E.7.1 Impedance Matching - Honeywell CM4 Technical Handbook

CM4 Four-Point Continuous Monitor

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.
Refer to the computer interface manual
to configure the Impedance Resistor.
Computer
(B) TX/RX+
(A) TX/RX-
Ground
Address
000
Figure E-4: Configuration showing Computer at one end of a Daisy-Chain
CM4 Impedance Resistor
See Figure E-6
CM4
2 (B) TX/RX+
3 (A) TX/RX-
7
Ground
Address
000
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.
CM4 Technical Manual
CM4 Impedance Resistor
See Figure E-6
CM4 CM4 CM4
2 2
2
3 3
3
7 7
7
Address Address Address
001 002 Last
CM4 Impedance Resistor
See Figure E-6
Computer CM4 CM4
2
2
3
3
7
7
Address Address Address
001 Last
002
Impedance
Termination
Resistor
Figure E-6: CM4 RS-485 Interface Board
Impedance Resistor Location.
E-33