Data Sheet and Application Notes NKM2401 Code/decoder chip
Part number 1-982
Description
The NKM2401 Radio Data Encoder/Decoder IC enables serial data to be easily transported over low
power radio links and therefore allows users to quickly and easily add short range “wireless” capabilities
to their systems. It is designed for use with licence exempt transmitter and receiver modules operating
in the VHF and UHF bands and provides the buffering, packetisation, encoding, error checking and ‘bit
bashing’ required for transmitters and the reverse processes for receivers.
It can be interfaced to any PC or microprocessor serial port to provide simplex (one way) transfer of
data with built in error checking ensuring data integrity. It is ideally suited for use with the Parallax
BASIC Stamp or other similar devices.
Features
♦
♦
♦
Applications
Simple to use
Transparent operation
Single chip solution
♦
♦
♦
Data acquisition
Alarm systems
Hand held terminals
Typical Application
Serial Data In
Serial Data Out
TxD
RxD
Data Ready*
Busy*
User
Send Data*
TxEn
NKM2401
Application
RF Transmitter
RF Receiver
NKM2401
* Optional handshake lines
User
Application
Introduction
Radio links comprising pre built and tested transmitter and receiver modules are now readily available
from a number of suppliers. These are easily connected to standard encoders and decoders allowing
simple On/Off remote control systems to be assembled.
However, sending data over these radio links is difficult as they are generally AC coupled, bandwidth
limited (100Hz to 5KHz typical) and may be subject to noise and interference. The receiver may
produce digital ‘white noise’ on its data output in the absence of any signal and in addition requires some
time to extract the data from the received audio signal. The format of the data must also be given
consideration as asymmetric data such as Serial ASCII (RS232) which is non DC balanced will cause the
‘data slicer’ in the receiver to give poor performance leading to errors after consecutive sequences of
ones or zeros. There may also be considerable pulse width distortion and ‘jitter’ produced by the link.
These radio links cannot therefore, usually be successfully connected directly to a UART, as such
devices will not tolerate these errors and almost certainly will produce erroneous data which may be
worse than useless !
NKM2401 Issue 1.1 Nov-98
Milford Instruments/nikam electronics www.milinst.demon.co.uk
Page 1 of 15
Data Sheet and Application Notes NKM2401 Code/decoder chip
Part number 1-982
Some form of communication protocol is required when using links over a noisy communication channel.
This needs to rugged, secure and have built in error checking. A number of complex standards do exist
but they are not really suited to simple radio links and they are not implemented on a simple to use IC.
The NKM2401 is intended to address this problem and provides the services required for an interface
between a data source and a radio module. It is intended for simplex (one way) communications and
transports short packets of data and is therefore suited to ‘control and sense’ (bytes) rather than ‘file
transfer’ (Megabytes) tasks.
The NKM2401 is configured as a ‘transmitter encoder’ or a ‘receiver decoder’ by the state of a single
mode pin.
It interfaces to the hosts by a simple serial interface operating at 2400 baud.
NKM2401 Issue 1.1 Nov-98
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Page 2 of 15
Data Sheet and Application Notes NKM2401 Code/decoder chip
Part number 1-982
Pin Out Schematic & Description
Vcc
Serial Data In
Busy
1
Gnd
Transmit Enable
NKM2401
Mode
High
Transmit Data
Test
Transmit Mode
Pin
1
2
3
4
5
6
7
8
Function - Mode Pin High - Transmit
Vcc +5V supply
Serial Data Input, 2400bps
Busy Output. Goes high whilst transmitting. Active High
Mode Pin high (Vcc) selects transmit mode
Test Input sends pre-set message. Active Low
Transmit Data Output to RF transmitter
Transmit Enable Output to RF transmitter. Active High
Ground. 0 Volt
Vcc
1
Data Ready Output
Rx Data Input
No Connect
Gnd
NKM2401
Serial Data Output
Send Data Input
Mode
Low
Receive Mode
Pin
1
2
3
4
5
6
7
8
Function - Mode Pin Low - Receive
Vcc +5V supply
Data input from RF receiver
No connection. Do not connect. Factory test point.
