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
User Interface
Interface
NC
STATUS
TXE
RESET
Vss (0V)
D0
TXD
RXD
OSC1
OSC2
Vdd
M3
M2
M1
D3
M0
serial data I/O
Remote control
Data bits
RSTXD/D1
mode select bits
Figure 2: package type, 18-Lead Plastic Dual In-line (PDIP - 300mil wide body)
Pin description:
Pin
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Name
NC
STATUS
TXE
RESET
0V
D0
D1/RSTXD
D2/RSRXD
D3
M0
M1
M2
M3
Vcc
OSC2
OSC1
RXD
TXD
I/O
-
out
out
in
-
in/out
in/out
in/out
in/out
Function
No Connection
High indicates data received or in rx FIFO buffer
Active low enable transmitter
A 10kΩ pullup to VCC must be provided
Supply ground
Data bit 0 value during Control44 encoder / decoder mode
Data bit 1 value during Control44 encoder / decoder mode
Inverted RS232 datastream in, 1200 baud
Data bit 2 value during Control44 encoder / decoder mode
Inverted RS232 datastream out, 1200 baud
Data bit 3 value during Control44 encoder / decoder mode
Mode select LSB -
Mode select
Mode select
Mode select MSB
5V (regulated power supply); Decouple with 0.01uF close to IC
connect to 3.58MHz crystal
connect to 3.58MHz crystal
Baseband input from receiver data output
Baseband output to transmitter data input
-
out
in
in
out
Notes:
1. No connections at all may be made to 'nc' pin
2. TXD, RXD, RSRXD, RSTXD and STATUS are 5V CMOS logic level
3. Some Radiometrix transmitters require 3V logic levels on their transit data inputs:
a divider (4.7kΩ series, 10kΩ to ground) on the TXD pin is needed
4. In RX operation, RXD becomes a high impedance
5. After pulling TXE low (active), the coder allows about 50mS for TX to power up and settle
6. RSTXD (7) has no pullup. If the device is only used for receive, then tie this pin to VCC
7. Vcc must be a 5v regulated supply (4.75 - 5.25V). At this oscillator speed the PIC will not operate
reliably at 3V
8. Pin 3 has no pullup. It is used to enter 'setup' mode (see below)
9. Without external loads the chip draws less than 5mA from 5v
10. STATUS goes high when valid data is present in the receive buffer.
11. OSC1, 2 require a 3.58MHz fundamental mode crystal, a series 100Ω resistor from OSC2,
and a pair of 15pF caps : from the crystal pins to 0V
Radiometrix Ltd
Page 2
1200buad Modem IC
Mode selection
The mode select bits M0-M3 put NBEK IC in to one of 16 operating modes, including modem mode.
MODE
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14 (Modem)
15
H=5V, L=0V
HHHH
HHHL
HHLH
HHLL
HLHH
HLHL
HLLH
HLLL
LHHH
LHHL
LHLH
LHLL
LLHH
LLHL
LLLH
LLLL
Receive mode
Un-modulated Transmission for testing carrier frequency, power, spurii
250Hz (500bps) square wave Modulated Transmission
1250Hz (2.5kbps) square wave Modulated Transmission
Pseudorandom NRZ stream modulated Transmission
Transmitter and Receiver turned ON and OFF periodically at 100ms interval
RESERVED for future use
RESERVED for future use
Continuous Control44 Transmission of Address=0 and Data=D3, D2, D1, D0
Control44 Transmission of Address=0 and Data=D3, D2, D1, D0
If either Data is changed or RESET (Trigger) switch is pressed.
Control44 Reception with Momentary Output. (paired with Mode 8)
Control44 Reception with Latched Output. (paired with Mode 9)
Transmits CTR44 Test Packet, Receive Echoed Packet (Radar Mode – Master)
Receive CTR44 Test Packet, Re-transmit it back to Sender (Echo Mode – Slave)
I1200 type 1200 baud Dumb Modem for bit balanced serial data
transmission
RESERVED for future use
Notes:
1. Mode 12 and 13 require Transceiver
2. In Mode 12, a test packet is transmitted, then the unit switches to Receive mode for 100ms before repeating.
3. Data sequence of 8 (D3=L, D2=H, D1=H, D0=H), 4, 2, 1 (D3=H, D2=H, D1=H, D0=L) is cycled through with
each transmission.
4. In Mode 13, the unit idles in Receive mode. When a valid CTR44 burst is received the unit keys up the
Transmitter and re-transmits this CTR44 burst back to sender in Mode 12.
Radiometrix Ltd
Page 3
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
Application Circuits
5V supply
10k
10k
10k
10k
NC
1k
STATUS
TXD
RXD
OSC1
OSC2
VDD
M3
M2
M1
M0
100nF
3.58MHz
3k2
2k2
EN
TXD
Vcc
GND
RF
Transmitter
NBEK-000-DIL
TXE
RESET
VSS
NC
TXD
µC
UART
RSTXD
RSRXD
NC
Figure 2: NBEK IC interfaced to a Transmitter (e.g. TX1)
Note:
Note
1. Transmit keyed when valid serial data is present at the RSTXD input, so no separate TX
control needed
2. OSC1,2 require a 3.58MHz ceramic resonator with internal capacitors like Murata
CSTLA3M58G55-B). If a 2 pin resonator or crystal is used, then two 15pF caps are needed: one
from OSC1 to 0V; one from OSC2 to 0V
5V supply
RF
Receiver
10k
NC
1k
STATUS
TXD
RXD
OSC1
OSC2
VDD
M3
M2
M1
M0
100nF
3.58MHz
RXD
Vcc
GND
NBEK-000-DIL
TXE
RESET
VSS
NC
RSTXD
RXD
µC
UART
RSRXD
NC
Figure 3: NBEK IC interfaced to a Receiver (e.g. NRX1)
Radiometrix Ltd
Page 5
1200buad Modem IC
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