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'[PIC] RS232, USART, and COM'
2005\05\12@203602 by Kenasw

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Dear all,
What would be the difference between rs232, usart, and pc com port.
I understand the the pc com port uses a usart, so why it is called com
or rs232 and not usart?
tnx

2005\05\12@205041 by Dave VanHorn

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At 07:35 PM 5/12/2005, spam_OUTkenaswTakeThisOuTspambtinternet.com wrote:
>Dear all,
>What would be the difference between rs232, usart, and pc com port.


What is the difference between a chicken, an egg, and an omlette?

A PC may use a UART or A USART as it's COM port, which then passes
through level shifters to comply with the RS-232 standard.

2005\05\12@205225 by John J. McDonough

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----- Original Message -----
From: <.....kenaswKILLspamspam@spam@btinternet.com>
Subject: [PIC] RS232, USART, and COM


> What would be the difference between rs232, usart, and pc com port.
> I understand the the pc com port uses a usart, so why it is called com
> or rs232 and not usart?

Actually, the PC com port might not use a usart, more likely it is a uart.

RS232 is basically an electrical interface.  It specifies the mark and space
voltages, and the meaning of a number of signals.  It can be used with
synchronous ports, but on the PC, the port is an asynchronous port.

'com' refers to the DOS name for the device, com standing for
communications.  It's easier to say COM port than "RS-232 asynchronous
serial port".

A usart is a universal synchronous/asynchronous device.  Typically, it's
output levels are either TTL or CMOS levels.  These are quite a bit
different than RS232 levels.  However, it's usart-ness isn't defined by the
voltages, but rather, by it's ability to take 8 bit data and send it one bit
at a time, or accept one bit at a time and stuff it into groups of 8 (or
so).  And it gets the extra S that a UART doesn't have by being able to send
and receive the data without the help of start and stop bits.

A usart, driven in asynchronous mode can communicate with a PC COM port
through level converters that take the usart voltages and translate them to
RS232 voltages.  Maxxim's MAX232 is probably the stereotypical RS-232 level
converter, but there are others, and transistors will do the job just fine,
too.

--McD




2005\05\13@055506 by Kenasw

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> Actually, the PC com port might not use a usart, more likely it is a uart.

Sorry, I mean UART, not USART!!

>
> RS232 is basically an electrical interface.  It specifies the mark and
space
> voltages, and the meaning of a number of signals.  It can be used with
> synchronous ports, but on the PC, the port is an asynchronous port.

Why does pc com port use RS232, is there a standard that requires this.
On the other hand, why do the pics use uart without rs232,  is this
a pic problem which will make it more complex to connect to pc?

>
> 'com' refers to the DOS name for the device, com standing for
> communications.  It's easier to say COM port than "RS-232 asynchronous
> serial port".
>
> A usart is a universal synchronous/asynchronous device.  Typically, it's
> output levels are either TTL or CMOS levels.  These are quite a bit
> different than RS232 levels.  However, it's usart-ness isn't defined by
the
> voltages, but rather, by it's ability to take 8 bit data and send it one
bit
> at a time, or accept one bit at a time and stuff it into groups of 8 (or
> so).  And it gets the extra S that a UART doesn't have by being able to
send
> and receive the data without the help of start and stop bits.
>
> A usart, driven in asynchronous mode can communicate with a PC COM port
> through level converters that take the usart voltages and translate them
to
> RS232 voltages.  Maxxim's MAX232 is probably the stereotypical RS-232
level
> converter, but there are others, and transistors will do the job just
fine,
> too.
>
> --McD
>

2005\05\13@060406 by Lindy Mayfield

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There is a standard called RS232.  
http://www.camiresearch.com/Data_Com_Basics/RS232_standard.html

Google gives tons of information when you type "rs232 standard"

(-:

> {Original Message removed}

2005\05\13@061911 by Jan-Erik Soderholm

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kenasw@btinternet.com wrote :

> Why does pc com port use RS232,...

To be able to communicate with *other* equipment with
serial (RS232) ports, of course.

> is there a standard that requires this.

Yes. And guess what,  the standard is called "RS232"...

> On the other hand, why do the pics use uart without rs232,  is this
> a pic problem which will make it more complex to connect to pc?