Mode Pin low (0V) selects receive mode
Send Data Output. Pull high to send the data in receive buffer
Serial Data Output. 2400bps
Data Ready Output flag. Goes low when valid data in buffer
Ground. 0 Volt
NKM2401 Issue 1.1 Nov-98
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Page 3 of 15
Data Sheet and Application Notes NKM2401 Code/decoder chip
Part number 1-982
Technical Specifications
Ambient Temperature 20° C
Supply Voltage
+5.0 V
Parameter
Min
Typical
Max
Units
Supply Voltage
Supply Current
4.75
1.8
5.0
5.25
2.4
Volts
mA
Serial In/Out Baud rate
Character time
RF transmit bit time
RF transmit time
Receive & decode time
Handshake delay
Serial data output time
Tx Enable source current
Operating temperature
Storage Temperature
2400
4.16
200
35
36
1
50
8.0
0
-40
20
20
bps
mS
uS
mS
mS
mS
mS
mA
+40
+85
deg. C
deg. C
Notes
1
2
3
4
Notes
1) Data is always ‘true’ i.e. same polarity as RS232 +/-12V levels but at 0v to +5V (Vcc) logic levels.
(Start bit is logic high, stop bit is logic low.)
2) 1 start, 8 data, 1 stop = 10 bits @ 416uS/bit = 4.16mS
3) Manchester encoded.
4) Eight bytes with handshake delay.
NKM2401 Issue 1.1 Nov-98
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Page 4 of 15
Data Sheet and Application Notes NKM2401 Code/decoder chip
Part number 1-982
System Timing Diagram
Approx 128mS Overall
Host sends 8 Characters
42mS
NKM2401 RF Transmit
and RF Receive/Decode
35mS
Data Ready Output
Send Data Input
NKM2401 Serial Data Out
50mS
Handshake delay 1mS
NKM2401 Issue 1.1 Nov-98
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Page 5 of 15
Data Sheet and Application Notes NKM2401 Code/decoder chip
Part number 1-982
Transmit Mode Interface
Host
Vcc
1
Serial Data
Busy
Mode
High
Gnd
Transmit Enable
NKM2401
Transmit Data
Test
Ground (0V)
Two Wire Interface
Transmit Mode Operation
In transmit mode the NKM2401 receives eight bytes of serial data from the sending host. It stores
this data in a local buffer and then calculates a CRC (Cyclic Redundancy Check) checksum on the data
only. It then sets the Transmit Enable line high to enable the RF transmitter module and a ‘busy’ line
high to indicate that no more data can be received. A pre-amble character is sent on the Transmit Data
line followed by a special synchronising character, the eight bytes of data and the CRC result. The
complete packet of transmitted data is Manchester encoded for optimum transmission over the RF link.
Notes
1.
2.
3.
4.
5.
Serial data format must be: 1 Start Bit, 8 data bits, 1 stop bit, No parity, 2400bps only. Data is
true. Any data byte format may be sent (Hex, decimal, ASCII)
The Serial Data In status is continuously tested, waiting for the positive going start bit of a
character. It must therefore normally be held low.
The NKM2401 accepts eight characters at a time. If more than this are sent, the internal buffer
overflows and the first eight are accepted and sent, the others may be lost. Radio packets always
contain eight bytes of data.
Characters can be sent to the NKM2401 character by character. There is no need for them to be in
a continuous stream. When the buffer has received eight characters they will be sent.
The use of the ‘Busy’ output line is optional. It goes high during transmission and may be monitored
by the host to provide a ‘handshaking’ function if required. i.e. Clear to send and NOT clear to send.
If handshaking is not used time must be allowed for the completion of the transmission and
reception of the message before sending any more data. (typically 120mS ) The ‘busy’ output can
also be connected to a LED to provide visual indication of ‘busy’.
Test Pin
To aid testing and diagnostics, a test pin is provided in transmit mode. This should normally be pulled
high by an external pull up resistor of 10k Ohm. If this pin is switched low by a pushbutton or jumper
the Serial Data Input is ignored and an internally generated test message is transmitted whilst the test
pin is held low. There is a 500mS delay between messages. The eight character message reads “123Test”.
