Manchester encoding using RS-232
Author: Adrian Mills
Date: 30-03-2009
www.summitelectronics.co.uk
Rev 2.1
Download this article Manchester_encoding_using_RS232.pdf
Introduction
The following takes a look at transporting Manchester
encoded data [1] using RS-232, over a radio frequency
(RF) link.
Those familiar with RS-232 / RS-485 etc will be aware
that when data is sent through a cable to remote equipment
it gets there with good reliability. If error detection
is required, this can take the form of a checksum as
part of a packet-based protocol. However, should you
want to send the same data over an RF link a number
of things have to be taken into consideration. Most
notably is the need to maintain a zero DC component
in the serial bit stream.
RS-232 / RS-485 is generally encoded and decoded using
a universal asynchronous receiver and transmitter (UART).
It is the UART we shall be using to transport Manchester
encoded data.
Manchester encoding
Manchester encoding (also known as Biphase Code) is
a synchronous clock encoding technique used to encode
the clock and data of a synchronous bit stream. In this
technique, the actual binary data to be transmitted
over the cable or RF link are not sent as a sequence
of logic 1's and 0's as in RS-232 (known technically
as Non Return to Zero (NRZ)). Instead, the bits are
translated into a slightly different format that has
a number of advantages over using straight binary encoding
(ie NRZ).
The main advantages of using Manchester encoding are:
1. Serial bit stream has a DC component of zero
2. Error detection is simple to implement
In general, when transmitting serial data to a radio
receiver, a DC component of zero must be maintained
(over a finite time). This is so the demodulator in
the receiver can properly interpret (discriminate) the
received data as 1's and 0's. Manchester encoding allows
us to do this.
Manchester encoding follows the rules:
1. If the original data is a Logic 0, the Manchester
code is: 0 to 1 (upward transition at bit centre)
2. If the original data is a Logic 1, the Manchester
code is: 1 to 0 (downward transition at bit centre)
It can be seen that there are two bits of Manchester
encoded data for each bit of original data. The penalty
for doing this is Manchester encoded data requires more
bandwidth than NRZ encoding.
Example of Manchester Encoding:
The pattern of 8-bits: " 0 1 1 1 1 0 0 1 "
encodes to " 01 10 10 10 10 01 01 10". The
least significant bit (LSB) is shown left most. Figure
1 shows a drawing of the encoded pattern as a waveform
in the time domain.
Figure 1 Waveform for the Manchester encoded bit
stream carrying the sequence 01111001
Alternatively the above can be represented as two 4-bit
nibbles
1. The pattern of bits " 0 1 1 1 " encodes
to " 01 10 10 10 " (see Figure 2a)
2. The pattern of bits " 1 0 0 1 " encodes
to " 10 01 01 10 " (see Figure 2b)
Figure 2 Waveforms for the Manchester encoded bit
stream carrying the sequence 0111 & 1001
This step leads us to sending 8-bits of original data
as two bytes of Manchester code.
Illegal codes
Given that a 0 encodes to 01 and 1 encodes to 10, it
follows that Manchester codes 00 and 11 are illegal
sequences. These illegal codes are used to error check
the data.
It is also possible to have the 4-bit illegal code
" 00 00 11 11 ", which would be an unlikely
occurrence. This 'illegal code' has the property of
having a DC component of zero and has no 1 to 0 transitions.
We can use this 'illegal code' (" 00 00 11 11")
as a unique start/stop pattern to identify the boundaries
of our Manchester encoded bit stream or data frame (see
Figure 3). It will also serve to initialise the radio
receivers' demodulator and synchronise the receiving
UART.
Figure 3 Unique start/stop pattern
RS-232 Wave Form
In RS-232, user data is sent as a number of frames
each a synchronous time-series of bits, typically eight
bits in length. The diagram in Figure 4, where B0 is
data LSB, shows the expected waveform from the UART
when using eight data bit, no parity and two stop bit
format, .
We can use RS-232 as a carrier for Manchester encoding
provided that RS-232 characters (bytes) are transmitted
end-to-end with no inter character delay. RS-232 has
a start bit, logical 0, and a stop bit, logical 1 which
have a DC component of zero. In our real life application
2-stop bits may be all that's available to us. What
2-stop bits means is that the DC component is no longer
zero, so here we make a compromise and say the DC component
is near zero (i.e. the extra stop bit has a 1/10 effect
on the DC component). However this can be compensated
for when the transmitter supports 9-bit data mode by
setting the ninth bit to zero and hence cancelling out
the second stop bit.
Figure 4 RS-232 Logic wave form
RS232 carries Manchester encoding
All that's required is to insert Manchester encoded
data into the RS-232 data bits. There are 4 original
data bits in each RS-232 byte.
Figure 5 shows the start/stop pattern inserted into
an RS-232 frame. This pattern is chosen for its property
of not having any 1 to 0 transitions. This means there
can be only one real start bit for the UART to synchronise
to.
Figure 5 Start/Stop pattern
Figure 6 a) and b) show the sequences from Figure 2
inserted into two RS-232 frames.
Figure 6 RS-232 frames for the Manchester
encoded sequence a) 0111 & b) 1001
Finally Figure 7 shows the waveform of the combined
frames from Figure 5 & Figure 6.
