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2.9 RS485 Communications
16116Install.fm
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
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
TB6-1
TB6-2
TB6-3
TB6-4
Table 4 RS485 Signal Levels
Data
Marking (idle)
Spacing
4, and
Figure 18
below.
Signal Name
Signal Ground
A
B
5 V
0 V
0 V
5 V
Section 2
A
B
Not Used
Logic Level
High
Low
Page 23
RS485 Communications
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