NKM2401 Issue 1.1 Nov-98
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Page 6 of 15
Data Sheet and Application Notes NKM2401 Code/decoder chip
Part number 1-982
Receive Mode Interface
+5V
LED (Option)
470R
Vcc
Host
Data Ready
Rx Data Input
NKM2401
No Connect
Serial Data
Send Data
Mode
+5V
Low
Ground (0V)
One Wire Interface (No Handshake)
+5V
LED (Option)
Vcc
470R
Host
Data Ready
Rx Data Input
NKM2401
No Connect
Serial Data
Send Data
Mode
Low
Ground (0V)
Three Wire Interface
+5V
LED (Option)
Vcc
470R
Rx Data Input
NKM2401
No Connect
Mode
Host
Data Ready
Serial Data
Send Data
Low
10k
Ground (0V)
NKM2401 Issue 1.1 Nov-98
Two Wire Interface
Milford Instruments/nikam electronics www.milinst.demon.co.uk
Page 7 of 15
Data Sheet and Application Notes NKM2401 Code/decoder chip
Part number 1-982
Receive Mode Operation
In receive mode the NKM2401 is continually listening for messages and searching the digital ‘white
noise’ on the receiver data output for the preamble followed by the unique synchronising character. If
it finds this character it “locks on” to the signal, decodes it and stores the data in a local buffer. The
CRC is calculated on the received data and if correct the ‘Data Ready Output’ line is set low.
Handshake Methods
One Wire Interface. (Beginners) This is the simplest method. The ‘Send Data’ line is permanently wired
high. Whenever the buffer is full (signalled by the ‘Data Ready’ line going low) it is immediately sent out
on the ‘Serial Data’ line. The host must therefore always be “ready and waiting” for this data.
Three Wire Interface. This full handshaking method allows complete flow control by the host at the
expense of using two extra I/O lines. The host monitors the ‘Data Ready’ line (by polling or interrupt)
until the NKM2401 signals that it has data in its buffer by setting the ‘Data Ready’ line low. When the
host is ready to receive data, it must set the ‘Send Data’ line high. After a 1mS delay (to allow settling)
the serial data is sent. After each character is sent there is a further 1mS delay and the ‘Send Data’
line is tested again. If it is still high another character is sent, if it is low no more characters are sent.
This allows the host to unload the buffer a character at a time, process the data (at its own pace) and
then collect another character. The ‘Data Ready’ line will only go high again when the buffer is empty.
The only disadvantage to this method is that the NKM2401 cannot receive any more new messages until
the buffer is empty.
Two Wire Interface. (Advanced) This is a refinement of the above method and saves one I/O line by
using a configurable bi-directional port in the host.. The ‘Data Ready’ output is connected to the ‘Send
Data’ input by a 10k resistor. The host I/O line is initially set up as an input and it is normally pulled up
to Vcc (+5V) via the 10k resistor from the ‘Data Ready’ line. When the buffer is full the ‘Data Ready’
line goes low as above. The host should detect this state, configure the I/O line as an output and set it
high when it is ready to receive the serial data. The series resistor allows the ‘Send Data’ input to be
set high overriding the ‘Data Ready’ low output. When the serial data has been sent (buffer empty) the
‘Data Ready’ line returns high awaiting the next packet.
Status
No data available
Data ready
Send data
Sending data
Done
Data Ready Output
Serial Data Output
Send Data Input
High
Low
Low
Low
Serial Data
Low
Set Low
Low
r Pull High
r Keep High
Set Low
o Goes Low
Low
Low
r Goes High
Notes
1.
The Data Ready line can drive an optional LED which indicates when the buffer has valid data
available. This is a useful diagnostic tool.
2. The Send Data input is tested after the transmission of each character and therefore must be held
high for the total duration of transmission (Eight characters)
3. The error checking (CRC) ensures (with 99.9% certainty) that any message received and flagged by
Data Ready will be error free. However if the CRC fails for any reason the message is lost forever.
