Hach 8450, Instrument Manual

Page: 23 / 66

Section 2
Page 23
16116Install.fm
RS485 Communications
2.9 RS485 Communications
Monitors with the RS485 and data logging option can communicate via this
digital network standard. RS485 signals and typical connection strategy is
indicated in
Table 3 , Table 4 , and Figure 18 below.
Operation under the RS485 standard requires that the following conditions
are met:
• Fewer than 32 instruments are connected.
• When receiving data, a logic high will be recognized when A > B by
200 mV.
• When receiving data, a logic low will be recognized when A < B by
200 mV.
• Maximum transmission line length is 4000 ft. using shielded, 26 AWG
twisted pair (Belden 9841).
• Shield connected to signal ground at all terminals.
• RS485 Line Termination—Each instrument is shipped with an internal
470
Ω
termination resistor (R7). This is the proper termination for a
network consisting of one host (with a standard 120
Ω
termination
resistor) and up to four instruments. When more than four instruments are
connected, R7 should be removed on all but the four instruments furthest
from the host.
Table 3 RS485 Connections
TB6 Signal Name
TB6-1 Signal Ground
TB6-2 A
TB6-3 B
TB6-4 Not Used
Table 4 RS485 Signal Levels
Data A B Logic Level
Marking (idle) 5 V 0 V High
Spacing 0 V 5 V Low