Motorola PL3307, Руководство по интеграции

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CHAPTER 7 SSI INTERFACE
Introduction
This chapter describes the system requirements of the Simple Serial Interface (SSI), which provides a
communications link between Motorola decoders (e.g., scan engines, 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 or scanner.
Communications
All communication between the decoder and host occurs over the hardware interface lines using the SSI
protocol. Refer to the
Simple Serial Interface Programmer’s Guide , p/n 72-40451-xx, for more information on
SSI.
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 that the SSI protocol allows
for any transaction is 257 (255 bytes + 2 byte checksum).
Decode data can be sent as ASCII data (unpacketed), or as part of a larger message (packeted), depending on
the decoder configuration.
SSI performs the following functions for the host device:
•
Maintains a bi-directional interface with the decoder
•
Allows the host to send commands that control the decoder
•
Passes data from the decoder to a host device in SSI packet format or straight decode message.
The working environment of the SSI consists of a decoder, a serial cable which attaches to the host device,
and in some instances, a power supply.
SSI transmits all decode data including special formatting (e.g., AIM ID). Parameter settings can control the
format of the transmitted data.
The decoder can also send parameter information, product identification information, or event codes to the
host.