Figure 7 Combined waveforms
Application
In this application it is assumed the 'C' functions
putc() and getc() directly output or input TTL levels
from a radio transmitter or receiver respectively to
a UART.
Start data frame
The data frame is started by transmitting a preamble
of start/stop patterns (SS) to the radio receiver. The
number of SS patterns depends on the characteristics
of the radio receiver.
In general, enough SS patterns are needed to:
1. Initialise the receiver demodulator.
2. Allow the RS-232 decoder to synchronise to the start
bits of each RS-232 byte.*
3. Indicate to the software communications handler that
an SS pattern has been received and data is expected
to follow.
* An additional stop bit is inserted at the end of
each SS pattern to allow for software implemented RS-232
decoders (UART) to 'catch up' with the start bit. This
may be omitted for hardware UARTs.
The unique SS pattern " 00001111 " (LSB left
most) is equivalent to the hex code 0xF0 (LSB right
most). The following 'C' function SendSS() will send
a preamble of four SS patterns.
SendSS( void ) {
{
putc(0xF0);
delay_us(500); // *
putc(0xF0);
delay_us(500); // *
putc(0xF0);
delay_us(500); // *
putc(0xF0);
delay_us(500); // *
};
* Additional 500us stop bit based on a bit rate of
2000 bits per second (bps).
The received stream will look similar to the waveform
shown in Figure 8 below.
Figure 8 Wave form of start/stop patterns
Note: In a real-life radio receiver, the absence of
a signal from the transmitter will generally result
in a received bit stream of noise (i.e. random 1's and
0's in no recognisable pattern). It's not until the
transmitter is sending a signal that the receiver can
'discriminate' the bit stream. This doesn't really matter
to us, as our Manchester decoder will reject any corrupted
data.
Send data
Let's for example send the data byte 0x41. This is
encoded into two Manchester encoded RS-232 bytes.
The least significant nibble is transmitted first,
working out long hand:
1. 0x41 represented as an LSB first bit stream is: "
1000 0010 "
2. Lower nibble " 1000 " Manchester encodes
to: " 10 01 01 01 " (i.e. 0xA9)
3. Upper nibble " 0010 " Manchester encodes
to: " 01 01 10 01 " (i.e. 0x9A)
putc(0xA9); // lower nibble
putc(0x9A); // upper nibble
In real life this would be carried out by a routine
similar to the one below:
void SendData(BYTE txbyte)
{
int i,j,b,me;
b = txbyte;
for (i=0; i<2; i++) {
me = 0; // manchester encoded txbyte
for (j=0 ; j<4; j++) {
me >>=2;
if (bit_test(b,0) )
me |= 0b01000000; // 1->0
else
me |= 0b10000000; // 0->1
b >>=1;
}
putc(me);
}
}
More data can be sent if required.
End data frame
To end a data frame, a series of 0xF0's are transmitted
using the function SendSS().
It could be argued that this is not necessary since
the preamble in the next data frame will reset the communications
handler. However the handler can use it as an indicator
that the data frame is complete.
Decoding data
Converting Manchester encoded data back to original
data is straight forward and error checking may be carried
out at the same time. Since data arrives as 4 pairs
of Manchester encoded data a simple shift and test algorithm
can be implemented to retrieve the original data.
The routine below decodes four bit-pairs to one nibble
of original data:
BYTE DECODE_DATA(BYTE encoded)
{
BYTE i,dec,enc,pattern;
enc = encoded;
if (enc == 0xf0) // start/end condition encountered
return 0xf0;
dec = 0;
for (i=0; i<4; i++) {
dec >>=1;
pattern = enc & 0b11;
if (pattern == 0b01) // 1
bit_set(dec,3);
else if (pattern == 0b10)
bit_clear(dec,3); // 0
else
return 0xff; // illegal code
enc >>=2;
}
return dec;
}
Other considerations
When software generated RS232 is used for receiving
serial data, it is important that the latency between
the start bit and the execution of getc() is kept to
a minimum. This is not an issue if getc() is waiting
for serial data, but becomes important when kbhit()
is used to determine the presence of a start bit.
Typically kbhit() is used in a tight loop (see below).
The latency is the time round the loop.
while(1) {
if ( kbhit() )
data = getc();
...
some other code executed here
...
}
For example if the bit rate is 2000bps, one bit time
is therefore 500us. A latency of no more than 50us is
suggested. Higher bit rates require less latency.
Conclusion
Manchester encoding is suitable for bit streams in
radio communications and can be transported using standard
RS232 functions. It is simple to encode, decode and
has good error detection negating the need for 'check-summed'
data. Manchester encoding is practical and easy to implement
on Microchip PIC devices.
References
1. Gorry Fairhurst, "Manchester Encoding";
www.erg.abdn.ac.uk/users/gorry/course/phy-pages/man.html
Bibliography
1. RF Solutions Ltd (www.rfsolutions.co.uk),
"AM Super Regenerative Receivers", Data sheet
DS016.
2. RF Solutions Ltd (www.rfsolutions.co.uk),
"AM Hybrid Transmitter", Data sheet DS013.
3. Custom Computer Services Inc (www.ccsinfo.com),
"C Compiler Reference Manual", July 2003.
4. QuickBuilder (www.quickbuilder.co.uk),
"AM RF Transponder"; www.quickbuilder.co.uk/qb/libs/comms.htm.
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