ETC CH1808

Telephone Line Status Detector
The CH1808 Telephone Line Status Detector can detect se-vered
lines, off-hook status, remote disconnect, and incoming call ringing. The device is ideal for security systems, alarm systems, or
critical instrumentation products where it is essential that the status of the telephone line be monitored. Be-cause
the detector accesses the public switched telephone lines, it must
be FCC Part 68 approved. This approval is provided with conveyed
registration. A sticker is supplied for mounting on the user equipment to indicate compliance with FCC Part 68 requirements.
The CH1808 operates on a single 5V supply and is housed in a
single in-line package. 1.4" long and 0.45" high requiring minimal
PCB area. It employes 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
the same line. The RI becomes active when the telephone company Central Office (CO) applies a ringing signal to the telephone
line. The OHD becomes active when current on the line is detected caused by the handset removal from the cradle of a telephone set that is connected to the Tip and Ring Out port. The OHD
can also be used to detect a remote line disconnect.
FCC Part 68 approved
Used on public switched telephone lines and “wet” leased
Detects off-hook on the local loop
Detects automatic off-hook for separate telsets
Detects a severed telephone cable; does not require
Detects incoming ringing
Optionally configured for remote off-hook detection
Requires minimum PCB area by using single in-line
package (SIP)
Operates at low power and uses a single +5 V supply
The operation of the CH1808 can be characterized by the fol-lowing functional blocks:
Local loop voltage loss detect
Ring detect
Off-hook/current detect
Figure 1 shows these blocks and their input and output func-tions.
In the Public Switched Telephone Network (PTSN), the wires from
a telephone set to the CO are referred to as the local loop. This
loop is connected to the CH1808 via the Telco port TIP and RING
pins. Drawing current from the local loop signals to the CO that the
telephone is off-hook. This occurs either when answering a call or
when placing a call.
The CH1808 detects a cut line or another device off-hook on the
local loop by the reduction of the line voltage between TIP and
RING terminals. This voltage is always present whether the telephone line is in use or idle. The line voltage must be at least 18 V
to indicate the presence of the line. All North American PSTN lines
and most “wet” active lease lines can use the Local Loop Voltage
Loss Detect function. “Dry” leased lines may not be used with the
CH1808 because there is no voltage present. When the line drops
below 18 V due to a cut line or other device that is off-hook on the
local loop, the LLM output is driven high.
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Figure 1. CH1808 Block Diagram
The LLM output has 45 kΩ pull-up resistance to +5 V and 2.2 µF
capacitance to ground. This is provided to filter out zero crossing
pulses that can occur during ringing. The residence source of
output drive current might not be sufficient for some applications.
In these cases, it is permissible to add a lower value resistor (not
less than 1 kΩ) in parallel between the output and the +5 V pin, if
the zero crossing pulses are not objectionable. If more drive current is needed while retaining zero cross filtering, an external capacitor must be added to keep the RC time constant, the same or
When using this Local Loop Monitoring circuit, external leakage
and capacitance on the Tip and Ring input must be minimized.
Differential leakage to Tip and Ring in excess of 0.1 µA can appear
like a CO battery voltage to the monitor circuit.
Document No. 607-1808 Rev. A (3/99)
This could result in the monitor circuit failing to det-ect a cut line.
Similarly, capacitance on the Tip and Ring in excess of 0.1 µF
could delay a cut line detection for tens of seconds. Potential
sources of capacitance include devices that share the local loop
such as modems and extension telephones.
The CO signals an incoming call by placing a ring signal on the
local loop. The CH1808 monitors the loop for this signal. The
Ring Indication output RI is normally HIGH. When ringing appears, the RI output follows the ringing cycle. It is set LOW during
the typical 2-second ring period and is restored to HIGH for the
typical 4 seconds between rings.
False ring indication may occur during the pulse dialing on the
same line. Therefore, the Ring Indication Signal should be ignored whenever any device is off-hook and especially when pulse
The CH1808 supports a telephone connection via the Tip Out
and Ring port. The CH1808 will provide a logic compatible indication to the host when attached telephone is taken off hook. The
Off-Hook Detect, OHD, output 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 offhook, the OHD will mirror this pulse by switching LOW and HIGH
at a typical rate of 10 pulses per second.
