Chassis Plans ATXP-875P, Technical Reference

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Appendix C: Communications Devices ATXP-875P – Technical Reference
72 Chassis Plans
Serial Ports
The ATXP-875P has four fixed RS-232 serial ports (RS-422/485 optional on SER C and SER D).
TIA/EIA-232
RS is the abbreviation for recommended standard. Usually, it is based on or is identical to other standards, e.g.,
EIA/TIA-232-F. TIA/EIA-232, previously known as RS-232 was developed in the 1960’s to interconnect layers of
the interface (ITU–T V.11), but also the pignut of the appropriate connectors (25-pin D-type or 9-pin DB9S) (ISO
2210) and the protocol (ISSUED-T V.24). The interface standard specifies also handshake and control lines in
addition to the 2 unidirectional receive data line (RD) and transmit data line (TD). The control lines data carrier
detect (DCD), data set ready (DSR), request to send (RTS), clear to send (CTS), data terminal ready (DTR), and the
ring indicator (RI) might be used, but do not necessarily have to be (for example, the PC-serial-mouse utilizes only
RI, TD, RD and GND). Although the standard supports only low speed data rates and line length of approximately
20 m maximum, it is still widely used. This is due to its simplicity and low cost.
Electrical
TIA/EIA-232 has high signal amplitudes of ±(5 V to 15 V) at the driver output. The triggering of the receiver
depends on the sign of the input voltage: that is, it senses whether the input is above 3 V or less than –3 V. The line
length is limited by the allowable capacitive load of less than 2500 pF. This results in a line length of approximately
20 m. The maximum slope of the signal is limited to 30 V/ms. The intention here is to limit any reflections that can
occur to the rise-and fall-times of the signal. Therefore, transmission line theory does not need to be applied, so no
impedance matching and termination measures are necessary.
Do not connect termination resistor when operating in RS-232 mode.
Protocol
Different from other purely electrical-layer-standards, TIA/EIA-232 defines not only the physical layer of the
interface (ITU-T V.11), but also the pinout of the appropriate connectors (25-pin D-type or 9-pin DB9S) (ISO 2210)
and the protocol (ITU-T V.24). The interface standard specifies also handshake and control lines in addition to the 2
unidirectional receive data line (RD) and transmit data line (TD). The control lines might be used, but do not
necessarily have to be.
RS-232 is Single-Ended Point-to-point Transmission
Single-Ended, Point-to-Point
Single-ended transmission is performed on one signal line, and the logical state is interpreted with respect to ground.
For simple, low-speed interfaces, a common ground return path is sufficient; for more advanced interfaces featuring
higher speeds and heavier loads, a single return path for each signaling line (twisted pair cable) is recommended.
The figure below shows the electrical schematic diagram of a single-ended transmission system.