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XE924M

Base Access Point for Multiple Remote Machines

Xecom 

XE924M COMMUNICATIONS CONTROLLER

Hardware:

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

communications controller in the XE924M. 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 XE924M’s I/O pins for

diagnostic purposes.

900 MHz Network

The 900 MHz network controlled by the XE924M

supports nodes on up to eight remote systems. All of

these systems communicate with the XE924M using

Xecom’s XE900M Smart Transceiver. The XE924M

and XE900M utilize a half-duplex channel on a single

carrier frequency, 916.48 MHz. The XE924M

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 XE900M and XE924M units in the

network listen for communications activity on the

916.48 MHz band before initiating a session.

Count-off Function

The count-off function permits the Base Access Point

to quickly check the status of all network nodes.

When the count-off command, AT#C: or AT#C=, is

broadcast by the Base Access Point each node

responds in a predetermined time slot. The time slot is

determined by the address assigned to the node. The

node’s response to the count-off command contains a

user-programmable 8 byte data packet. This data

packet is programmed with the AT#E: or AT#E=

command. The count-off function can also be initiated

from an XE900M acting as the network hub.

Data Packets

The XE924M places all data incoming across the

modem link into packets. Likewise the XE900M 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 XE900M and

XE924M 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 XE900M serial port.

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 XE900M. The XE924M

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 XE900M 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 XE924M Communications Controller

waits for entry of the desired node address before

directing the call to the proper node.

XECOM

(9)

XE924M

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