allows the host to send commands which can control the decoder
•
Passes data from the decoder to a host device in the formatted SSI packet format or
straight decode message.