In simplex (one way only) systems such as these it may therefore be prudent to send the message
multiple times.
NKM2401 Issue 1.1 Nov-98
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Page 8 of 15
Data Sheet and Application Notes NKM2401 Code/decoder chip
Part number 1-982
Low cost modules usually share the same RF channel. Whilst it is possible for one transmitter to
transmit to many receivers, care should be taken if multiple transmitters are being used in vicinity of
each other to ensure that they do not transmit simultaneously. Messages may “clash” and therefore
destroy each other. There is no addressing built into the NKM2401. This should be done at the
application layer embedded in the users message. E.g. first character is the address. This should be
inspected by the host to determine if the intended destination has been reached.
NKM2401 Issue 1.1 Nov-98
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Page 9 of 15
Data Sheet and Application Notes NKM2401 Code/decoder chip
Part number 1-982
Typical Application Circuit
+5V
100n 10k
AERIAL
NKM2401
1
2
3
4
SERIAL
DATA IN
BUSY
VDD
GP5
GP4
GP3
8
7
6
5
VSS
GP0
GP1
GP2
1
2
RF GND
RF OUT
3
4
5
VCC
GND
D IN
TXEN
TXD
MODE
HIGH
470R
TXM-418F
BUSY
TEST
5V VERSION ONLY
0v
10k
PB SW
LED
10R
AERIAL
RF
GND
100n
LED
1
2
+5V
470R
10k
RXD
TP
DTCT
GND
VCC
AF
DATA
100n
1
2
3
4
MODE
LOW
3
4
5
6
7
NKM2401
VDD
GP5
GP4
GP3
VSS
GP0
GP1
GP2
8
7
6
5
DATA READY
SERIAL DATA OUT
SEND DATA
10k
SILRX-418F
0V
100n
Notes
1. The LEDs are optional but very useful.
2. The TXM module must be the 5V (low voltage) version. External buffering may be required to drive
other modules.
3. The TP is for factory use only.
NKM2401 Issue 1.1 Nov-98
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Page 10 of 15
Data Sheet and Application Notes NKM2401 Code/decoder chip
Part number 1-982
Connection to PC RS-232 Port
100n
+5V
10k
AERIAL
DCD
DSR
RXD
RTS
TXD
CTS
DTR
RI
GND
1
6
2
7
3
8
4
9
5
NKM2401
SERIAL
DATA IN
47k
1
2
3
4
RXD
BUSY
MODE
HIGH
PC AT
VDD
GP5
GP4
GP3
VSS
GP0
GP1
GP2
8
7
6
5
TXEN
1
2
RF GND
RF OUT
3
4
5
VCC
GND
D IN
TXD
470R
TXM-418F
TEST
5V VERSION ONLY
0v
10k
PB SW
BUSY LED
10R
100n
AERIAL
LED
100n
+5V
DATA READY
RF
GND
1
2
470R
10k
RXD
TP
1
2
3
4
NKM2401
VDD
GP5
GP4
GP3
VSS
GP0
GP1
GP2
8
7
6
5
1
6
2
7
3
8
4
9
5
SERIAL DATA
SEND DATA
1k
MODE
LOW
DTCT
GND
VCC
AF
DATA
3
4
5
6
7
DCD
DSR
RXD
RTS
TXD
CTS
DTR
RI
GND
PC AT
SILRX-418F
0V
100n
The NKM2401 can be directly connected to a PC RS232 port as shown above. A simple terminal
programme can be used to drive the devices.
In transmit mode the +/-12V PC RS232 output is connected to the RXD input via a series resistor of
47k. Internal protection diodes clamp the input voltage to ground and +5V.
In receive mode the serial data output is connected to the RXD input of the PC. The 0-5V logic levels
will usually drive most PC serial inputs (but not all!) A series resistor of 1k provides some short circuit
protection to the NKM2401.
The polarity of the signals is correct for these direct connections. If longer lines need to be driven a
logic level to RS232 level converter should be used. These are usually inverting so extra inverters
should be connected in the signal lines.
