CH1808
Telephone Line Status Detector
INTRODUCTION
FEATURES
The CH1808 Telephone Line Status Detector detects
FCC Part 68 approved.
severed
lines,
the
off-hook
condition,
remote
Used on public switched telephone lines and “wet”
disconnects and incoming call ringing. This device is
leased lines.
ideal for security systems, alarm systems, or critical
Detects off-hook on the local loop.
instrumentation products where it is essential that the
Detects automatic off-hook for separate telsets.
line status be monitored.
Because the CH1808
Detects a severed telephone cable; does not
accesses the Public Switched Telephone Network
require polling.
(PSTN), it is FCC Part 68 approved. This approval is
Detects incoming ringing.
provided to the end user with conveyed registration.
Optionally
configured
for
remote
off-hook
Each unit is delivered with a label to be mounted by
detection.
the user on the host equipment indicating compliance
Requires minimum PCB area by using single in-
with FCC Part 68 requirements.
line package (SIP).
Operates at low power and uses a single +5V
The CH1808 operates on a single 5V supply and is
supply.
housed in a single in-line package 1.4" long and 0.45"
high. This package requires minimal PCB area. The
CH1808 provides three logic outputs: Local Loop
Monitor (LLM), Ring Indication (RI), and Off-Hook
Detect (OHD). The LLM output becomes active when
the CH1808 detects a loss of voltage resulting from a
cut line or when another device goes off-hook on a
shared line.
The RI becomes active when the
telephone company Central Office (CO) applies a
ringing signal to the PSTN line. The OHD becomes
active when current flow on the line is detected caused
by the use of a shared handset. The OHD can also be
used to detect a remote line disconnect event.
PRINCIPLES OF OPERATION
Operation of the CH1808 can be characterized by the
following functional blocks:
Local loop voltage loss detect
Ring detect
Figure 1.
CH1808 Block Diagram.
Off-hook/current detect
Basic Operation. Figure 1 shows these blocks and
All North American PSTN lines and most “wet” lease
their input and output functions.
The PSTN wires
lines can use the Local Loop Voltage Loss Detect
extending from the users equipment to the CO are
function. “Dry” leased lines may not be used with the
referred to as the local loop. This loop is connected to
CH1808 because there is no voltage present. When
the CH1808 via the TIP and RING pins.
Drawing
the line voltage between TIP and RING drops below
current from the local loop signals to the CO that the
18V, the LLM output is driven high.
equipment is off-hook. The off-hook condition occurs
either when answering a call or when placing a call.
The LLM output has 45K Ω pull-up resistance to +5V
When not drawing current, the equipment is said to be
and 2.2 µ F capacitance to ground. This is provided to
on-hook.
filter out zero crossing pulses that occur during ringing.
If the CH1808 output drive current is not sufficient, it is
Local loop Voltage Loss Detection. A voltage is
permissible to add a lower value resistor (not less than
always present whether the telephone line is in use
1K Ω ) in parallel between the output and the +5V pin, if
(i.e., off-hook) or idle (i.e., on-hook). The line voltage
the zero crossing pulses are not objectionable. If more
must be at least 18V to indicate an operational line.
drive current is needed while retaining zero cross
2002 Cermetek Microelectronics, Inc.
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CH1808 Telephone Line Status Detector
filtering, an external capacitor must be added to either
APPLICATIONS
match the existing RC time constant or to increase it.
The following are typical CH1808 applications.
When using the LLM circuit, external leakage and
Off-Hook Detect.
The CH1808 can be used to
capacitance on the TIP and RING input must be
deactivate a shared line TESLET device when on off-
minimized.
Differential leakage in excess of 0.1 µ A
hook condition is detected in a remote device.
In
may appear as a CO battery voltage to the CH1808.
Figure 2, the CH1808 activates a relay which
This could result in the CH1808 failing to detect a cut
disconnects Telset 1 from the TELCO TIP line when
line. Similarly, capacitance on the TIP and RING lines
Telset 2 is off-hook. The relay is activated when OHD
in excess of 0.1 µ F could delay a cut line detection for
goes low.
tens of seconds.
Potential sources of capacitance
include devices that share the local loop such as
Ring Detection . The CH1808 may be used to create
modems and extension telephones.
a conditioned envelop wave form of the incoming ring
signal. Referring to the circuitry shown in Figure 3, the
Ring Detection. The CO signals an incoming call by
4.7 µ F capacitor supresses the RI return to HIGH
placing a ring signal on the local loop. The CH1808
during zero-crossing of the ring pulse and holds RI to
monitors the loop for this signal. The Ring Indication
approximately 0.2V. The comparator circuit maintains
output RI is normally HIGH.
When the CH1808
a stready output when RI is active. A NAND Gate may
detects ringing, the RI output follows the ringing cycle.
be substituted for the comparator circuit.
