Reference Guide

183
SSI Interface
Introduction
This chapter describes the system requirements of the Simple Serial Interface (SSI), which provides a
communications link between Zebra 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 digital scanner 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 digital scanner 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 digital scanner configuration.
SSI performs the following functions for the host device:
• Maintains a bi-directional interface with the digital scanner
• Allows the host to send commands that control the digital scanner
• Passes data from the digital scanner to a host device in SSI packet format or straight decode
message.
The working environment of the SSI consists of a digital scanner, 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 digital scanner can also send parameter information, product identification information, or event codes
to the host.
All commands sent between the digital scanner and host must use the format described in the SSI
Message Formats section. <bl_blue><em_Emphasis>SSI Transactions on page 13-185 describes the
required sequence of messages in specific cases.
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