Johnson Controls tyco iSTAR Ultra, Installation And Configuration Manual, Page: 95 / 118

Reader - Voltage Requirements and Distance
iSTAR Ultra Installation and Configuration Guide 5–29
Reader - Voltage Requirements and Distance
To operate properly, each reader requires the following minimum voltage:

A standard RM Series Reader or RM-4 board requires at least 7.5 volts
.

An RM-4E board requires at least 11 volts
.
The iSTAR Ultra supplies 12 volts at its reader connectors; however, the amount of voltage
that reaches the reader is affected by the following:

Number of devices on the bus

Current draw of each device

Wiring length between the devices and iSTAR Ultra

Wire gauge that connects the devices

State of the battery (if running on apS)

Tolerance if sourced by an external power supply.
To determine the maximum distance of a reader from an iSTAR Ultra, calculate the voltage
that reaches each reader. If the voltage is insufficient, you can shorten the wire length, use a
heavier wire, or add UL294 power-limited power supply.

Wire resistance is as follows:
• 24 AWG = 26.0

per 1000 ft.
• 22 AWG = 16.5

per 1000 ft.
• 20 AWG = 10.2

per 1000 ft.
• 18 AWG = 6.5

per 1000 ft.
Table 5-9 on page 5-30 lists the general wiring requirements for an iSTAR Ultra and its
components.
NOTE
You must consider the wire length both to and from the reader, in the calculation.
Example:

The RM-4 is 500 feet away. (1000' round trip)

The RM-4 draws 250 mA. and is connected with 20 AWG wire.
The equation to calculate voltage drop is:
E= I*R
I = .25 A (250 mA)
R= 10.2 ohms (from the table)
E= .25 * 10.2
E= 2.55 (voltage drop)
12 Vdc - 2.55 Vdc = 9.45 Vdc
The connection meets the requirement.