Introduction
The EIA standard has left some unspecified areas regarding what constitutes a
compliant cable-connector implementation. One area is specifying the connector
itself, while another area is defining for the 21 circuits in the standard which
is optional and which is required. This was done on purpose, since this standard
cable was intended for simple local terminal interface through a multiplexed,
synchronous, dedicated line that is shared by a cluster of remote terminals and
is equipped with automatic dialing units.
Let's look at some sample configurations :
- Transmit only
- Transmit only with RTS
- Receive only
- Half Duplex
- Full Duplex
- Full Duplex With RTS
- Special
RS-232-C standard configuration
RS-232-C interchange circuit (1) (2) (3) (4) (5) (6) (7)
-------------------------------------------------------------------------------
1 Protective Ground - - - - - - o
7 Signal Ground X X X X X X X
-------------------------------------------------------------------------------
2 Transmitted Data X X X X X o
3 Received Data X X X X o
-------------------------------------------------------------------------------
4 Request to Send X X X o
5 Clear to Send X X X X X o
6 Data Set Ready X X X X X X o
20 Data Terminal Ready S S S S S S o
22 Ring Indicator S S S S S S o
8 Received Line Signal Detector X X X X o
-------------------------------------------------------------------------------
X = required for any configuration
S = required for using PSDN (public switched telephone network)
o = specified by cable designer
Notice that only one circuit is absolute requirement for
any such cable, this is the Signal Ground on Pin 7. That means that as long as
Signal Ground is included on pin 7 this cable is "RS-232-C compliant"".
Although microcomputer system have one-way devices such as transmit-only
joysticks, or receive only printers, the most common situation is a full-duplex,
two-way communication. The classical application for the above is a send/receive
terminal where characters are transmitted from the keyboard to a microcomputer
and echoed back to a display screen. The data is traveling in both directions
from the DTE (keyboard and screen), to the DCE (computer serial I/O port). There
are two configurations for full-duplex, one with and one without the Request to
Send line implemented. A "safe" strategy is to always include it.
The figure below shows a schematic full-duplex configuration
RS-232-C Standard Full Duplex Cable
- The Signals present in a standard full-duplex RS-232-C cable are :
- For a modem over a switched telephone network we must add two more signals
To understand the function of each signal lets look at the events taking
place to eventually exchange data. Each event will cause a transition from state
to state from the idle state through the data exchange and communicating state
to finally back to idle state.
The events and transitions are grouped in phases :
Alerting
In the Alerting phase, the call originating station dials the phone number of
the call-answering station. The remote telephone begins to ring, and the Ring Indicator
Signal of the answering DCE make an OFF to ON transition. This ends this
phase.
Equipment Readiness
The Equipment readiness phase is involving events associated with turning
Data Set Ready and Data Terminal Ready to ON, and thus completing this phase.
Circuit Assurance
The Circuit Assurance consists of events associated with turning ON the
Received Line Signal Detector signals at both communicating stations.
Channel Readiness
The Channel Readiness Phase uses Request to Send and Clear to Send
handshaking to arrive at the target state of active data-exchange state. All
these events result in the equipment being disconnected from the telephone
network and getting back to original idle state.
The following diagram illustrates the above :
Other (Non Standard) Common Configurations
Three-Wire Economy Model
This model involves only minimum number of circuits for full-duplex
communications.The circuits present are Transmitted Data on pin 2, Received Data
on pin 3, and Signal Ground on pin 7. There are configurations for which this
cable is entirely adequate, but many common microsystems components use the RTS
and CTS circuits. This equipment will not transmit, until it received an
asserted CTS signal. For this we use the next model to trick the USART-based I/O
ports into transmission.
The following diagram illustrates the above :
Three-Wire with Luxury Loop-Back
This cable has the following loop-back jumpers:
By jumpering Data Set Ready, the Equipment Readiness
phase is completed as soon as the DTE asserts its Data Terminal
Ready line. This is achieved when the DTE is powered-up. Also, when the DTE
is powered-up the Request to Send
is asserted and the Circuit Assurance
phase is completed, since the Request to Send
is jumpered to the Received Line Signal
Detector. Since Request to Send
is jumpered to Clear to send, it
is also implies the completion of the channel readiness
phase. The bottom line is that Data Terminal
Ready and Request to Send
are the only two events required to achieve the target data-exchange state.
Notice that this implementation omits some features from the full
implementation. Most of them concerning preventing overrun errors.
The following diagram illustrates the above :
Null Modem with Luxury Loop-Back and the Null Modem with Double-Cross
This model is designed to answer the requirement to trick two DTEs into
communicating over a strictly local RS-232-C interface, with no modems
or DCEs. This concept is identified as the "crossover technique" The word
luxury is used since there are some modest model in which some
loop-backs are omitted. When none of the loop-backs is present it is simply the
three-wire economy model. Notice that this model is exactly the three-wire with
luxury loop-back model with null modem.
The double Cross variant involves a crossover between the following two pairs
of control signals:
The following loop-back are included:
And finally a crossover for null-modem function:
The following diagram illustrates the null modem with luxury loop-back :
