Siemens 505-2571 User Manual page 11

As control systems become more complex and dispersed, the demand for
attaching serial devices has increased. Serial devices are typically used to
provide operator interface, to run supervisory control and monitoring
packages, and to facilitate controller programming support.
Most Series 505 controllers today are equipped with two serial ports to
accommodate the minimum requirement. However, these ports can be
consumed with the supervisory control package and one operator device.
Some controllers, like the SIMATIC 525, provide only an RS-232 port,
limiting reliable communications to about 50 feet at higher baud rates.
The PPX:505-2571 provides two additional RS-232 ports and 2 additional
RS-422 ports in addition to the standard controller ports. These additional
ports may be used to attach various types of serial devices which conform to
the Non–Intelligent Terminal Protocol (NITP). Devices requiring the
Transparent Byte Protocol (TBP) must be connected to the CPU or the RBC
serial port.
The simplest of these types of devices are COROS TD/OP operator panels.
These operator interfaces provide for direct access to controller memory
locations for read and/or write capability through an RS-422 direct
connection. Devices attached through the RS-422 interface may be located
up to 4000 feet away.
SIMATIC 505-2571 Program Port Expander Manual 505–2571 Module Description 1-3