Background information:
RS232 Interface
Basics
The RS232 standard describes the serial connection between a data terminal device (DTE) and a data communication device (DCE) with their electrical and mechanical properties. Although the standard only defines this connection type, the RS232 interface has established itself as a general standard for serial data transmission over short distances.
In interface documentation for many devices the designation V.24 is often used instead of RS232. Although there are various standards which differ from each other in the details, the terms are usually used synonymously in practice.
RS232 signals can be converted using a variety of interfaces into nearly any other interface type, including Ethernet, USB, RS422, RS485, 20mA, glass and plastic fiber optics, etc.
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Signals and Connectors:
DTE and DCE interfaces differ basically in their pin configurations: PCs, printers, plotters or the main port of a terminal are equipped with a DTE configuration, whereas modems and printer ports on terminals use DCE configurations. Some plotter types represent a unique situation when they are equipped with both a DCE and a DTE interface.
The RS232 standard defines a 25-pin SUB-D connector as the standard. In addition, the 9-pin SUB-D has found wide use especially in the PC world.
From the table it can be seen that for example pin 2 on the connectors is always designated TxD (Transmit Data) regardless of the direction of the data signal. The additional information for DTE or DCE configuration is needed to ascertain the function of the described pin:
Physical Transmission Procedure
In the RS232 interface the individual data bits for a character are sent one after the other as voltage states over a send or receive line. A logical "1" thus corresponds to a negative voltage level of -15..-3V, and a logical "0" to a positive voltage level of +3..+15V referenced to commond ground.
The data sender must generate a minimum level of +/-5V under load, whereas the receiver will recognize a level of +/-3V as a valid signal. The permissible resistive load must be greater than 3 KOhm, and the capacitive load represented by the transmission cable is limited to 2500 pF.
When insufficient signal levels are present, the RS232 Isolator for DIN rail mount can restore them to standard levels. In contrast to typical RS232 isolators, this device is not powered by the signal lines, but rather uses active, externally powered RS232 interfaces to perform signal regeneration.
Cable length
The achievable distance between two RS232 devices is - as in the case of all serial transmission procedures - mainly determined by the cable used and by the baud rate. RS232C defines the maximum distance as 15 meters without consideration of the transmission speed.
Newer versions of the RS232 standard define the maximum cable length based on the cable capacitance, which must not be exceeded. By selecting a low-capacitance cable (approx. 50pF/m), a distance of max. 50 meters can be attained without additional means.
Longer distances can be realized by using RS232 line drivers or by parallel use of an existing network infrastructure with Com-Servers, which can transport the RS232 signals over an Ethernet.
Galvanic isolation
When estimating the maximum cable length, the problem of possible potential differences always need to be taken into account. With growing cable lengths and in industrial environments galvanic isolation should be provided.
This can be easily accomplished using RS232 isolators plugged into existing connections or by sending the RS232 signals over a fiber optic connection.
Data Flow Control:
RS232 interfaces have a variety of handshake lines which however are only needed in general for connnecting a modem to a data terminal device.
The much more common case of connecting two data terminal devices is generally handled with no problem by a reduced number of handshake lines. Unused handshake inputs are simply set to an enable level by tying them to their own handshake outputs.
Newer applications increasingly use software handshaking with the "XON" and "XOFF" characters. Another option is to eliminate traditional flow control altogether and instead to use a block protocol. In addition, the data will be checked using checksums which the receiver must monitor and the sender must acknowledge.