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The article presented focus on the practical aspect of USART
and RS232. For technical details, I would strongly recommend the
following website from beyond logic,

http://www.beyondlogic.org/serial/serial.htm

As what I experience, the data transmission length of the
cable can determine by many factors. The factors include the following,

– data
transmission speed

– quality of the
cable, noise (unwanted signal)

– transmitted
voltage

– receiver
sensitivity

– etc…

We have to remember that electronics are still analog in
nature. Communication distance using RS232 can be increase further if
the cable is of better quality, a shield or coaxial cable for example.

The most significant factor is still the data transmission
speed. The following is a reference that I found in one website
regarding the relationship between data baud rate and cable length.

http://www.lammertbies.nl/comm/info/RS-232_specs.html

The transmission cable should be twisted as a pair for your
+ve & -ve (or ground/reference signal). The reason for having it
twisted is to ensure that the pair of wire is as close to each other as
possible. Why? This is because the signal energy (or refer as
integrity) is contained between the +ve & -ve wire. Any gap between
the two wire can result in signal distortion (losses). The gap
represent a change in the cable impedance (capacitance/inductance)
affecting the signal integrity on the wire. Electromagnetic, it is
about how the field interact with one between the gap.

I had once wiring up two RS232 communication line without
using twisted wire. In order to save the trouble to lay another set of
cable, I tried to squeeze the two RS232 line to the cable. It end up
with a lot of communication problem. The data I send on com1 is able to
trigger the devices connected to com2. The signal on com1 is actually
coupled over to com2, causing com2 to think that some data is being
received. The data is the corrupted version of the data from com1.

The higher the frequency, the worst is gets. This is also why
our network CAT5e CAT6 cable are all twisted inside, protected by
aluminum foil shield. No sharp bending should be allow, as this will
cause the twisted pair to open up a gap in between. A typical cable
bending radius as specify in the manufacturer datasheet is about
25-50cm. All this details comes into the picture when your
communication speed is high. I see many contractor laying the network
cable without any of these consideration. The effect is negligible, for
low speed communication. Most of us might not even realized it too,
because minor transmission error is already resolved through the TCP/IP
protocol. A coaxial cable is a better form of cable structure to
contain the integrity of the signal. The energy is contain on the
dielectric, between the inner conductor core and the outer wire mesh.

DB9
male socket on DTE (data terminal equipment), example: a computer.

DB9
female plug on DCE (data communication equipment), example: a modem.

RS-232D is defined as RS232 being
terminated with the RJ45 plug. They are used on
cisco network switch equipment for command control input, and also on
RS232 to Ethernet server for Lantronix products. The advantage of RJ45
compare to DB9 is the size. More ports can be connected to the
equipment with a much smaller panel interface.

Loop Back RS232 Connector

–
Short
Pin 2 to Pin 3 (if no hardware control)

–
see the
following diagram, Loop Back Plug (for hardware control RS232
communication)

MAX232 IC and schematics

Click here for MAX232 datasheet

MAX232 circuit layout
reference.

Note: Input pin
11, 13 can be left unconnected. There is a internal pull-up resistor,
pulling pin 11 to 5V and pin 13 to 0V

IC chip maker has come up with the integrated circuit for
interfacing RS232 with TTL logic (5V for logic 1, 0V for logic 0),
making the interfacing work very simple. MAX232 is one of the many IC
in the market which helps to convert between RS232 -/+10V and TTL +/-
5V. It is a simple voltage level converter in short. The charge pump
design allows the circuit to generate +/-10V from a 5V supply, with the
help from the four capacitor. With charge pump to double up the supply
voltage for RS232 transmitter, there is no need to design a power
supply for +/-10V.

The diagram on the left shows the schematic of the MAX232 IC
circuit. It consist of only 4x 1uF 16V electrolytic capacitor, and the
MAX232 IC itself. It is that simple. I have include a layout which I
always use for PC to PIC16F877a microcontroller, RS232 interface.

MAX232 alternative:
LTC1386

MAX232 (3.3V version):
MAX3232

– Software programming using serial com.

– Window OS HyperTerminal (from WinXp)
	For WinXP user, Click>> Start>> Programs>> Accessories >> Communication>> HyperTerminal.exe
Docklight_Scripting.zip
Virtual Serial Ports Emulator.zip (ETERLOGIC.COM) emulator allows you to Connector – create a virtual com port which can be opened twice. Allows two application program to communicate to each other via the same serial port number. Data Splitter – create a virtual com port which allows multiple application to share a single existing com port. Pair – Create 2x new virtual com port which is cross connected to each other. (A null cable). Allows two application program to communicate with each other. Mapper – Remap a physical com port to another com port number. Useful for old software which does not allow com port number to be changed. TcpServer – convert a physical com port to a TCP port as a server, so that multiple client can be connected and access to the physical com port. TcpClient – convert a physical com port to a TCP port as a client. If connection is lost, this client will auto reconnect to the server. Serial Redirector – connects up between to com port. (A null cable) UDP Manager – convert a physical com port to a UDP port as a server, so that multiple client can be connected and access to the physical com port. Bridge – Connects up two data stream. Spy – VSPE device to spy on a data stream.Other Software Tools – BillSerialMonitor.zip
– EZTerminal.exe
– RS232HD30.exe
– RS232 data logger.exe
– COM Inspector (detects serial communication settings)

