CM4 Four-Point Continuous Monitor
CM4 Technical Manual
E-33
E.7.1 Impedance Matching
The communication interfaces in the first and last units in the daisy-
chain configuration must have matching impedance. This is true
whether the computer is at the end of the daisy chain or within the daisy
chain. Check the computer’s interface to determine the impedance
termination resistor’s value and how to install/set. The illustrations
below show two daisy chain schemes, the first with the computer on
the end of the daisy chain, and the second with the computer within
the daisy chain. The impedance termination resistors on all interfaces
between the first and last units are removed.
CM4
Computer
(B) TX/RX+
(A) TX/RX-
Ground
2
3
7
2
3
7
2
3
7
CM4
CM4
Address
000
Address
001
Address
002
Address
Last
CM4 Impedance Resistor
See Figure E-6
Refer to the computer interface manual
to configure the Impedance Resistor.
Figure E-4: Configuration showing Computer at one end of a Daisy-Chain
Computer
CM4
(B) TX/RX+
(A) TX/RX-
Ground
2
3
7
2
3
7
2
3
7
CM4
CM4
Address
000
Address
001
Address
002
Address
Last
CM4 Impedance Resistor
See Figure E-6
CM4 Impedance Resistor
See Figure E-6
Figure E-5: Configuration showing Computer in the middle of a Daisy-Chain
The CM4 unit’s impedance termination resistor is located on the
RS-485 interface as shown below. The resistor plugs into sockets on
the interface board for easy removal or insertion; there is no soldering
required.
Impedance
Termination
Resistor
Figure E-6: CM4 RS-485 Interface Board
Impedance Resistor Location.
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