It is set LOW during the 2-second (typical) ring period
and is restored to HIGH for the 4 seconds (typical)
Alternatively, the RI output from the CH1808 may be
between rings.
applied directly to a microprocessor input port and RI
signal debouncing can be performed with software.
False Ring Indication may occur during pulse dialing or
Note that the RI signal frequency during ringing is
when any device being monitored is off-hook.
twice that of the incoming ringing signal provided by
Therefore, RI should be ignored whenever OHD is
the CO. The CO signal is typically 20Hz but older CO
active or whenever pulse dialing is taking place.
systems range from 15.3Hz to 68Hz.
Off-Hook/Current Detection. The CH1808 supports
a telephone handset connection via the TIP OUT and
RING Pins.
The CH1808 will provide a logic
compatible indication to the host when an attached
telephone is taken off-hook.
The Off-Hook Detect,
OHD, will be active LOW any time the handset is
removed from the telephone cradle. Because pulse
dialing is achieved by repeatedly switching the telset
on and off-hook, the OHD will mirror this pulse by
switching LOW and HIGH at a rate of 10 pulses per
second (typical).
NOISE SUPPRESSION
The CH1808 interfaces to the PSTN line and, as such,
is subject to transients produced by electrical
discharge and ring voltage transitions.
This may
cause momentary false status change indications.
While some suppression is built into the CH1808 and
is adequate for most applications, additional external
capacitance on the outputs may be required.
Any
added
capacitance
should
be
determined
by
appropriate analysis and testing in the particular
Figure 2. Dual Telset Control.
application. Cermetek suggests that the outputs be
level detected as opposed to edge detected to
minimize the false indications on excessively noisy
lines.
2002 Cermetek Microelectronics, Inc.
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Cermetek Microelectronics, Inc.
CH1808 Telephone Line Status Detector
DESIGN
CONSIDERATIONS
FOR
FCC
CONFORMANCE
The CH1808 includes interface circuits that couple to
the PSTN line and provide FCC required isolation and
protection.
The following guidelines should be
followed to maintain FCC compliance:
1. CH1808 must be mounted away from hazardous
voltages.
2. Connecting the CH1808 to PSTN lines should be
made through a standard RJ-11 jack or other
approved connector device.
Figure 3.
Ring Indicator Envelope Filter.
3. Circuit board traces to the CH1808 TIP, RING,
and TIP OUT pins must have a spacing to
adjacent traces exceeding 0.10 inch. TIP, RING,
Remote Disconnect Detect.
The Off-Hook Detect
and TIP OUT traces should have a nominal width
circuit may be utilized to signal when a remote call has
of 0.020 inches.
terminated. To use the CH1808 in this mode, connect
the user equipment to RING and TIP OUT as shown in
4. TIP, RING, and TIP OUT traces should be as
Figure 4.
short as possible to prevent coupling with other
signals. Mount the CH1808 close to the PSTN
During a telephone call, the OHD pin is driven LOW.
line connection.
Once
the
CO
detects
a
disconnect,
the
CO
intentionally removes the loop current momentarily.
5. No additional circuitry shall be connected between
This event can be used to signal the disconnect. The
the CH1808 and the PSTN line’s RJ-11C jack
loss of loop current usually occurs within 10 to 12
except as shown in Figure 4.
seconds of the disconnect and persists for 800 to
1,000ms.
The OHD output, detecting the loss of
6. The FCC registration label included with the
current, is driven HIGH during this event.
CH1808 must be affixed to the outside of the host
product.
Figure 4.
Disconnect Detect for Telset Unit connected to TIPOUT and RING.
2002 Cermetek Microelectronics, Inc.
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CH1808 Telephone Line Status Detector
USER MANUAL
In the event repairs are ever needed on the (insert
The following information should be included in the
your product name), they should be performed by
host product user manual.
(insert your company name) or an authorized
representative of (insert your company name).
For
Type of Service : The (insert your product name) is
information contact: (insert your company address).
designed to be used on standard device telephone
lines. It connects to the telephone line by means of a
Changes in Attestation procedure for Plugs and
standard jack called USOC RJ-11C or (USOC RJ45S).
Jacks: (Name of applicant) attests that the network
Connection to the telephone company provided coin
interface plugs or jacks used on this equipment comply
service (CO i mplemented systems) is prohibited.
with and will continue to comply with the mechanical
Connection to party line service is subject to state
requirements specified in Part 68, Subpart F,
tariffs.
specifically the dimensions, tolerances, and metallic
plating requirements.
The compliance of these
Telephone Company Procedures : The goal of the
connectors will be assured by purchase specifications
telephone company is to provide you with the best
and incoming inspection.
Documentation of such
service it can. In order to do this, it may occasionally
specifications and/or inspections will be provided to
be necessary for them to make changes in their
the FCC within 30 days of their request for same.
equipment, operations, or procedures.
