B&B Electronics 3PCIU4 - Manual (Page 63)

RS-232 Connections/Operation
Manual Documentation Number 3PCIoUx-1008 Chapter 7 57
RS-232 Signal Levels
RS-232 signal lines are referenced to ground, and each signal can alternate
above and below ground. The RS-232 standard specifies output voltages
must be no less than +5 volts and no greater than +25 volts to represent a
Space on a transmit line (or an asserted handshake line). Output voltages
must be between –5 volts and –25 volts to represent a Mark on a transmit line
(or an de-asserted handshake line).
The standard specifies that RS-232 inputs accept voltage levels between –3
volts and –25 volts for a Mark (asserted handshake) and between +3 volts to
+25 volts for a Space (de-asserted handshake). Typically RS-232 voltages
will be between +3V to +11V and –3V to –11V.
Handshaking
To control the flow of data between two devices, some software uses
hardware handshaking. RS-232 devices which require hardware handshaking
require connection of RTS/CTS and/or DTR/DSR lines in order to operate
properly.
Programs for RS-232 may chose to use only the RS-232 TD and RD data
lines and ignore hardware handshaking inputs (set up as None in the port
configuration). In some cases the software may require connections to verify
that a cable is connected and the devices are ready for access (typically
DTR/DSR is used).
Some devices may use software handshaking in which data characters (e.g.
Xon/Xoff ) are sent to start and stop the incoming or outgoing data. These
unprintable characters have the decimal value (17) and (19), and can often be
generated in a terminal program with CTL+Q or CTL+S.
RTS Control In RS-232 Mode
In RS-232 mode, MIport cards support software and hardware handshaking.
Handshake lines can be controlled from the communications software used
with your application. When operating a MIport card port in RS-232 mode,
set the
RTS Control driver setting to Normal . This ensures that the RTS line
will be free to be controlled by your software. If your communications
software uses software handshaking, hardware handshake lines will not be
required.