Very few (if any at all) uart's has RS232 builtin. Today the RS232
interfacing is done with chips like the MAX232. Earlier (say more
then 10 years ago), one had a separate +/- 15V power and simple
"RS232-drivers". The MAX232 generates the +/- 10-15 V internaly.

But why not just google around on the net or check your local
library. There have been a lot of books written on serial
communication over the years. I'm not sure of how old the
RS232 standard is, but at least as old as I've been in the
business, about 25 years. So it pre-dates most of the
technology we use today.

Jan-Erik.



2005\05\13@062240 by Alan B. Pearce

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>Why does pc com port use RS232, is there a standard that requires this.

Yes, the RS232 standard. This is a standard originally set up for computer
equipment to talk to modem equipment, and has become the defacto voltage
interface specification for serial interfaces that connect outside computer
cabinets. There are other standards, but this is the generally the
commonest.

>On the other hand, why do the pics use uart without rs232,  is this
>a pic problem which will make it more complex to connect to pc?

So does the UART inside a PC. There are other chips that do the conversion
to RS232 voltage levels, just like using a MAX232 with a PIC.

2005\05\13@072348 by olin_piclist

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kenasw@btinternet.com wrote:
> What would be the difference between rs232, usart, and pc com port.

RS-232 is the name of the "standard" (RS stands for "recommended standard")
that defines the low level electrical details.  USART stands for Universal
Synchronous Asynchronous Receiver Transmitter, and is the name of the PIC
peripheral intended to make it easy to ultimately communicate via RS-232.  A
PC COM port is a physical implementation of RS-232 on a PC.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\05\13@081121 by Russell McMahon

face
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> Why does pc com port use RS232, is there a standard that requires
> this.
> On the other hand, why do the pics use uart without rs232,  is this
> a pic problem which will make it more complex to connect to pc?

RS232 is intended for intercommunication between pieces of equipment
over distances liable to be found in a computer room. The interface is
intended to be reasonably robust and reasonably well protected against
real world conditions.

Inter-Integrated circuit interfacing is done at a level that makes
most sense for the technology in use, whether TTL, CMOS, ECL or
whatever. The interfaces are (hopefully) robust enough for their
intended on-PCB environment. It would make little sense to invariably
engineer them for "real world" environments - especially so when many
are used only within the PCB environment.

When there is a need to transfer signals between real-world and PCB
environments suitably engineered interfaces are provided. These may be
simple and passive and use only eg resistors and capacitors or may be
more complex and may use special ICs or isolation equipment. When it
is necessary for a PIC (or any other processor) to talk RS232 an RS232
interface is added. similarly RS422, 20 mA current loop or any other
specific standard interface. If a PIC was equipped with an RS232
interface and one wanted to use RS422 it would be even more annoying
than it is now to do so.

Using the right thing in the right place and knowing which is which is
part of what engineering is about.



       Russell McMahon



2005\05\13@085011 by Kenasw

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----- Original Message -----
From: "Russell McMahon" <apptechspamKILLspamparadise.net.nz>
To: "Microcontroller discussion list - Public." <.....piclistKILLspamspam.....mit.edu>
Sent: Friday, May 13, 2005 1:07 PM
Subject: Re: [PIC] RS232, USART, and COM


{Quote hidden}

I do understand now, i.e. RS232 is usefull to extend the u(s)art interface
over a long distance, i.e. 1 or 2 or 3 meters, but when our signals travel
only a few centimeters on a pcb, we do not need rs232 and in fact rs232
will simply be a hinderance.

tnx

>
>
>         Russell McMahon
>
>
>
> --

2005\05\13@085138 by Kenasw

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----- Original Message -----
From: "Alan B. Pearce" <EraseMEA.B.Pearcespam_OUTspamTakeThisOuTrl.ac.uk>
To: "Microcontroller discussion list - Public." <piclistspamspam_OUTmit.edu>
Sent: Friday, May 13, 2005 11:22 AM
Subject: Re: [PIC] RS232, USART, and COM


> >Why does pc com port use RS232, is there a standard that requires this.
>
> Yes, the RS232 standard. This is a standard originally set up for computer
> equipment to talk to modem equipment, and has become the defacto voltage
> interface specification for serial interfaces that connect outside
computer
> cabinets. There are other standards, but this is the generally the
> commonest.

Yes, you are right, tnx.