reference source:

http://www.simplecomtools.com

Click
onto the text field on the bottom of the apps. Key in and ascii data,
and click on the send button on the right (logo of an arrow pointing
right). Immediately after you click on the send button, you should be
able to see on the display screen, the same text that you
have sent. If you disconnect the loop back connection, you will
not be able to see the text that you have sent.

This apps is
good. It can display the data in hex, which is used very often in
hardware development work. There are also memory which allows you to
save frequet sent data.

MAX485 schematics

MAX485 IC and schematics

Click
here for MAX485/MAX488 datasheet

Click
here for MAX3485/MAX3488 datasheet

MAX488 IC and schematics

RS485 is a half duplex configuration. RS485 using only 2 wire
to communicate in both direction. With only two wire, it means that
when one side is transmitting, the other side of the communication line
will be receiving. Both side cannot be transmitting at the same time.
For RS485 transceiver, use MAX485 or MAX3485. They have the same pin
out except that MAX485 uses 5V supply, MAX3485 uses 3.3V supply.

RS422 can be connected to work with RS485 to either receive or
transmit date, but not both. RS422 can be wired directly using a pair
of wire, +ve to +ve, -ve to -ve terminal. For RS422 transceiver, use
MAX488 or MAX3488. They have the same pin out except that MAX488 uses
5V supply, MAX3488 uses 3.3V supply.

MAX488 alternative (pin8+,pin7-pin6+,pin5-,5V, version):

twisted pair wire (with shielding)

twisted pair wire (without any shielding)

Notice the gap between a normal pair of wires. By twisting the
cable, the gap can be reduce, therefore minimizing the interference.

Unlike RS232 signal, RS485 implements
differential voltage as the physical signal to communication. More
information on the physical RS485 signal is illustrated in the next
section. The cable for RS485 or RS422 works as a pair to transmit and
another pair to receive. This is important, because it means that the
cable need to work as a pair in order to transmit the signal properly.
Proper type of cable is required.

The picture on left is a twisted pair wire. It is
recommended to use twisted pair cable to transmit RS485/RS422 signal.
The reason being twisted wire can help minimize the gap between the 2
wire, therefore minimizing interference and signal distortion. This can
be explain through the electromagnetic theory. The gap or loop between
the two wire actually acts as the antenna intercepting nearby signal,
as well as coupling it’s own signal to wire next to it. These results
in distortion in the signal, and ultimately reduces the data rate you
can transmit over the wire pair. The twisted cable also maintain a
distance, make it difficult for another wire to slip in between the
wire pair. For further immunity against interference, the twisted pair
is wrapped with aluminum foil acting as a shield.

http://www.lammertbies.nl/comm/info/RS-485.html

Note that you should deploy twisted pair on a
signal wire pair. You can consider one wire as signal while the other
one as reference or ground. If the twisted group of wires contains more
than one signal, it will make the interference worst.

Attempting to twist the bunch of wires for RS232
will not improve the performance. The wires for transmit (TX) and
receive (RX) will interfere. It is possible that you might receive some
rubbish data immediately after you perform a data transmit. This
problem will be more obvious when you send the data at very high baud
rate and longer wiring distance, eg 115.2kps at 20 meters. It is due to
the signal on the TX wire being coupled onto the RX wire, triggering
the device, thinking that there are incoming data on the opposite line.

For RS232, you might like to twist between TX(pin
3)/ Gnd(pin 5), and another twisted pair RX(pin 2)/Gnd(pin 5) to
improve the transmission. There is only a common pin, therefore the two
wire will have to share the pin 5 ground.

The signal level from the output of MAX485 IC depends on the
load from the communication line. Typically the open circuit output of
the MAX485 IC with/without a 120Ω
termination resistor has ?V1 = 5Vdc, ?V0 = 0.8Vdc. When the line
includes the inline resistors and the pull down/up resistor for the
RS485 bus,  ?V1 = 3.2Vdc, ?V0 = 0.6Vdc. These open circuit reading
is taken from the output of MAX485 IC using an oscilloscope. Some note
is observed when attempt to watch the RS485 communication from the
oscilloscope. When the probe get into contact with the signal, the
communication fails. The receiver device is able to decode the signal.
It is believe that the ground reference of the probe might be connected
to earth and will affect RS485 signal

The baud rate setting is set at 9600bps, data bits of 8, no
parity bit, 1 stop bit.

