User's Manual

Protocol Operation
Crescendo VHF Half-Duplex User Manual Page 33 of 74
6. Protocol Operation
6.1 Overview
Both of the Crescendo‟s serial ports can be independently configured with different protocol modes.
Protocol modes serve two purposes:
Provide methods for configuring the radio for operation, and for interrogating it in order to
determine current operational status.
Allow the Crescendo radio to determine how data received on its serial ports is to be converted into
RF packets.
In addition to the protocol modes, each serial port can be configured with a packetiser timer, to maintain
compatibility with protocols which cannot handle the inter-character delays introduced by the Crescendo
block allocation scheme. The use of packetiser timers is discussed in section 4.3.2 on page 18.
Section 7 on page 43 provides some example applications using these protocol modes to achieve different
data communications requirements.
6.2 Data Driven Protocol
The data driven protocol provides a low latency connection between the radios in a network. When data
driven protocol is enabled, the packet driven nature of the Crescendo is disabled, changing the radio
behaviour to the following:
Addressing and routing are not used.
Retries are disabled, but error checking is still utilised.
All data presented to the main serial port is transmitted immediately over the air, and appears on the
main serial port of all units in range which have data driven protocol configured.
There are two configurable parameters which affect the way the data driven protocol operates:
Data Timeout: The period, in milliseconds, for which the radio will continue to transmit after all
data in the serial buffer has been transmitted. The data timeout can be configured between 0 and
255ms.
Lead-in Count: The number of lead-in bytes the Crescendo will discard and not transmit over the
air. Using a Lead-in Count and lead-in bytes gives the radio modems time to connect to each other
before the data to be transmitted over the air arrives on the serial port. This can reduce end-to end
latency. The lead-in count can be configured between 0 and 255 bytes.
The following restrictions apply when using the data driven protocol:
The data driven protocol can only be configured on the main port.
While data driven is configured on the main port, Hayes dial-up protocol can be configured on the
auxiliary port. The dialling capability of the Hayes dial-up protocol on the auxiliary is disabled.
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