RTU´s

[ketal]

Could somebody make a clear picture between the global idea behind
rs-232 * handshake * four wire * half duplex * modem * protocol *
polling * as a whole? Am I missing someting?
Maybe a place where this could be clearly analyced?
Thank You.

[Tom Jenkins]

What you are really asking for is a full course on serial communications. Here are some high points, but please get some text books or manufacturers info for additional info.

1) rs-232 is shorthand for Electronics Industry Association Standard RS-232C which defines the ELECTRICAL connection between two serial devices, including standardized pin functions and voltage levels

2)handshake is simply two serial devices making sure they are on the same link, verifying that the receiver is ready to receive when the sender is ready to send, the baud rate and data formats are the same, etc. It can be done by hardware like CTS wired to RTS (Clear To Send wired to Request To Send), or by software like Header/Acknowledge, XON/XOF etc.

3)four wire is not usually used with RS-232, but is typically meant to indicate RS-485 or RS-422. These are serial communications electrical specs that are more noise immune that RS-232 and used industrially for longer distances.

4)half duplex means the data transmission can go in either direction but only one direction at a time (when the sender is sending, the receiver must shut up and listen). This is differentiated from full duplex, where communications can occur in both directions simultaneously.

5) modem means MODulate/DEModulate and is generally used in reference to changing bits in electrical serial format to tone signals for transmission over voice or leased line telephone lines.

6) protocol is simply a fixed pattern for establishing communications between two devices, usually by means of rigid software rules for the data format and addressing. A protocol must identify the header (a character(s) that says wake up, I'm talking), the addressing of the destination device (Hey number 52, I want YOU), the type of transaction (and I'm going to read integer registers) the register address (V2020 through V2040) and a checksum (it the total bits in this transaction aren't 27 then the signal is garbled so let me know and I will re-transmit). Lots of other factors also go into a protocol, and they can be master/slave, peer to peer, token passing, Carrier Sense Multiple Access Collision Detect, etc. etc. etc.

7) polling is when a single unit (often the master in a master/slave protocol) goes through each device on the communications link one at a time to get data from it. After the last one is done the cycle is repeated - not too different from political opinion polling, really!

8) "Am I missing someting?" Oh yeah, and this space is too short to fill in all the gaps. additional reading, in Phil's book first and then in other references, is required!

[rsdoran]

I am not sure what you mean by the "global idea" but I can clarify what the items are.

RS232: Short for recommended standard-232C, a standard interface approved by the Electronic Industries Association (EIA) for connecting serial devices. Almost all modems conform to the EIA-232 standard and most personal computers have an EIA-232 port for connecting a modem or other device. In addition to modems, many display screens, mice, and serial printers are designed to connect to a EIA-232 port. In EIA-232 parlance, the device that connects to the interface is called a Data Communications Equipment (DCE) and the device to which it connects (e.g., the computer) is called a Data Terminal Equipment (DTE).

Handshaking: The process by which two devices initiate communications. Handshaking begins when one device sends a message to another device indicating that it wants to establish a communications channel. The two devices then send several messages back and forth that enable them to agree on a communications protocol.

Modem:Short for modulator-demodulator. A modem is a device or program that enables a computer to transmit data over telephone lines. Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves. A modem converts between these two forms.

Protocol:The special set of rules that end points in a telecommunication connection use when they communicate. Protocols exist at several levels in a telecommunication connection. There are hardware telephone protocols. There are protocols between each of several functional layers and each corresponding layer at the other end of a communication. Both end points must recognize and observe a protocol. Protocols are often described in an industry or international standard.

Half-Duplex: Half-duplex data transmission means that data can be transmitted in both directions on a signal carrier, but not at the same time. For example, on a local area network using a technology that has half-duplex transmission, one workstation can send data on the line and then immediately receive data on the line from the same direction in which data was just transmitted

Polling Making continuous requests for data from another device. For example, modems that support polling can call another system and request data. In a master/slave scenario, the master queries each slave device in turn as to whether it has any data to transmit. If the slave answers yes then the device is permitted to transmit its data. If the slave answers no then the master moves on and polls the next slave device. The process is repeated continuously.

When you refered to FOUR WIRE I had to assume you were referring to RS485 four wire usage, instead of discussing this I am offering tutorial links to RS 485
http://www.arcelect.com/485info.htm

http://www.hw.cz/english/docs/rs485/rs485.html

A good site overall to get Serial Port info
http://www.lvr.com/serport.htm


Geez beat to the draw again

[David_Emmerich]

Good Grief!!!
You folks come up with such great info, and so fast!....
rasdorn, we'll have to meet one of these days....maybe at the next Industrial Show in Nashville.....Thanks for the links. David

[gbradley]

Tom and Ron
You guys rock!

I guess Ketal asked the question in the correct manner, because both of those answers were excellent.
I wish I could remember this stuff that well.

When I need that kind of information I always have to look it up, and it is nearly impossible to find it spelled out so straightforward.

Heck I still confuse the printer port and the Com port.
Which one is is the parallel and which one is the serial port?

Lpt --- Parallel Printer
COM--- Serial ???
There I go working without tools again.

Thanks again Phil.
George Bradley

[ketal]

TOM & RSDORAN:
IT IS A KIND OF HEAVEN TO KNOW THAT KNOWLEDGE IS SHARED
IN SUCH A CLEAR FAST AND UNSELFISH DIRECT WAYS; SO MANY OURS OF STUDY AND
PRACTICE PUT IN PLAIN SYNTHESIS. THAT SPEAKS A LOT ABOUT
NORTH AMERICANS , MAYBE I
SHOULD STICK JUST TO TECHNICKS!.
WELL THANK YOU! beerchug

[Tom Jenkins]

Aw shucks, fellers, 't warn't nuthin'.

Seriously, Ketal, the objective is to have a new generation coming up to carry us old farts when we start to fall down!