7828 Density Transmitter Technical Manual Electrical Installation
78285000_AB 3-7
3.4 CONNECTIONS
The connections to the 7828 Density Transmitter are quite straightforward:
•= Power
•= RS485 (Modbus) Communications A/B
•= Analog (4 - 20mA).
It is recommended that you install all three connections (six cores) at installation, to avoid
the possibility of expensive alterations to the cabling at a later date.
3.4.1 POWER
Connect a suitable 24V power supply to pins 1 and 2 of the transmitter. Ensure that the
loop resistance of the cables is such that the voltage at the transmitter terminals is greater
than 20 volts. (The maximum voltage at the 7828 terminals is 28V.)
3.4.2 RS485
The Modbus standard uses the RS485 electrical standard. This uses the difference
between the two signal cores to transmit and detect logic levels, and is therefore able to
tolerate significantly higher levels of common mode noise than the more familiar RS232
standard, which utilises the voltage between the signal core and a common earth. A brief
summary of some typical characteristics of the two standards is given below.
RS485 RS232
Signal detection Differential Single-ended
Receiver threshold 200mV +1.5V
Transmitter output swing 0 to +5V (no load)
+2 to +3V (120 ohm load)
± 8V
A converter is required for communication between the two standards. Further details are
given in the next section.
Only two signal connections are required for RS485, usually called A and B, sometimes ‘+’
and ‘-‘.
Note: Unfortunately, different manufacturers have interpreted the standard in different
ways. Some have a ‘logic 1’ represented by signal A being more positive than signal B,
others have made the opposite interpretation. If you encounter communication difficulties
with RS485, the first remedy is to swap over the A and B connections at one end of the
network.
For areas which may experience high common mode signals, a third conductor can be
used as a ground reference for the communications signals. If used, this should be
connected to Pin 2 (Power supply negative) on the 7828.
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