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EIA RS232d

Posted on 1997-03-15
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what is the difference between a RS232d and a RS2323c?
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Question by:mister031397
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elsberry earned 10 total points
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RS-232

The standard sets out:

The connectors: Female connector on the DCTE ( ie Modem) The length of cable: 50 feet max. ( about 15 metres)
Cable Capacitance: 2500 picofarads ( this is related to certain technical issues)



RS-232-D also says that the 25-way 'D' Connector should be used and says which pin should be used for what. Some companies such as IBM, may not fully have adopted the RS-232-D and may use a different connector such as the 9-way 'D' connector.

The voltages to be used:

Mark: -25v to -5v at the transmitter; Less than -3v at the receiving end.
Space: +5v to 25v at the transmitter; More than 3 volts at the receiver.



Notice that the two data symbols are called 'Mark' and 'Space'. Mark means 1 for data lines and 0 for control lines.

Each of the circuits in the standard has a name and a particular use. RS-232-D ties this down to individual pins on the connector, and these are the ones used here. Not all the pins are used for all modems: there is no point in connecting the 'Ring Indicator' circuit on a modem which can't answer the phone.

These are the pin connections, circuit names and descriptions:

Pin 1 - Protective Ground

This pin is used to connect to any screen on the cable or equipment. It helps to reduce the effect of electrical noise on the equipment..

Pin 7 - Signal Ground

This is the pin against which all the signal voltages are measured. Without this circuit nothing else will work.

Pin 2 - Transmitted Data(TD)

This is the circuit which carries the data being output by the DTE ( the computer) - it is an output from the computer and an input into the modem.

Pin 3 - Received Data (RD)

This is the circuit on which the DTE (Computer) receives data.

Pin 4 - Request to Send (RTS)

Pin 5 - Clear to Send (CTS)

These two pins need to be considered together. They provide a means for the two communicating DTE's (computers) to tell one another about their ready state. A computer which is ready to receive data can send RTS to the modem (it is up to the modem to forward this signal). A computer cannot send data until it receives Clear-To-Send (CTS).

Pin 6 - DCE Ready (also known as Data Set Ready)

This signal indicates that the Modem is ready to pass data i.e. The Modem is powered up and not in test-mode.

Pin 20 - Data Terminal (Equipment) Ready

This signal can be used by the computer to force the modem to answer an incoming call - it places the modem in a 'off-hook' state.

Pin 8 - Received Line Signal Detector or Data Carrier Detector(DCD)

The modem places a signal on this circuit if it is receiving a sufficiently strong carrier (tone). Some computers will not accept or send data unless this signal is present.

Pin 22 - Ring Indicator

The modem puts a signal on this line if the phone is ringing. This is how the computer knows about incoming calls.

The other pins all have assigned uses put are not normally used in simple applications of modems. These connections may not all be used on some modems and signals may have to be forced into the correct states to convince the computer that all is well. This is often done by extra loops of wire inside the plug linking pins 4&5 and 8,6&20 together.



Non-Standard Uses for RS-232-D

RS-232 was intended for connecting DTE and DCTE together. Unfortunately, it is also used (frequently) in ways it was never intended for - such as connecting two computers together or for connecting a computer to a printer. The connection of a computer to a printer by this method is falling out of fashion as this type of circuit is considered to be too slow for many modern printers.

On a printer, intended for 'RS-232' type connections, you should read the documentation carefully as it is often possible to set a switch in the printer to make it mimic ( emulate) a DCTE - this is often the simplest method.

To connect two computers or a computer to a printer (without DCTE emulation) you will need a special cable which mimics the behavior of a modem. This specially wired cable (or small box ) is known as a null-modem.
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