RS-232

RS-232 er en gammel grænseflade for seriel digital datakommunikation, som blev introduceret i 1962. RS-232 muliggør lag-1 kommunikation mellem datakommunikationsudstyr (DCE) (f.eks. et modem) og dataterminaludstyr (DTE) (f.eks. en telegraf). De blev tidligere anvendt til PC tastaturer og mus.

De serielle mekaniske, elektriske og funktionelle karakteristikker (det fysiske lag) er beskrevet i:

ITU-T V.24, V.28^[2]
RS-232-C (Recommended Standard 232), 1969^[3]
TIA^[4] (EIA-232), TIA-232^[5]

Herefter benævnt RS-232.

I 1960'erne (og før) var elektrisk digital logik dyr, pga. de mange diskrete komponenter, der skulle til for implementere de funktioner man havde brug for. Det er nok grunden til at en RS-232 grænseflades mange out-of-band-signaler. En RS-232 har to serielle simplex forbindelser (tilsammen typisk full-duplex):

Lederen TxD (og stel, jord) som sender data til den anden ende.
Lederen RxD (og stel, jord) som modtager data fra den anden ende.

Herudover haves mange dedikerede signaler:

Request To Send (RTS)
Ready To Receive (RTR)
Clear To Send (CTS)
Data Terminal Ready (DTR)
Data Set Ready (DSR)
Data Carrier Detect (DCD)
Ring Indicator (RI)

Eventuelt dedikerede synkroniseringssignaler:

Send timing (ST)
Clock signal, transmitter timing (TT)

Forskelligt

En RS-232 dataforbindelse kan anvendes på flere måder:

med flere eller færre dedikerede styresignaleringer - også kaldet hardware-styresignalering (fx RTS, CTS og DSR, DTR)
uden dedikerede styresignaleringer - men så bør den indlejrede styresignalering XON/XOFF anvendes i begge ender, for at undgå datatab. XON-styrekoden signalerer transmit off - dvs. "stop midlertidigt med at sende". XOFF-styrekoden signalerer transmit on - dvs. "genoptag sending".

Den oprindelige måde at anvende RS-232 var at låse datasignaleringshastighederne fast i hver retning. Fx 75 baud i den ene retning - og 1200 baud i den anden retning. Men det er besværligt, da man skal konfigurere begge dataforbindelses enders hardware.

Der er ingen standard for, hvordan de to RS-232 automatisk kan forhandle hastighed, i de to retninger. Det er grunden til at hastighedsforhandlingen tit går galt. Hvis den ene endes hastigheder låses fast, kan den anden endes hastighedsforhandling typisk fungere.

Blandingen af datavært-typer (kommunikationsudstyr/DCE, terminaludstyr/DTE), individuelle datavært styresignaleringskrav, forskellige RS-232 kabler ("standard" og null-modem="krydset kabel"), manuelle og/eller automatiske konfigureringer gør at etablering af RS-232 dataforbindelser kan være meget besværligt.^[6]^[7] Det ultimative RS-232 værktøj at anskaffe, er en RS-232 break out box til at teste sig frem til en kabelforbindelse som fungerer, når alle andre forsøg på at få en (stabil) forbindelse fejler. (se illustration)

Stik

RS-232 blev oprindeligt specificeret til 25 eller 9 polede sub-D-stik. Men RS-232 bliver også anvendt over 8P8C (forkert benævnt RJ-45) i flere varianter. ^[8] ^[9] ^[10] ^[11] ^[12] Der er også en forbindelsesvariant til 10P10C (også forkert benævnt RJ-45). ^[13] ^[14]

