Amplicon USB232U, Instruction Manual

Page: 21 / 33

USB232U/485Ui
Page 15
4.2.2.1 RS232 Electrical Levels
When testing serial communications circuits, it is useful to know what signal levels to expect.
The specified voltage levels of RS232 data and control signals are summarised in the following
table:-
Interchange Voltage –15 to –3 V +3 to +15 V
Data Binary State
1 0
Data Signal Marking Spacing
Control Function (RTS, DTR)
OFF ON
The levels are specified relative to signal ground at 0 volts, (the remote device signal ground in
the case of isolated ports), and the region between –3 and +3 volts is the transition region. The
voltage levels transmitted by the USB232U data and control lines are about ±10 V.
4.2.2.2 RS232 Data Rates and Cable lengths
The RS232 standard specifies the data signalling rate up to 20 kilobaud, but an RS232
interface is often used up to 1 MBaud. The specified electrical characteristics allow a typical
maximum cable length between transmitter and associated receiver of about 15 m (50 ft). In
practice, cable lengths can often be increased at lower rates and may need to be kept shorter
at rates above 20 kBd.
RS422 and RS485 standards specify data signalling rates up to 10 MBaud, with decreasing
cable length maxima at the higher rates. At a signalling rate of 1.5 Mbaud by the USB485Ui,
the maximum cable length is specified as 80 m (260 ft) see section 4.2.3.5.
4.2.2.3 RS232 Cable Adapters
With the variety of available interpretations of the RS232 interface, it is often necessary to
employ adapters, or make up suitable cables to ensure that the proper connections are made.
The following diagrams show typical adapters that may be required. These adapters can
normally be made from the two appropriate connectors wired back to back, with the main
interconnecting cable being a standard 25 or 9-way pin to pin type.
Various plug-in adapter and test modules are available from
Amplicon Liveline Ltd to ease
the problems of interconnecting RS232 interfaces. See the current catalogue for full details and
descriptions.
4.2.3 RS422/485 Application Notes
RS422 and RS485 have very similar characteristics, with RS485 being compatible with, and
offering all the advantages of RS422 plus the additional capability of multi-transmitter
operation, higher device capacity and better common mode performance. RS422 pre-dated
RS485 as a standard, but RS485 is now more popular and used in new system design.
For the USB485Ui the on board circuitry for the two modes is identical, the application deciding
the mode of interface operation.