902B Abs Piezo
Operation manual
11
Serial user interface
The 902B is as standard supplied with RS232 or RS485 user interface. The user interface allows change of
transducer parameters like set point settings and calibration.
The serial interface uses the following data format: 8 data bits, 1 stop bit and no parity bit.
RS232 user interface
The 902B is DCE (Data Communication Equipment) and can
be connected to DTE (Data Terminal Equipment), typically a
PC
.
The serial communication does not use hardware handshake.
The RS232 standard does not specify the maximum cable
length, but length depends on environment, cable quality and
communication speed. In general cable spans shorter than
15m (50ft.) does not require any extra precautions.
The RS232 connection on transducers delivered with 6 pin Hirschmann and 8 pin RJ45/FCC68 connector is
available at a separate connector. Refer to Accessories and Replacement part number page 51 for RS232
programming cable. The connector is located under the label on the top of the transducer.
RS485 user interface
RS485 is a network communication system that enables the
user to communicate with several units on the same
communication line.
RS485 is a balanced communication system, because signal
on one wire is ideally the exact opposite of the signal on the
second wire. Compared to RS232 communication RS485
allows significantly longer cable span. The maximum length
of cable span depends on the environment, cable quality and
communication speed, but relative long cable spans up to
1,200m (4,000 ft.) is possible.
There are 2 wires other than ground that are used to transmit
the digital RS485 signal. The 902B uses half duplex
communication.
Always use high quality shielded data cables for serial communication. For long cable runs
use twisted pairs. See also Accessory and Replacement part number page 51.
The EIA-485 and NMEA standards specification states that signal A is the inverting “-“ and signal B is the
non-inverting or “+”. This is in conflict with the A/B naming used by a number of differential transceiver
manufacturers which is incorrect, but their practice is used throughout the industry. Therefore, care must be
taken when using A/B naming. In addition to the A and B connections, the EIA standard also specifies a third
interconnection point called C, which is the common ground.
At high communication baud rates and when using long cable runs, a termination resistor of typical 120 Ohm
should be connected between pin 1 and 2 at the 902B DSUB connector and between pin A and B at the data
communication equipment. The termination resistors provide low impedance that reduces the sensitivity to
electrical noise and prevents data reflection that can cause data communication corruption.
RS485 twisted pair cable run with 120 Ω terminator resistors (902B with 15 pin connector)
When connecting multiple devices in a RS485 network, make sure that proper guidelines and
specifications are followed to ensure optimal communication performance of the 902B.
Improper network design can cause data communication interruption and data collision.
