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Appendix A • Glossary
Automation PC 820 User's Manual V 1.00
Real time
A system is operating in real time or has real-time capability if the input sizes (e.g. signals, data)
are received and processed in a defined time period, and the results are made available in real
time for a partner system or the system environment. See also "real-time demands" and "real-
time system".
ROM
An abbreviation for " R ead- O nly M emory". Semiconductor memory where programs or data were
permanently stored during the production process.
RS232
R ecommended S tandard N umber 232 . Oldest and most widespread interface standard, also
called a V.24 interface. All signals are referenced to ground making this an unbalanced interface.
High level: -3 ... -30 V, low level: +3 ... +30 V. Cable lengths up to 15 m, transfer rates up to 20
kBit/s. For point-to-point connections between 2 participants.
RS422
R ecommended S tandard N umber 422 . Interface standard, balanced operation, increased
immunity to disturbances. High level: 2 ... -6 V, low level: +2 ... +6 V. 4-wire connection
[inverted/not inverted], cable lengths up to 1200 m, transfer rates up to 10 Mbit/s, 1 sender can
carry out simplex communication with up to 10 receivers.
RS485
R ecommended S tandard N umber 485 . Interface standard upgraded from RS422.
High level: 1.5 ... -6 V, low level: +1.5 ... +6 V; 2-wire connection [half duplex operation] or 4-wire
connection [full duplex operation. Cable lengths up to 1200 m, transfer rates up to 10 Mbit/s. Up
to 32 participants can be connected to an RS485 bus [sender/receiver].
RTS
An abbreviation for " R equest T o S end". A signal used in serial data transfer for requesting send
permission. For example, it is sent from a computer to the modem connected to it. The RTS
signal is assigned to pin 4 according to the hardware specifications of the RS-232-C standard.
RXD
An abbreviation for "Receive ( RX ) D ata". A line for transferring serial data received from one
device to another, e.g. from a modem to a computer. For connections complying with the RS-
232-C standard, the RXD is connected to pin 3 of the plug.