Line monitoring

In security systems where intercom stations are used in emergency situations only, maybe just once per year, it’s extremely important that the equipment works when it’s finally used. To make a security system useful in practice, it’s necessary that the equipment has built-in fault monitoring

ASLT subscriber lines

Wire test

Error detection

The AlphaCom’s line boards ASLT (4-wire TouchLine station) and ATLB (2-wire telephone) monitors station wires continuously based on the fact that certain currents flows during the stations when everything is OK, and that either wire break or short-circuit creates abnormal currents that are detected and reported. The line boards has an on-board microcontroller which performs the line monitoring along with other tasks. This means fast response (errors are reported after 2-4 seconds), and there is no load on the functional software that runs on the AMC board.

NOTE: The ASLT board determines automatically whether a station has display or not depending on the ab-wire current (display stations draw a constant 10 mA to power the display). Some line error conditions may be detected to be wrong station type rather than a line problem. When the exchange is in doubt, a so-called “extended check” is performed - see detailed description below.

Error state: Station down

When a station wire is detected to be faulty, the station is assumed useless and is blocked by the exchange software. It’s no longer possible to dial to the station, i.e. attempts are rejected with a failure tone. (Group Calls will be connected, as there is no check of each station’s error state due to group set-up speed). Other station tests does not cause station down, they are just reported.

ASLT

ASLT performs continuous line monitoring, which can NOT be disabled. Trick: If you need a “portable” station that can be plugged in and out without causing line error reports, you can connect resistors close to the exchange to form a “dummy station”, and then modify the portable station so that it lacks the corresponding resistors (contact support for details).

TouchLine station without display

• Break of all wires (station un-plug) is detected, but needs 1 to 3 minutes before report.
• The ab-wires (speaker) encodes the functions Off-hook, On-hook, M-key, C-key where off-hook is close to wire break, and C-key is close to short-circuit. <br\>

- Off-hook is not possible to separate from break, and is reported if the handset is off for more than one hour and no dialling takes place. (AMC feature, from version 07.20. Timer can be modified using TST, .ex_profile.timeouts.off_hook_too_long = 36000 (0x8ca0), new value e.g. 200 for 20 seconds.).<br\> - M key in idle for more than 10 minutes (from AMC 07.12/07.60: 1 hour 50 min) causes “station down”, then change to display-station mode.<br\> - Short-circuit close to the station may not be detected on long lines (1 km) as the wire loop resistance limits the current to the normal C-key area.

• The cd-wires (microphone, keyboard) encodes the functions OPEN, PRIVATE, digit-key where OPEN is close to wire break, and digit-key is close to short-circuit. Single error on cd is not reported.
• Note that some cross-connects like c-wire shorted to b-wire is not possible to detect as the line currents change very little.

TouchLine station with display

• In this case the ab-wire is supplying power for the display, by adding a constant current of 10 mA to the ab-wires.

<br\> - Break is here detected immediately as there is a guaranteed 10 mA current in normal Off-hook. <br\> - Short circuit on long lines is even more difficult to detect here as the C-key current is higher.

• From AMC 07.20: You can select a special station type “Interguard”, which means that line monitoring shall report deviations from display station limits immediately. Station type CRM 3 & 4 is included also.

ASLT Extended Line Check

In order to distinguish between the otherwise indistinguishable "station disconnected" and "non-display station with off-hook" states, a so-called "extended check" is initiated every 23 seconds if the station is presumed disconnected or in off-hook idle.

PS: The station is still considered idle when passively listening to audio programs, simplex (program) conferences or group calls.

The extended check utilises 4 closely spaced polarity reversals on the CD wire pair (double flash in station LED) to do this. As a side effect there will be double flashes on a station every 23s when the handset is left (or has fallen) off-hook so long that the handset dial tone has timed out. This is not a bug -- it is a feature! Consequently, this is done for display stations also.

When the station is actively used or a change of station is being detected, no such checks are done. All activities will, however time out and return to idle, allowing the plugging in or out of a non-display station with handset off to be detected eventually, no matter what. <br\> Exception: Conversation does by default not time out -- time-out must be programmed explicitly.

ASLT error codes

The ASLT reports a code value to pinpoint a problem. Note that only the first detected error is flagged, and that there may be additional errors on the same station.<br\> 80 (1000 0000) = AB Shorted<br\> 40 (0100 0000) = AB Open<br\> 20 (0010 0000) = CD Shorted<br\> 10 (0001 0000) = CD Open

ATLB

You can activate line monitoring of (modified) telephones in AlphaPro per physical number. It’s possible to select the limit for the current, which is necessary to handle various line voltages and line lengths.

Telephone with parallel resistor

• An on-hook telephone draws very little current. To be able to detect a wire break, it’s necessary to connect a resistor in parallel close to (or inside) the telephone.
• An off-hook telephone draws a lot of current which is not possible to separate from short-circuit, and is reported if the handset is off for more than a one hour and no dialling takes place.

ATLB Error Codes

Currently, ATLB reports identically to ASLT. (As there is little information in that, this may be changed in the future to the 8-bit line current value which is compared to the value entered in AlphaPro as monitoring limit).

40 (0100 0000) = AB Open

Supervision of IP stations

The IP substation is supervised in two different ways.

1. Each IP-sub-stations have its own unique TCP socket for data communication. This socket send TCP-alive frames every 5 second, thus a broken socket is discovered within 10 seconds and reported as line error from AlphaCom.
2. From the Application layer AlphaCom is sending a "PING" approximately every 90 second. The stations reply with a "PONG". If one PONG is missing the station is reported as line fault.

IP station down is now reported to the event handler "faulty station line" ON/OFF and reported on the log port in the two following ways:

930: Feb 13 16:17:58 src@Node1 AlphaLog: N001.D065:.Exch:@D65.B25-1  Line Down: IP ST Data Com St 20  "1001 -"

(Data communication is lost, no PING reply or socket closed)

933: Feb 13 16:21:34 src@Node1 AlphaLog: N001.D065:.Exch:@D65.B25-1  Line Down: IP ST License St 20  "1001 -"

(This happens if you enter a new license key with no IP stations licenses or not enough licenses, then some stations will be out of business)

The system station will report like ordinary line faults with reference to master board and first port since the IP station is not related to any physical AMC board position: