Service manual

B-1
April 2001
Part No. 001-9800-203
EXTERNAL SERIAL CONTROL PROTOCOL
APPENDIX B EXTERNAL SERIAL CONTROL PROTOCOL
B.1 INTRODUCTION
B.1.1 GOALS OF THIS PROTOCOL
The goals of this protocol are as follows:
• To use a standard type of signaling that is supported
by typical embedded microprocessors (or their
support chips).
• To support single- or multi-drop operation.
• To provide a method of verifying message integrity.
These goals are realized in the protocol by using
“packets” (or “messages”) that use EIA232 protocol
(at TTL voltage levels) and contain the following:
• Framing characters
• Source and destination addressing
• A checksum
B.1.2 PROTOCOL LAYERS
The protocol has the following three “layers”:
• The Physical Signaling layer is not covered in this
document. It may vary somewhat depending on the
hardware implementation.
• Message Framing is discussed in the next section.
• The Application Commands comprise the rest of the
document.
NOTE: While the protocol has been designed to oper-
ate in a multi-drop environment, the current platform
radio software does not
support multi-drop operation.
B.2 MESSAGE FRAMING AND FORMATTING
B.2.1 FRAME FORMAT
• The message (packet) level information exchange
protocol uses a variable length message format.
• The protocol uses a SYNC character to indicate the
start of a frame and an ESCape character to signal
the end of a frame.
• A DLE character and a replacement character from
Table B-1 are used when the message byte contains
an embedded SYNC, ESCape, or DLE character.
• SYNC and ESCape patterns are not permitted at any
time within the message.
• On transmission, embedded control characters are
detected and replaced with an index character.
• On reception, the replacement character is restored
by using the replacement value as an index into a
replacement table.
The format of the messages is shown in
Table B-2.
Application Command
Message Framing
Physical Signaling
Table B-1 Control Character Replacement
Index
Control Character Replacement Index
SYNC (0xba) 0x00
ESCape (0x7e) 0x01
DLE (0x10) 0x02