If these
changes affect your service or the operation of your
equipment, the telephone company will give you
written notice to allow you to make any changes
necessary to maintain uninterrupted services.
If you have any questions about your telephone line,
such as how many pieces of equipment you can
connect to it, the telephone company will provide this
information upon request.
In certain circumstances, it may be necessary for the
telephone company to request information from you
concerning the equipment you have connected to your
telephone.
Upon request by the telephone company, provide the
FCC registration number and the ringer equivalence
number (REN) of the equipment connected that is
connected to your line; both of these items are listed
on the equipment label. The sum of all the RENs on
your telephone lines should be less than five in order
to assure proper service from the telephone company.
In some cases, a sum of five may not be usable on a
given line.
If Problems Arise : If any of your telephone equipment
is not operating properly, you should immediately
remove it from your telephone line because it may
cause harm to the telephone network. If the telephone
company notes a problem, they may temporarily
discontinue service. When practical, they will notify
you in advance of this disconnection.
If advance
notice is not feasible, you will be notified as soon as
possible. When you are notified, you will be given the
opportunity to correct the problem and informed of
your right to file a complaint with the FCC.
2002 Cermetek Microelectronics, Inc.
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CH1808 Telephone Line Status Detector
Table 1.
CH1808 Pin Descriptions.
Pin
Name
I/O
Function
1
RING
I/O
Telco/Telset Port . Direct connection to the RING lead of a telset and RING lead of
the telephone line through a standard RJ11 jack. PSTN or two-wire “wet” leased
line connection required.
2
TIP
I/O
Telco Port . Direct connection to the TIP lead of the telephone line through a
standard RJ11 jack. PSTN or two-wire “wet” leased line connection required.
3
TIP OUT
I/O
Telset Port . Direct connection to the TIP lead of a telset.
4
OHD
O
Off-Hook Detect . Active LOW when telset device is connected to TIPOUT and
RING. When the telephone handset is removed from its cradle, OHD is driven
LOW. Once active, OHD will momentarily be driven HIGH after disconnect of telset
connected to TIPOUT and RING.
5, 9, 10
GND
I
Ground . Signal and power common ground. Pins 5, 9, and 10 are internally
connected.
6
RI
O
Ring Indication . Active LOW. RI is driven LOW during the 2 second (typical) ring
period and HIGH during the 4 seconds(typical) between rings.
RI is otherwise
HIGH (i.e., when no ring is present).
7
V CC
I
+5V: Positive Supply . This supply is +5.0VDC ± 10%.
8
LLM
O
Local Loop Monitor . Active HIGH. LLM is driven HIGH when the local loop is
severed or when another device on the local loop is off–hook.
Table 2.
CH1808 Electrical Specifications.
V CC = +5V ± 10%
T A =0° to 55°C UNLESS OTHERWISE SPECIFIED
Parameter
Symbol
Conditions
Min.
Typ.
Max.
Units
LOGIC
Output high
V OH
IOH = 40mA
2.4
V
Output low
V OL
IOL = 1.0mA
0.4
V
TELEPHONE LINE
INTERFACE
Line Resistance
R L
Tip to Ring
7
M
ohms
Input Resistance
R I
Tip or Ring to any other pin
30
1500
M
ohms
Surge Protection
Conforms to all FCC Part 68 surge,
V
hazardous
voltage,
and
leakage
requirements
LLM Threshold Voltage
V LLM
Voltage across Tip and Ring
16
18
20
V
LLM Detection Time
I D
CX = 0.001 µ F (Tip or Ring to GND)
1
sec
VCX = 50V (Tip to Ring)
OHD Threshold Current
I D
Current from Tip to Tip Out
5
10
mA
Loop Current
I LOOP
Current from Tip to Tip Out
2
100
mA
Ring Detect Threshold
15.3 to 68Hz
20
130
V rms
Leakage Current
I L
Leakage to Tip to Ring
40
0.1
mA
POWER
Leakage Current
I CC
Supply Voltage = +5V
2
5
mA
2002 Cermetek Microelectronics, Inc.
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Document No. 607-0012 Revision B (01/02)
Cermetek Microelectronics, Inc.
CH1808 Telephone Line Status Detector
Table 3.
Summary of CH1808 Family of Products.
Model
Summary of Features
Operating Temperature
CH1808
FCC Part 68 Approved
0°C to +70°C
Figure 5. CH1808 Physical Dimensions and Pin Functions.
Cermetek reserves the right to make changes in specifications at any time and without notice. The information furnished by Cermetek
in this publication is believed to be accurate and reliable. However, no responsibility is assumed by Cermetek for its use, or for any
infringements of patents or other rights of third parties resulting from its use. No license is granted under any patents or patent rights
of Cermetek.
Printed in U.S.A
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WEB SITE: http:/www.cermetek.com | EMAIL: sales@cermetek.com
2002 Cermetek Microelectronics, Inc.
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Document No. 607-0012 Revision B (01/02)