Amplicon USB232U, Инструкция по эксплуатации

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USB232U/485Ui
Page 14
4.2 Application Notes
As previously noted, many applications packages are designed to support data transfer over
the standard serial communications ports. When set up as specified and conventional multi-
way cables and connectors are in use, then the interface will be transparent to the user.
However, for special applications, particularly when use of the RS485 interface is required, the
following notes will be helpful.
4.2.1 Applicable Standards
4.2.1.1 RS Standards
The prefix 'RS' signifies Recommended Standard, and the three interface types, RS232,
RS422 and RS485, supported by the Amplicon range of devices are all well defined in the
appropriate standards publications. The functions and features of the data and control
interfaces presented by the USB232U/485Ui series generally conform to normal industry
practice employing a subset of the full standards.
The RS232 serial interface standard is now at revision E and its full title is EAI/TIA RS232E,
"Interface Between Data Terminal Equipment and Data Communication Equipment Employing
Serial Binary Data Interchange".
Similar European standards CCITT V.24 and V.28 define an
interface which is operationally compatible with RS232.
The RS232 port of the USB232U are designated as DTE (Data Terminal Equipment) in
accordance with normal practice for a computer driven serial port. The current RS232 standard
specifies the 25 way male D-type connector for DTE. The full 25 lines are not required to
support the common functions of RS232, and IBM and other manufacturers frequently adopt a
9-way D-type connector that provides all the necessary functionality. Where this 9-way system
is used, manufacturers conform to interface standard EIA 574, with the pin-out configuration as
shown in Figure 3.2.
The RS422 interface is defined in the EIA/TIA standard RS422-B,
"Electrical Characteristics of
Balanced Voltage Digital Interface Circuits" . As stated, this standard defines the electrical
characteristics but does not specify a connector type or pin-out configuration. The USB485Ui
meets the RS422 standard specification with communication via a 9-way D-type connector
wired as shown in Figure 3.3 with the exception of RTS and CTS control lines.
The RS485 specification is also published as EIA standard EIA 530 pt 87 and is closely allied
to the RS422 standard. The USB485Ui meets the requirements laid down in the RS485
standard.
4.2.2 RS232 Application Notes
RS232 was approved as a standard interface in 1969. Before and since that time this interface
in its various revisions has been very popular despite its many shortcomings and usage outside
its intended role. Most small computers now have an RS232 interface as standard, or easily
added, and many peripherals and instruments support this serial interface, but not all in the
same way. Hence the application of an RS232 connection needs careful study of the
equipment manufacturers connections and methods before the devices are linked. However
RS232 lines are quite safe if wrongly connected, so the experimental use of a break-out box or
'hit and miss' methods of establishing communication are often employed.