Kilder/referencer

^ CT-1024 Terminal System Kit Arkiveret 3. juli 2008 hos Wayback Machine Citat: "...The baud rate or speed at which the serial data is transmitted or received, is 110 baud, or if the optional parts are installed, 110, 150, 300, 600 and 1200 baud...The input/output connections to the interface are RS-232 compatible..."
^ ITU-T Recommendations: V Series (kan frit downloades)
^ Electronics Industries Association, "EIA Standard RS-232-C Interface Between Data Terminal Equipment and Data Communication Equipment Employing Serial Data Interchange", August 1969, reprinted in Telebyte Technology Data Communication Library, Greenlawn NY, 1985, no ISBN
^ Telecommunications Industry Association (TIA), Electronic Industries Alliance (EIA)
^ interfacebus.com: EIA-232 Bus
^ Eksempel hvor besværlig det kan være: Understanding RS232 Serial Port Communication Citat: "...If RS232 is a standard why can't I just use a standard lead to connect together two RS232 ports and expect them to talk to one another? That's a good question...You've plugged your instrument into your computer's COM port, installed your data acquisition software, but no data appears. You suspect it is a problem with your RS232 communications. What do you do?...Obtain all the information available about your instrument or device. You will need to know the number of the com port into which you have plugged your device, the device baud rate, number of bits in the data byte and parity. Enter this information in ComDebug. If you have information about the Stop Bits then use it, otherwise set them to 2. This will at worse slow the message down slightly. See Making Measurements through the COM Port for a detailed explanation of each of these settings..."
^ Youtube: How to troubleshoot serial RS232 communications Arkiveret 24. maj 2018 hos Wayback Machine
^ epanorama.net, Tomi Engdahl: Wiring RS-232 to RJ45 connector Citat: "...There is no standard, but many applications use the pinout of the 9-pin D-connector and just forget about pin 9..."
^ jneuhaus.com: Neuhaus "Standard" for Interconnection of Serial Devices. RJ45 8-pin modular connectors for RS-232
^ nullmodem.com: RJ-45 Pinout for RocketPort Et stykke nede: Citat: "...The following chart shows the pinout for RJ-45 connectors used on certain RocketPort serial interface cards (manufactured by Comtrol)..."
^ yost.com: Yost serial device wiring: RS232 on RJ45 (I stedet for RJ45 burde der have stået 8P8C), yost.com: Yost Serial Device Wiring Standard
^ Cisco: Cabling Guide for Console and AUX Ports
^ EIA 232 Modem Cables RJ 45 10 Pin to DB 9 (10P10C forkert benævnt RJ-45)
^ EIA 232 Modem Cables RJ-45 10pin to DB-25 (10P10C forkert benævnt RJ-45)

[1] CT-1024 Terminal System Kit Arkiveret 3. juli 2008 hos Wayback Machine Citat: "...The baud rate or speed at which the serial data is transmitted or received, is 110 baud, or if the optional parts are installed, 110, 150, 300, 600 and 1200 baud...The input/output connections to the interface are RS-232 compatible..."

[2] ITU-T Recommendations: V Series (kan frit downloades)

[3] Electronics Industries Association, "EIA Standard RS-232-C Interface Between Data Terminal Equipment and Data Communication Equipment Employing Serial Data Interchange", August 1969, reprinted in Telebyte Technology Data Communication Library, Greenlawn NY, 1985, no ISBN

[4] Telecommunications Industry Association (TIA), Electronic Industries Alliance (EIA)

[5] terfacebus.com: EIA-232 Bus

[6] Eksempel hvor besværlig det kan være: Understanding RS232 Serial Port Communication Citat: "...If RS232 is a standard why can't I just use a standard lead to connect together two RS232 ports and expect them to talk to one another? That's a good question...You've plugged your instrument into your computer's COM port, installed your data acquisition software, but no data appears. You suspect it is a problem with your RS232 communications. What do you do?...Obtain all the information available about your instrument or device. You will need to know the number of the com port into which you have plugged your device, the device baud rate, number of bits in the data byte and parity. Enter this information in ComDebug. If you have information about the Stop Bits then use it, otherwise set them to 2. This will at worse slow the message down slightly. See Making Measurements through the COM Port for a detailed explanation of each of these settings..."

[7] Youtube: How to troubleshoot serial RS232 communications Arkiveret 24. maj 2018 hos Wayback Machine

[8] rama.net, Tomi Engdahl: Wiring RS-232 to RJ45 connector Citat: "...There is no standard, but many applications use the pinout of the 9-pin D-connector and just forget about pin 9..."

[9] uhaus.com: Neuhaus "Standard" for Interconnection of Serial Devices. RJ45 8-pin modular connectors for RS-232

[10] ullmodem.com: RJ-45 Pinout for RocketPort Et stykke nede: Citat: "...The following chart shows the pinout for RJ-45 connectors used on certain RocketPort serial interface cards (manufactured by Comtrol)..."

[11] yost.com: Yost serial device wiring: RS232 on RJ45 (I stedet for RJ45 burde der have stået 8P8C), yost.com: Yost Serial Device Wiring Standard

[12] Cisco: Cabling Guide for Console and AUX Ports

[13] EIA 232 Modem Cables RJ 45 10 Pin to DB 9 (10P10C forkert benævnt RJ-45)

[14] EIA 232 Modem Cables RJ-45 10pin to DB-25 (10P10C forkert benævnt RJ-45)

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