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NBEK-000-XXX: DescriptionNBEK Controller As 1200 Baud Modem; File Size222KB，7 Pages; ManufacturerRadiometrix Ltd.; Websitehttp://www.radiometrix.com/

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NBEK-000-XXX Overview

NBEK Controller As 1200 Baud Modem

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EW

N

16 April 2008

Radiometrix

Hartcran House, 231 Kenton Lane, Harrow, HA3 8RP, England

Tel: +44 (0) 20 8909 9595, Fax: +44 (0) 20 8909 2233

NBEK-000-xxx

NBEK-000-

NBEK Controller As 1200 Baud Modem

The Narrow band Evaluation Kit (NBEK) IC can be used as 1200baud half-duplex serial modem

suitable for narrow band receivers, transmitters and transceivers (or any RF modules with >5kbps data

rate). It takes care of preamble, synchronisation, bit

balancing and error checking and enables a

transparent radio data link to be established

between radio devices

This is a half duplex unit, so collisions between

transmitted and received packets must be dealt

with by the user.

Provided no two devices attempt to transmit

simultaneously no further restrictions on data

Figure 1: NBEK-000-DIL

transmission need be made, as all transmit timing,

valid data identification and data stream buffering is conducted by the unit. Synchronisation and

framing words in the packet prevent the receiver outputting garbage in the absence of wanted RF

signal or presence of interference.

Supports 1200 baud asynchronous data: 1 start bit, 8 data bits, 1 stop bit.

With typical microcontrollers and UARTs, direct connection is usable as shown on figures 2 & 3.

To connect to a true RS232 device, inverting level shifters must be used (MAX232 type are ideal, but

simple NPN/PNP transistor switches often suffice as used in the application circuits figures 4 & 5).

Features

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Operating voltage (temperature): 5V for standard version (-40°C to +85°C)

Maximum usage of the range capability of an RF module

Adequate preamble to settle data slicer in the receiver

Extra wake up preamble to allow for transmitter power up time requirements

Differential Manchester encoding of address, data and checksum

Synchronisation codes and checksum to reduce false triggering on noise

Suitable to be used with Narrow Band FM radio modules

Serial modem baud rate at 1200bps (half-duplex)

Addressable point-to-point

Applications

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PDAs, organisers & laptops

Handheld / portable terminals

EPOS equipment, barcode scanners

In-building environmental monitoring and control

Remote data acquisition system, data logging

Fleet management, vehicle data acquisition

Radiometrix Ltd

Page 1

1200buad Modem IC

Operating principle of modem

This device is a simplex link handling a 1200 baud asynchronous datastream (1 start, 8data, 1stop).

Buffering in the transmit end handles TX startup timing requirements, while the presence of sync codes

in the transmitted bursts prevents the receiver from outputting garbage in the presence of noise. No

error correction or retransmission of lost packets is supported. To operate with proper +/-12V RS232

levels an external buffer (such as a MAX232) will be needed

Operation: Serial interface

Both transmit and receive modems implement a 32 byte software FIFO. At the transmit end this is

used to allow for the transmitter start up time (about 32mS), while on receive it buffers arriving packets

to the constant output data rate. All timing and data formatting tasks are handled by the software. The

user need not worry about keying the transmitter before sending data: the link is entirely transparent.

For transmission across the link data is formatted into packets, each comprising 3 bytes of data and a

sync code. If less than 3 bytes are in the transmit end FIFO then a packet is still sent, but idle state

replaces the unused bytes. When the transmit end FIFO is completely emptied, then the transmitter is

keyed off .

Operation: Radio interface

Raw data is not fed to the radios. A coding operation in the transmit mode, and decoding in the receive

mode, isolate the AC coupled, potentially noisy baseband radio environment from the datastream.

The radio link is fed a continuous tone by the modem. As in biphase codes, information is coded by

varying the duration consecutive half-cycles of this tone. In our case half cycles of 500µs (a long, or L)

and 250µs (a short, or S) are used.

In idle (or 'preamble') state, a sequence of Ls is sent (resembling a 1kHz tone).

A packet comprises the Synchronising (or address) part : LSSSSSSSSSSSSL followed by the Data part,

made up of twelve Groups (of four half cycles duration). Each Group encodes 2 data bits, so one byte is

encoded by 4 Groups.

Although there are 16 possible states for a four half cycle group (from SSSS to LLLL), only four of the

possible states are used for valid data:

All other possible group combinations (except LLLL) are void, and result in the entire packet being

rejected by the receiver software as a noise artifact. Idle state tone (LLLL) is decoded as null data, but

does not void the packet, as a packet containing less than 3 bytes still needs to be decoded.

The advantage of using the above coding technique:

1.Ease of decoding:

2. Inherent error detection:

Unlike biphase, where a bit is coded as either a long half cycle or a

short cycle, here all half cycles are detected separately.

Although only 14 sync/address 'bits' (halfcycles) are used, the

following 48 'bits' (halfcycles) only code 24 real data bits, leading to

enhanced noise/error discrimination (the equivalent of 5 more sync

bits).

For a maximum transmitted baseband frequency of 2kHz, a 3 byte

packet is sent in 22ms. An equivalent biphase coded packet

(comprising 19 sync + 24 data + 3 null flags) at 2kbits/sec takes

23ms

3. Comparable spectral efficiency:

Radiometrix Ltd

Page 4

1200buad Modem IC

NBEK-000-XXX Related Products

	NBEK-000-XXX	NBEK-000-DIL	NBEK-000-SS	NBEK-000-SO
Description	NBEK Controller As 1200 Baud Modem	NBEK Controller As 1200 Baud Modem	NBEK Controller As 1200 Baud Modem	NBEK Controller As 1200 Baud Modem

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