Fixed-mount Imager Integration Guide

Chapter 8 SSI Interface
Overview
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 communications between the decoder and host are done over the hardware interface lines using the SSI
protocol. See 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 allowed by the SSI protocol 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.
The SSI performs the following major 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 the formatted 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). The format of the transmitted data can be
controlled via parameter settings.
The decoder may also send parameter information, product identification information or event codes to the host.
All commands sent between the decoder and host must use the format described in the SSI Message Formats
section. The SSI Transactions section describes the required sequence of messages in specific cases.