No handshaking is used in these examples as PC UARTs will usually accept the eight byte packets. The
hardware handshake lines of the PC should be tied as shown to defeat any hardware handshaking that
may be used on some PCs or .checked by software.
NKM2401 Issue 1.1 Nov-98
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Page 11 of 15
Data Sheet and Application Notes NKM2401 Code/decoder chip
Part number 1-982
Connection to Stamp
+5V
100n 10k
AERIAL
+9V
PWR
GND
PC0
PC1
+5V
RES
P0
P1
P2
P3
P4
P5
P6
P7
1
2
2
4
5
6
7
8
9
10
11
12
13
14
NKM2401
SERIAL
DATA IN
1
2
3
4
BUSY
VDD
GP5
GP4
GP3
VSS
GP0
GP1
GP2
8
7
6
5
1
2
RF GND
RF OUT
3
4
5
VCC
GND
D IN
TXEN
TXD
MODE
HIGH
470R
TXM-418F
STAMP1
BUSY
TEST
5V VERSION ONLY
0v
PB SW
LED
10R
AERIAL
RF
GND
100n
1
2
LED
+5V
+9V
470R
10k
RXD
TP
DTCT
GND
VCC
AF
DATA
100n
1
2
3
4
NKM2401
VDD
GP5
GP4
GP3
VSS
GP0
GP1
GP2
8
7
6
5
MODE
LOW
3
4
5
6
7
DATA READY
SERIAL DATA OUT
SEND DATA
1
2
2
4
5
6
7
8
9
10
11
12
13
14
PWR
GND
PC0
PC1
+5V
RES
P0
P1
P2
P3
P4
P5
P6
P7
STAMP1
SILRX-418F
0V
100n
The NKM2401 provides an ideal interface to Stamp modules. It frees valuable resources and allows
users to quickly and easily add ‘wireless’ capabilities to their systems. The low current consumption
allows the NKM2401 to be powered from the 5V regulated supply available from the Stamp.
Stamp Software Examples
The following code snippets demonstrate the ease of use of these modules. The familiar SERIN and
SEROUT commands hook in directly to the Stamp and no special coding is required.
Transmit Routines
Send:
serout 0,N2400,("12345678")
pause 500
goto Send
' Send message on Pin 0
‘do something useful here
' send message again
Receive Routines
Receive:
serin 0,N2400,b1,b2,b3,b4,b5,b6,b7,b8
pause 500
goto Receive
NKM2401 Issue 1.1 Nov-98
‘Receive message on Pin 0
‘Store data in variables b1 to b8
‘do something with the data
‘get some more data
Milford Instruments/nikam electronics www.milinst.demon.co.uk
Page 12 of 15
Data Sheet and Application Notes NKM2401 Code/decoder chip
Part number 1-982
No ‘handshaking’ is used here so adequate time must be allowed for the complete transmission,
reception and handling of the message before sending another one. ( approx. 130mS) If handshaking is
used the characters may be unloaded a character at a time into a single variable.
Real World Stamp Application
The following circuits illustrate a working example of a Stamp based data acquisition system. An ADC
converter and a temperature sensor transmit their data to a remote LCD display. Full working software
listings for the Stamp code are available on disk.
