The Keypad Data Bus and the Voltage at the Far End

Why this matters

The keypad at the far end of the house is the device most likely to fail during a power outage, and it fails at the worst moment: hours into the outage, on battery, when the panel's output has sagged and the keypad's backlight and sounder are pulling their hardest. On AC it looked fine at install. The call comes in as "the garage keypad goes blank and the system says it lost a keypad", usually after a storm, and the cause is not the keypad. It is copper.

What the bus is

Most panels run their keypads and other addressable devices on a four-conductor bus:

• Auxiliary power (+), conventionally red, the panel's regulated output.
• Common (-), conventionally black, the return for both power and data.
• Two data conductors, conventionally green and yellow, carrying the panel's polling and the devices' replies.

Colors and terminal names vary by maker; land them per the panel's documentation, not by habit. Everything on the bus shares the same power output and the same common: keypads, zone expanders, wireless receivers, output modules, sometimes a cellular communicator. The data side is a conversation the panel holds with each device by address; the power side is just wire, and wire has resistance.

Two limits come from the documentation, and they are separate: the total current the auxiliary output may supply (everything on the bus plus anything else powered from aux, like motion detectors), and the wire length the bus may run (often stated as a maximum per leg and a maximum total across all legs). A layout can pass one and fail the other.

The layout in this example

A two-story house with the panel in the basement. Device currents are illustrative maximums read off each device's spec sheet (keypad figures are with backlight on and sounder active, which is how a keypad draws during entry delay):

PANEL AUX  13.6 V on AC, 12.0 V late in an outage
   |
   +-- 30 ft 22/4 home run ---------> KP1 hall keypad
   |                                   0.15 A
   +-- 60 ft 22/4 home run ---------> RCV wireless rcvr
   |                                   0.06 A
   +-- 110 ft 22/4 --> EXP zone expander
                       0.04 A
                         |
                         +-- 90 ft 22/4 --> KP2 garage kp
                                            0.15 A
BUS TOTAL  0.15 + 0.06 + 0.04 + 0.15 = 0.40 A

That 0.40 A comes off the panel's auxiliary rating and goes into the standby battery calculation; the battery math belongs to How to Size a Standby Battery for an Alarm Panel.

Working the drop, leg by leg

Twenty-two gauge copper is about 16.1 ohms per 1,000 ft of single conductor. Current goes out on the red and comes back on the black, so the loop length is twice the cable length. Voltage drop is current times loop resistance, and on a daisy chain each segment carries the current of everything beyond it.

KP1, home run, 30 ft. Loop 60 ft, so 60 x 16.1 / 1,000 = 0.97 ohms. Drop 0.15 A x 0.97 = 0.15 V.

RCV, home run, 60 ft. Loop 120 ft, 1.93 ohms. Drop 0.06 A x 1.93 = 0.12 V.

Daisy chain to KP2.

• Segment 1, panel to expander, 110 ft: loop 220 ft, 3.54 ohms, carrying the expander plus KP2, 0.04 + 0.15 = 0.19 A. Drop 0.19 x 3.54 = 0.67 V.
• Segment 2, expander to KP2, 90 ft: loop 180 ft, 2.90 ohms, carrying KP2 only, 0.15 A. Drop 0.15 x 2.90 = 0.44 V.
• At KP2: 0.67 + 0.44 = 1.11 V down from the panel.

Check it the other way. Treat the whole 200 ft to KP2 as if it carried 0.15 A: 400 ft of conductor is 6.44 ohms, times 0.15 A is 0.97 V. The real figure is higher by the expander's 0.04 A across segment 1, 0.04 x 3.54 = 0.14 V, and 0.97 + 0.14 = 1.11 V. The two methods agree.

Now the two operating points. On AC the panel's aux reads 13.6 V, so KP2 sees 13.6 - 1.11 = 12.49 V. Late in an outage the aux has followed the battery down to about 12.0 V, so KP2 sees 12.0 - 1.11 = 10.89 V. Say this keypad's documentation gives a minimum operating voltage of 11.0 V (illustrative; read the real one off the spec sheet). On AC it has 1.49 V of margin. On battery it is 0.11 V below its floor, and it drops off the bus exactly when the customer most needs it to show them what is going on.

KP1 on the same outage sees 12.0 - 0.15 = 11.85 V and stays up. That is why the call names one keypad, and why swapping it fixes nothing.

Three fixes and what each one costs

Heavier wire on the long leg. Eighteen gauge is about 6.4 ohms per 1,000 ft. Rerun the daisy chain on 18/4: segment 1 is 220 x 6.4 / 1,000 = 1.41 ohms, drop 0.19 x 1.41 = 0.27 V; segment 2 is 1.15 ohms, drop 0.15 x 1.15 = 0.17 V; total 0.44 V. On battery KP2 now sees 12.0 - 0.44 = 11.56 V, 0.56 V above its illustrative floor. Costs a pull of new cable, which in a finished house is the expensive part.