>
> >On the other hand, why do the pics use uart without rs232,  is this
> >a pic problem which will make it more complex to connect to pc?
>
> So does the UART inside a PC. There are other chips that do the conversion
> to RS232 voltage levels, just like using a MAX232 with a PIC.
>
> --

2005\05\13@085445 by Kenasw

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----- Original Message -----
From: "Lindy Mayfield" <@spam@Lindy.MayfieldKILLspamspameur.sas.com>
To: "Microcontroller discussion list - Public." <KILLspampiclistKILLspamspammit.edu>
Sent: Friday, May 13, 2005 11:03 AM
Subject: RE: [PIC] RS232, USART, and COM


> There is a standard called RS232.
> http://www.camiresearch.com/Data_Com_Basics/RS232_standard.html

It is good, tnx

>
> Google gives tons of information when you type "rs232 standard"
>
> (-:
>
> > {Original Message removed}

2005\05\13@093420 by Dave VanHorn

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At 05:19 AM 5/13/2005, Jan-Erik Soderholm wrote:
>RemoveMEkenaswTakeThisOuTspambtinternet.com wrote :
>
> > Why does pc com port use RS232,...
>
>To be able to communicate with *other* equipment with
>serial (RS232) ports, of course.
>
> > is there a standard that requires this.
>
>Yes. And guess what,  the standard is called "RS232"...

Careful.. Nothing requires that the PC use RS-232, other than the
fact that a lot of other equipment uses it too.
232 was originally intended for industrial equipment in very noisy
environments, like a steel mill.
It only went to 9600 baud IIRC, and 25' distance.

232 was used by most of the early computers, because it was
relatively simple, and companies like Zilog made the SIO-0 which was
a two channel UART, and a very nice one at that.
The PC's parallel port grew more or less out of an intel PIA chip
(parallel interface adaptor), though similar implementations had been
done for years on the Zilog PIO chip,. and other similar devices.

Apple used RS-422 or 485, I forget which, which was maybe a more
sensible standard, but it never caught on outside their little enclave.
This is another serial comms standard, also intended for industrial use.

> > On the other hand, why do the pics use uart without rs232,  is this
> > a pic problem which will make it more complex to connect to pc?
>
>Very few (if any at all) uart's has RS232 builtin. Today the RS232
>interfacing is done with chips like the MAX232.

Max 3100's are available with the 232 drivers built in. It's an SPI based uart.


2005\05\13@094442 by Dave VanHorn

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>
>I do understand now, i.e. RS232 is usefull to extend the u(s)art interface
>over a long distance, i.e. 1 or 2 or 3 meters, but when our signals travel
>only a few centimeters on a pcb, we do not need rs232 and in fact rs232
>will simply be a hinderance.

Exactly.

I would also argue that async serial is a waste of resources at that level.
You can talk clocked serial over a couple of general I/O pins,
letting the slower device be the master.
This has many advantages. In async serial, you need to maintain
precise timing, and you need to process the char you've received
before the next one arrives.  You can add handshaking pins, but it's
pretty much guaranteed that you will receive at least one more char
after you assert the handshake line to get the other guy to shut up.

In clocked serial, the recipient toggles a clock line, and the sender
puts the data bits out when it sees the clock change.
So, the recipient can go off and do something that takes even several
seconds, and come back and resume the conversation right where it
was, even in the middle of a char.  SPI is a hardware based version
of clocked serial, but some chips are not blessed with those internal
peripherals, and they have to "bit-bang" it, doing the protocol in
software as opposed to hardware.

You can bit-bang async serial as well, but you have to keep those
timings accurate, and that can be pretty difficult without a timer,
and you may not want to use your timer for that.

Engineering is the art of compromise. :)

2005\05\13@111552 by Kenasw

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----- Original Message -----
From: "Dave VanHorn" <spamBeGonedvanhornspamBeGonespamdvanhorn.org>
To: "Microcontroller discussion list - Public." <TakeThisOuTpiclistEraseMEspamspam_OUTmit.edu>;
"Microcontroller discussion list - Public." <RemoveMEpiclistspamTakeThisOuTmit.edu>
Sent: Friday, May 13, 2005 2:45 PM
Subject: Re: [PIC] RS232, USART, and COM


>
> >
> >I do understand now, i.e. RS232 is usefull to extend the u(s)art
interface
> >over a long distance, i.e. 1 or 2 or 3 meters, but when our signals
travel
> >only a few centimeters on a pcb, we do not need rs232 and in fact rs232
> >will simply be a hinderance.
>
> Exactly.

