Johnson Controls METASYS ASC, Networking And Troubleshooting Manual

Page: 13 / 48

Introduction—ASC and N2 Bus Networking and Troubleshooting Guide 13
The ASCs have electrical protection built into the N2 Bus transceiver
circuit. It prevents the N2 Bus circuitry from being damaged if someone
inadvertently connects a voltage source greater than 5 VDC/VAC between
any two or the three N2 Bus terminals. Refer to Table 2.
Table 2: Voltage Protection on ASCs
Controller Voltage N2 Bus Circuitry Can Withstand
AHU < 30 VDC/VAC
LCP
< 6 VDC/VAC (before 3/1/92)
< 12 VDC/VAC (after 3/1/92)
MM-CVT101-0 < 30 VDC/VAC
UNT < 30 VDC/VAC
VAV < 30 VDC/VAC
The following table summarizes the rules and maximums allowed for
installing the N2 Bus. You may wish to photocopy this table and keep it
handy.
Table 3: N2 Bus Rules
Category Rules/Maximums Allowed
General Only one PC/Panel/
LTD Version Companion per
N2 Bus
Only daisy-chained devices
Maximum Number of Devices 100 devices per PC/Panel/ LTD Version Companion
50 devices per repeater
2 repeaters cascaded
Line Length and Type 5000 feet between repeaters
15,000 feet from PC or Panel/ LTD to farthest ASC
(3 segments of 5000 feet each)
26 AWG twisted pair minimum
(stranded 22 AWG or heavier recommended)
Terminations 2 switched EOL per segment (preferred)
1 switched EOL per segment (required)
The N2 Bus uses End-of-Line (EOL) terminations through the use of
jumpers on the Companion Panel/LTD, RS232-to-N2 Bus converter
(MM-CVT101), and repeater. N2 Bus circuitry in the ASCs is self-
terminating. Figure 5 shows an electrical drawing of the end-of-line
wiring for the MM-CVT10-1 and the Panel/LTD Companion.
N2 Bus Rules
N2 EOL
Termination