Micromod MOD: MODCELL Multiloop Processor 2002N Model C and Associated Hardware - Manual (Page 53)

MODCELL Multiloop Processor
COMMUNICATIONS CONNECTIONS
4 - COMMUNICATIONS CONNECTIONS
4.1 ICN CONNECTIONS
A typical Instrument Communications Network (ICN) configuration is shown in Figure 4-1.
4.1.1 Cable Requirements
The total length of the ICN includes the total length of the physical two-wire bus between
each node on the ICN and the length of any MOD 30 instrument cables between the
nodes and the instruments. This length can be up to 2000 ft (609.6 m). Cable
requirements for an ICN are dependent upon the length of the ICN as described below.
• When the total length is 500 ft (150 m) or less, use 18 AWG (1 mm) twisted pair
cable.
• When the total length exceeds 500 ft (150 m):
- Entire length of the ICN must be at virtually the same potential and voltage drop
between any two points on ICN must not exceed 3V.
- Cable capacitance for an ICN must be between 18 and 25 pf/ft (60-83 pf/m).
- Any MOD 30 instrument termination node must include R9 (on 1722F and 1730F
boards) or R31 (on 1723F board).
• When the total length exceeds 1500 ft (450 m) or if the ICN must be routed through
high noise (EMI/RFI) environments, use 22 AWG (0.64 mm) shielded cable. If an
ICN must be run next to power lines or other unusual noise frequencies, contact your
service representative for assistance.
4.1.2 ICN Addresses
Each ICN supports addresses 0 through F (0 through 15). The address is set at the ICN
module as shown in Figure 4-1.
4.1.3 Termination
One set of ICN termination resistors must be installed, Figure 4-1, on an ICN to prevent
noise from being picked up by the ICN circuitry and generating a diagnostic alarm. The
ICN termination scheme requires a nominal 24 volt DC power supply that can supply
15.4 mA (the Model B ICN module provides this). When selecting a termination
scheme consider the following:
• ICN wires must be shielded and connected directly to chassis ground at one end
only.
• If an external power supply is being used, the negative rail should be connected to
chassis ground when possible.
• If the Local Control Panel (LCP) is being used, the LCP ICN resistors kit 175S0024
can be installed to ensure that the ICN is properly terminated. The LCP uses 24 Vdc
power anyway and redundant power supplies can be connected with this kit.
• If MOD 30 instruments are on the network (and no LCP), the ICN can be terminated
using resistors R7, R8 and R9 on any single 1722F Field Input Board or resistors
4 7