XE924 Просмотр технического описания (PDF) - Xecom

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XE924

Base access point for multiple remote machines.

Xecom 

XE924 COMMUNICATIONS CONTROLLER

Hardware:

A low-cost micro-controller performs the duties of the

communications controller in the XE924. This micro-

controller manages the 900 MHz network and the data

translation between the 900 MHz transceiver and the

embedded modem.

The embedded modem is Xecom’s XE2420, 2400 BPS

modem. This modem includes its own dedicated

controller for the modem functions including the AT

commands. The serial interface signals from the

XE2420 are brought out to the XE924’s I/O pins for

diagnostic purposes.

900 MHz Network

The 900 MHz network controlled by the XE924

supports nodes on up to eight remote systems. All of

these systems communicate with the XE924 using

Xecom’s XE900 Smart Transceiver. the XE924 and

XE900 utilize a half-duplex channel on a single carrier

frequency, 916.48 MHz. The XE924 communications

controller manages this channel preventing collisions,

correcting errors, and reversing channels.

Carrier Sense Multiple Access (CSMA)

The primary tool used to prevent collisions between

systems is a CSMA protocol for each of the 900 MHz

transceivers. All XE900 and XE924 units in the

network listen for communications activity on the

916.48 MHz band before initiating a session.

Wireless Link to Modem Link Translation

Data on the wireless link travels in packets at 9600 BPS.

The communications controller strips the data from the

packets before transmitting it through the modem.

Because the maximum data rate of the modem connection

is 2400 BPS, the communications controller buffers the

data, and sends it to the modem at 2400 BPS. While this is

happening any incoming data from the modem has to be

buffered and held until the wireless channel can be

reversed. When the wireless channel is available to

transmit, the data in the buffer is put into packets and

transmitted over the 900 MHz transceiver at 9600 BPS.

The communications controller also manages the modem’s

connections. AT commands entered at the remote node

are placed in packets and passed across the wireless link

by the XE900. The XE924 Communications Controller

removes the command from the packet, passes it to the

modem, and waits for a modem response. The modem

response is put in a packet and returned to the XE900

equipped node.

The routing of incoming modem calls will also be handled

by the communications controller. After the modem

answers the incoming call and establishes the

connection, the XE924 Communications Controller waits

for entry of the desired node address before directing the

call to the proper node.

Data Packets

The XE924 places all data incoming across the modem

link into packets. Likewise the XE900 places all of the

data arriving through its serial port into packets.

These packets providing addressing and error

correction for the wireless communications link. The

communications controllers in the XE900 and XE924

place the data in packets only for the wireless link. The

data is extracted from the packets before being sent to

the modem or to the XE900 serial port.

XECOM

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XE924

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