Double the power conductors. Where the existing run is a 22/6 or there is a spare 22/4 alongside, put two conductors in parallel on aux and two on common. That halves the loop resistance of the power path: drop to KP2 falls from 1.11 V to about 0.56 V, and on battery KP2 sees 11.44 V. The data pair stays as it was. Cheaper than a pull if the spare copper is already there.

A local power supply at the far end. A listed, supervised power supply near KP2 powers the keypad, with its common tied to the panel's common and only data and common coming from the panel. KP2's drop on the bus disappears and the panel's aux load falls by 0.15 A. The trade is a second battery to maintain and a supply that must report its own AC loss and low battery back to the panel, so it belongs on a zone or an output the documentation says to use.

What does NOT fix it: turning down the keypad backlight in programming (it helps a little and the next tech turns it back up), or moving KP2 to a home run of the same 22/4. A home run straight from the panel is still 200 ft of cable: the drop falls only by the expander's 0.14 V share, to 0.97 V, and KP2 sees 11.03 V on battery. That clears the illustrative floor by 0.03 V, which is a margin a cold night or a tired battery eats.

Home runs, daisy chains and fault isolation

Home runs make each leg's voltage drop independent, so the far keypad is not also carrying an expander's current. They also make faults easier to isolate: lift one leg at the panel and see whether the bus comes back. On a daisy chain, a broken data conductor at the expander takes KP2 off line with it, and a short anywhere takes down everything.

Both topologies share one hard fact: the bus is one circuit. A short between aux and common on any leg, home run or chain, pulls the whole output down and every keypad goes dark together. A bus that died all at once after a drywall job is a staple, not a dead panel.

Mind the panel's length limits more on home runs than on chains, because four short home runs can add up past a stated total that one long chain would never reach.

How to measure it, under load, at the device

The calculation predicts; the meter decides. Measure at the device's own terminals, not at the panel.

• Put the account on test before you start pulling AC or tripping zones; see related: How to Put an Alarm Account on Test and Take It Off.
• Load the bus the way an alarm does. Start an entry delay so the keypads are beeping, with backlights on. A keypad at idle draws less and reads better than it will when it matters.
• Read aux and common at KP2, and at the panel's aux terminals at the same moment. The difference is the real drop.
• Then take AC away. Unplug the panel's plug-in transformer to run on battery. If the transformer is hardwired to a 120 V circuit, leave that circuit to an electrician where local rules require, and simulate on battery only if the panel offers a test. Read both points again with the battery carrying the load. Plug the transformer back in and confirm the keypad clears the AC-loss trouble before you leave.
• To read bus current, lift the aux conductor at the panel and put a meter in series on its amps range. Return the conductor to the terminal afterwards and confirm the keypads come back on line. Take the account off test only after the station confirms the AC restore and a test signal.

Write the readings on the work order: "KP2 after the 18/4 rerun: 13.16 V on AC, 11.55 V on battery, entry delay running." A far-end voltage on the record is the one piece of data that answers the outage callback without another visit.

When it is the data, not the volts

A keypad that drops off the bus with healthy voltage at its terminals has a different problem, and three causes cover most of them:

• Two devices on one address. A replacement keypad left at its factory address collides with one already there; both act up, often intermittently. Check every device's address against the panel's device list before blaming wire.
• Data conductors swapped or split. Green and yellow reversed at one device, or one data conductor landed on a splice that is loose. The device may power up and show nothing, or show a "no communication" message the panel's manual names.
• Induced noise. A bus run strapped alongside line-voltage cable for a long stretch, or sharing a bored hole with it, picks up interference. Keep low-voltage cable separated from power wiring as the panel's documentation and your adopted electrical code require, and cross line-voltage runs at right angles.

What changes the answer

Cold matters: a sealed lead-acid battery gives less voltage under load when it is cold, so a panel in an unheated garage loses margin in winter. Adding devices later changes every segment upstream of them: a cellular communicator or an output module added at the expander raises segment 1's current, and KP2 loses margin without anyone touching its wire. Recalculate the leg whenever a device goes on the bus.

References

• Panel manufacturer's installation manual (auxiliary current rating, bus length limits, wiring and terminal assignments)
• Device spec sheets (standby and maximum current, minimum operating voltage)
• Published copper conductor resistance tables (ohms per 1,000 ft by AWG)
• See related: How to Size a Standby Battery for an Alarm Panel; Alarm Cable Types and Which Run Gets Which; Security + Alarm Systems Reference