The top display the actual RS232 signal from a computer
system’s serial com port. This signal is tapped from the TX (+ve
connected) and ground line (-ve connected). It has loaded input from
the converter device. The higher voltage level represents logic 0 at
about 6Vdc, while the lower voltage level is a logic 1 at about -7Vdc.
When there is no transmission, the signal idle at -7Vdc.

The bottom display the RS485 differential signal converted
from the RS232 signal using a converter SNA10A. This signal is tapped
from the A (+ve connected) and B terminal (-ve connected). This is an
open load signal from the output of the converter device. The higher
voltage level represents logic 1 at about +4Vdc, while the lower
voltage level represents logic 0 at about -4Vdc. When there is no
transmission, the signal idle at about 1V.

References:

http://en.wikipedia.org/wiki/List_of_well-known_ports_(computing)

The Ethernet looks very remote to me when I first try to
understand it. It is very complex. After a few years of experience, I
slowly gain enough confident to talk about this type of communication.

In fact, Ethernet is another form of serial communication. The
hardware aspect is similar to RS485, with pin 1 & 2 handling the
transmission of the serial data, while pin 3 & 6 is for the
receiving of serial data. I have not find any concrete information, but
I imagine it closely to a RS422. One pair of wire for TX, the another
pair for RX. The physical signal for RS422 is the same as RS485. The
signal are interchangeable. RS422 is a duplex while RS485 is a half
duplex communication line (see above for further information).

The beautiful part of Ethernet is on the streaming bytes
riding on the serial communication line. This stream of bytes is also
define as the data packets or protocol. It is the data protocol that
make Ethernet so special. Which is why there is so little information
on the physical aspect on the Ethernet. Ethernet is all about protocol.
Protocol consist of the data and the header. The header contains the
MAC, IP, PORT and other information which will helps the data packet to
be routed to the correct destination. You can imagine a letter to be
delivered. Letter contains address which helps the post man to deliver
the letter.

What we typical see on the cover of a letter:

Att: David

myCompany Pte Lte
(business registration number J123456789)

Street 3, Blk 3,
#03-33.

myCompany denote the company name which can be quite unique.
You can imagine the MAC address as the company name or as a business
registration number. The MAC address is a set of number (6 bytes, 0x11
0xAA 0x22 0xBB 0x99 0xFF) which uniquely identify the electronics
hardware Ethernet device. It is the basic number the hardware will
have, which identify itself from the rest from other gadgets. Most
computer have at least one network card/adaptor. Each card is a unique
communication device, and therefore have it’s own MAC address. If a PC
has 3 network card installed, it would have 3 different MAC address.
MAC address is the 6 bytes ID of the Ethernet hardware.

Next comes the IP address. You can think of it as your home
address. The number for IPv4 (IP version 4) is 4 bytes long
(192.168.1.255). Unlike a MAC address, IP address can be configured by
the user/programmer. You can imagine that while the company name
remains no change, no matter where it shifted it’s address. IP address
can be changed. Like a letter, the IP address will allows your data
packet to be delivered to the correct place. Each Ethernet hardware
(network card) will have it’s own MAC address. Each MAC address will be
assigned the IP address. Your company will be identified by it’s unique
number and the location address. A typical computer installed with 1
Ethernet card, can be identified with it’s MAC and IP address.

PORT is quite tricky to me at first. What is exactly a port.
You can relate it to the name of the person on our letter example. The
letter may be delivered to the correct address, but it does not
indicate who should receive the letter. Port identify the person that
should receive the data packet. In our computer example PORT is a
number which defines the application software that receive the data.
When you open up your web browser, the browser software will open it’s
port number 80. Any incoming data packet indicating port 80, will be
passed to the browser software. The browser software will then render
the data onto the screen for you to read. Similarly other network
application works in this way. Some common application’s protocol and
it’s port number are as follows,

IPv6 (Internet Protocol version 6)

TCP (TCP Transmission Control Protocol)

UDP (User Datagram Protocol)

ICMP (Internet Control Message Protocol)

The data packet route to the correct IP address, after which
it will check if it arrive to the correct MAC ID. When the packet
reaches into the computer, the packet is further route to the port,
where the appropriate application software will read and further
interpret the data packet.

Ethernet has more features, but the basic concept is still as
easy to understand as a RS232 serial communication.

What we have discuss so far is only the protocol on IPv4
header. Protocol or header is just bytes of information that describe
about the data it carries. The data itself may also contains its own
header which interpret another data within. It is like layers and
layers of onion skin. Just as you peel off the header for the data, you
notice another header to peel. Layer after layer, we finally got our
data.