Transmitter Circuit (Available as part number 1-484)
+9-12V DC INPUT
+5V
10k 10k
PROG
PORT
+5V
CLOCK
DATA
10k
ADC0831
1
2
3
4
CS
VCC
+VIN CLK
-VIN
D0
GND VREF
8
7
6
5
1
2
3
4
BS1-1C
1
2
3
4
5
6
7
8
9
10
11
12
13
14
PWR
GND
PC0
PC1
+5V
RES
P0
P1
P2
P3
P4
P5
P6
P7
DS1620
D0
CLK
RST
GND
VCC
T(HI)
T(LO)
T(COM)
8
7
6
5
AERIAL
+5V
100n 10k
NKM2401
1
2
3
4
VDD
GP5
GP4
GP3
VSS
GP0
GP1
GP2
8
7
6
5
RF GND
RF OUT
3
4
5
VCC
GND
D IN
TXEN
TXD
MODE
HIGH
0-5V
1
2
470R
TXM-418F
5V VERSION ONLY
0v
ANALOGUE INPUT
100n
NKM2401 Issue 1.1 Nov-98
100n
SW1SW2
BUSY 10k
TEST
100n
Milford Instruments/nikam electronics www.milinst.demon.co.uk
Page 13 of 15
Data Sheet and Application Notes NKM2401 Code/decoder chip
Part number 1-982
Receiver Circuit (Available as part number 1-485)
10k
+5V
10k
1N4001
PROG
PORT
1N4001
RA2
1k
470R
470R
1k
LM7805
+9-12V DC INPUT
VO G VI
N
D
BS2-1C
1
2
3
4
5
6
7
8
9
10
11
12
RA1
470R 220u
SOUT PWR
SIN GND
ATN RES
GND +5V
P0
P15
P1
P14
P2
P13
P3
P12
P4
P11
P5
P10
P6
P9
P7
P8
24
23
22
21
20
19
18
17
16
15
14
13
LED
1
2
3
SERIAL LCD DISPLAY
+
-
LED BC184L
BC184L
LED
SW1 SW2
10R
AERIAL
100n
RF 1
GND 2
0V
LED
+5V
470R
10k
RXD
TP
DTCT
GND
VCC
AF
DATA
PIEZO
1
2
3
4
NKM2401
VDD VSS
GP5 GP0
GP4 GP1
GP3 GP2
8
7
6
5
DATA READY
SEND DATA
SERIAL DATA
MODE
LOW
3
4
5
6
7
SILRX-418F
0V
100n
RF Environment
Receivers are very sensitive and like all radios can be subject to interference. Care should be taken to
avoid local sources of interference such as noisy switch mode power supplies or external bus micro
systems. Such devices can generate harmonics that have considerable energy in the UHF band. This will
desensitise the receiver and may cause loss of range. Aerials should be kept clear of large metallic
objects or surfaces and should ideally be surrounded by “free space”.
The module manufacturers data sheets detail these requirements and should be carefully studied.
NKM2401 Issue 1.1 Nov-98
Milford Instruments/nikam electronics www.milinst.demon.co.uk
Page 14 of 15
Data Sheet and Application Notes NKM2401 Code/decoder chip
Part number 1-982
Further Reading
Radiometrix Data Sheets. TXM-418, SILRX-418, BiM Transceiver. See www.radiometrix.co.uk
RS-232 Simplified. Everything YOU need to Know About Connecting, Interfacing & Troubleshooting
Peripheral Devices.
Author: Byron W.Putman. Publisher: Prentice Hall. ISBN 0-13-783499-3
Product Order Codes
Description
Order Code
NKM2401 Modem IC
RF AppKit
Assembled Receiver Unit
Assembled Transmitter Unit
1-982
1-939
1-484
1-485
Document History
Issue
Date
Revision
1.0
1.1
1.2
Oct-98
Oct-98
Nov-98
Preliminary
Minor corrections
Stamp App Circuits added
Copyright
The information contained in this data sheet is the property of Milford Instruments/nikam electronics
and copyright is vested in them with all rights reserved. Under copyright law this documentation may
not be copied, photocopied, reproduced, translated or reduced to any electronic medium or machine
readable form in whole or in part without the written consent of Milford Instruments/nikam
electronics.
Disclaimer
Milford Instruments has an on going policy to improve the performance and reliability of their
products, we therefore reserve the right to make changes without notice. The information contained in
this data sheet is believed to be accurate however we do not assume any responsibility for errors nor
any liability arising from the application or use of any product or circuit described herein. This data
sheet neither states nor implies warranty of any kind, including fitness for any particular application.
Milford Instruments-England
Tel: +44 1977 683665, Fax: +44 1977 681465
[email protected]; www.milinst.demon.co.uk
End of Data Sheet
NKM2401 Issue 1.1 Nov-98
Milford Instruments/nikam electronics www.milinst.demon.co.uk
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