Well, another difference (?) is also that RS232 has other signals apart from
TX and RX which if I understand correctly belong to the UART, i.e.
TX and RX come from the UART standard, the others (CTS,RTS,DSR....)
are only RS232 signals, i.e. they do not exist in UART interfaces.

>
> I would also argue that async serial is a waste of resources at that
level.
{Quote hidden}

> --

2005\05\13@112240 by Dave VanHorn

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At 10:15 AM 5/13/2005, kenaswEraseMEspam.....btinternet.com wrote:

{Quote hidden}

They are routinely implemented by general purpose I/O bits, but
earlier uarts like the SIO-0 did implement a lot of those signals,
and they even would handle the handshaking for you.

http://www.z80.info/z80sio.htm

2005\05\13@114838 by William Chops Westfield

face picon face

On May 13, 2005, at 4:24 AM, Olin Lathrop wrote:

>> What would be the difference between rs232, usart, and pc com port.

COM is a logical device name, more or less specific to windows/dos.
(it would be /dev/tty under most unix systems, for instance.)

Normally, COMn connects to to a particular type of asynchronous serial
communications port.  "async serial communications" implies a standard
for bit ordering, timing, synchronization, and error conditions that
allow for sending computer data over a single wire (in each direction.)
(X.20 describes it, I think, although a uart manual probably does
better.)
A UART is the common name for the device that interfaces to a computer
on
one side and the serial port on the other.  However, a uart is digital
logic device, so the serial output is still at digital logic levels.

To move serial data across significant distances, digital logic levels
are not ideal, so there are additional standards that describe how to
shift those levels for more reliable long-distance use.  rs232 is one
of these standards.  It's a rather complete standard defining how two
devices talk to each other with serial ports.  It is the most widely
violated standard in the world, and includes lots of bits and functions
that no one uses any more (it specifies a 25 pin connector, and has a
use for nearly all of the 25 pins.)  When most people say rs232, they're
talking about the most basic pieces - the voltage levels representing
a zero or one on each signal line.  These voltages are "inconvenient"
for
digital logic to produce, and so there's usually a separate chip to
do the conversion.

The three are completely orthogonal; you can have com ports that are
are actually network connections, serial ports that aren't "async",
serial ports that aren't rs232, and rs232 ports that aren't async
serial.

BillW

2005\05\15@152654 by Bill & Pookie

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Mr. William Chops Westfield gives a very clear and well written explanation
below.

>From posts you (kenasw) have made since then I see there are still some
things that you have questions about.

There is no UART standard.  The U in UART stands for Universal  (fits all
requirements).  The UART was originally a chip that was used to connect a
Data Terminal Equipment (DTE)  or computer, to a Data Communications
Equipment (DCE) or modem, using the RS232 Protocol.  The modem was usually
hooked to a phone line.

Here are some good details.
RS232, cables and UART's
http://www.lammertbies.nl/sitemap.html

RS232  http://yarchive.net/comp/rs232.html

I will take a big chance and say that RS232 was not designed to connect
computer to computer.  So when connecting equipment using what each
equipment maker calls RS232 is not always plug and play.  Some will need
some wires crossed and or signals jumpired.  A null modem cable takes care
of most of these problems along with a couple of gender changer plugs.

The UART is one of the mile stones in computer development.  It was the
first I/O chip for a microprocessor that had it's own intelligence.  The
computer software set it up and pumped data, the UART would send the byte,
and interrupt the computer when it was ready for another byte.  It also
checked for certain error conditions.

The UART being universal, was capable of RS422 and RS485 protocol.  These
standards are used for better noise immunity over longer distances than
RS232.  It could also use the 5 bit baudot code.

What you do with the signals in and out of the UART depends on what they go
to and how far it goes to get to where it hopes to go to.

Baud rates are up to you also.  The baud rate for the MIDI musical devices
are a odd one until you realize that they are derived from the 1 mHz cpu
clock from olden times and therefore did not require a separate crystal for
the UART.

So all you need to do is to match what you are communicating with and if you
"roll your own", be concerned with noise the father you go.

Bill

{Original Message removed}

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