The first layer is IPv4 (still quite common in this era dated:
July 2009) or IPv6 header (new protocol).

Going deeper, we have another layer typically TCP or UDP. They
defines the manner, the data is being exchanged across the
communication channel. TCP/IP protocol means that the data is
transported using the IP and TCP header. Two layer actually.

References:

– http://www.fatpipe.org/~mjb/Drawings/

– rfc792 (ICMP protocol)

– icmp protocol

– an-139 (how to route ethernet PCB)

– all about Ethernet

Loop
Back Ethernet Connector

–
Short
Pin 1 to Pin 3

–
Short
Pin 2 to Pin 6

After so much theory, let’s talk more
physical stuff on Ethernet. As shown on the bottom and left, these are
the typical wiring on the network. Straight cable between the network
equipment and PC, and also cross cable between PC to PC or equipment to
equipment.

Ethernet Pin Out from your computer device

I have been looking for hours a confirmation
on the network pin 1 & 2 (TX±).
Whether the pin out is referring to a DTE (a computer) or DCE (network
switch/hub). Most website indicates the TX RX pin out scheme, but did
not indicate whether the description is for a DTE or DCE. After
searching for so long, I finally found it.

I
would like to give credit to this website for providing the information.

http://www.duxcw.com/digest/Howto/network/cable/cable5.htm

CAT7, SSTP, individual shielding, solid core	1 – Jacket 2 – Shield-braid 3 – Shield-foil 4 – Solid twisted pair 5 – Drain wire
CAT6a, STP, individual shielding, solid core	1 – Jacket 2 – Shield-foil 3 – Drain wire 4 – Solid twisted pair
CAT5e, FTP, shielding, solid core	1 – Jacket 2 – Shield – foil 3 – Drain wire 4 – Protective skin 5 – Solid twisted pair
CAT5, UTP, no shielding, solid core	1 – Jacket 2 – Solid twisted pair

Cable manufacturers

About Network
cable

Notice that a
network cable contains 4 pair of twisted cable, and the pair is
differentiated by the 4 sets of color pair. If you remember the RS485
wiring as mention in the earlier section, these twisted wire will look
very familiar to you again. The twisted wire provides a better
protection against possible interference. Notice how the twisted pair
is assigned to the RX signal± and the
other pair assigned to TX signal±.

There are many
type of cat cable to choose from. Depending on your deployment need,
these are some of the selection that you may have to consider.

– CAT5e, CAT6a,
CAT7, CAT8

– FTP, SSTP/SFTP,
STP/USTP, UTP

– Solid or
Stranded wire core

– Indoor or
Outdoor

– AWG wire size

Some people may
refer the network cable as RJ45, which is not correct. RJ45 is actually
the name of the plug. Another name for the plug is 8P8C connectors.

The twisted pair
cabling standard is refer as the CAT standard. The term is typically
being referred to when selecting the type of network cable. The CAT
define the signal frequency that the cable is able to carry for a
distance of 100m. High frequency signal gets filter away as the cable
becomes longer, which also means that the data rate will be reduced.
This means that a short CAT5e cable is able to transmit as fast as a
longer CAT7 cable. Therefore 100m is a normalize distance to compare
between the cable quality.

Once you got the hang out of it, it is fairly managable.

SDA—____—___—___——————————__________–
SCL—-____—___—___—___—___—___—___—___—___–___
……^….^…..^…..^…..^…..^…..^…..^…..^…..^
……1….2…..3…..4…..5…..6…..7…..8…..9…..10

The command/data that was sent to MCP79410 are as follows (Write data to EEPROM),
– start
– 0xAE (write command to MCP79410)
– 0x00 (address for the following data)
– 0xAA (data to be written into the address)
– end

1- START signal. SCL is held high when SDA perform a negative edge signal (logic high to low).

2- Logic 1 is sent. The SDA is loaded with logic 1 just before SCL is clock positive edge.

SDA signal/data line should remain stable during the HIGH period of the clock pulse. Any changes during the HIGH clock pulse duration are inteprested as control signals (START or STOP command).

3- Logic 0 is sent. (at the +ve edge of SCL)
4- Logic 1 is sent.
5- Logic 0 is sent.
6- Logic 1 is sent.
7- Logic 1 is sent.
8- Logic 1 is sent.
9- Logic 0 is sent.
10- The SDA and SCL lines are release at this moment in time. SCL
returns to logic high, but SDA is still at logic low even when both
lines are release. This shows that the slave device is actually holding
the SDA line down. The holding of this SDA line is the slave
acknowledging the data received. This is important because it shows
that the slave is responsing to the command send. The slave is working.
The SDA line is release shortly.

Program hang when executing a read command.

Problem was resolved. The microcontroller was able to read the data properly from MCP79410.

After resolving the problem, the micrcontroller was able to read from MCP79410 successfully.