Figure 2. Dual Telset Control
The CH1808 interfaces a telephone line and as such is subject
to transients produced by electrical discharge and ring voltage
transition, which could cause momentary false status change
detections. While some suppression is built in and is adequate
for most applications, additional external capacitance on the outputs may be desirable. The capacitance value that may have to
be added to each output should be determined by appropriate
analysis and testing for each particular application. It is suggested that the outputs be level detected as opposed to edge
detected to indicate line status. This will minimize the possibility
of false indications on excessively noisy lines.
The following are some typical applications for the CH1808.
In Telset 2 on the Tip Out port is off-hook, the OHD is driven LOW
as shown in Figure 2. This activates a relay disconnecting Telset
1 from the Telco Tip line.
The RI output is in raw form from an opto-isolator. The circuitry
shown in Figure 3 converts this raw output to a clean logic output.
The 4.7 mF capacitor supresses the RI return to HIGH during zerocrossing of the ring pulse and holds RI around 0.2 V. The comparator circuit maintains a stready output when RI is active. The
comparator circuit may be sub-stituted with a NAND gate.
The RI output from the CH1808 can be applied directly to a microprocessor input port and the signal debouncing can be done in
software. The pulse frequency during ringing is twice that of the
signal from the CO. The CO signal is usually 20 Hz but older CO
systems may provide signals that range from 15.3 to 68 Hz.
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Figure 3. Ring Indicator Envelope Filter
The Off-Hook Detect circuit can be utilized to signal when the
remote party has disconnected the call. To use the CH1808 in
this mode, connect the local telset or data device to Ring and Tip
Out as shown in Figure 4.
During a telephone call, the OHD pin is driven LOW. Once the
telephone system detects a remote disconnect, a signal is sent
to the local loop. This usually occurs about 10 to 12 seconds
after the remote party has hung up. This issues a break in the
local loop current for 800 to 1,000 ms. The OHD output, detecting the loss of current, is driven HIGH during this break.
Changes in Attestation procedure for Plugs and Jacks
(Name of applicant) attests that the network interface plugs or
jacks used on this equipment comply with and will continue to
Document No. 607-1808 Rev. A (3/99)
Figure 4. Remote Telset Disconnect Detect
comply with the mechanical requirements specified in Part 68.
Subpart F, specifically the dimensions, tolerances, and metallic
plating requirementsThe compliance of these connectors will
be asured by purchase specifications and incoming inspection.
Documentation of such specifications and/or inspections will
be provided the FCC within 30 days of their request for same.
The CH1808 includes interface circuits that couple it to the telephone line and provide FCC required isolation and protection.
The following guidelines should be followed to maintain FCC
CH1808 must be mounted away from hazardous voltages.
Connecting the CH1808 to telephone lines should be
made through a standard RJ-11 jack or other approved
connector device.
3) Circuit board traces to the CH1808 Tip, Ring, and Tip
Out pins must exceed 0.10 inch spacing from all other
traces. Tip, Ring, and Tip Out traces should have a
nominal width of 0.020 inches.
4) Tip, Ring, and Tip Out traces should be as short as possible to prevent coupling from other signals. Mount the
CH1808 close to the telephone line connection.
No additional circuitry should be connected between the
CH1808 and the telephone line’s RJ-11C jack except as
shown in Figure 4.
6) The FCC registration label included must be affixed to
the outside of the host product.
In your user manual, the following information should be
Type of Service: The (insert your product name) is designed
to be used on standard device telephone lines. It connects to
the telephone line by means of a standard jack called USOC RJ11C or (USOC RJ45S). Connection to the telephone company
provided coin service (CO implemented systems) is prohibited.
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Connection to party line service is subject to state tariffs.
Telephone Company Procedures: The goal of the telephone
company is to provide you with the best service it can. In order
to do this, it may occasionally be necessary for them to make
changes in their 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.
In the event repairs are ever needed on the (insert your product name), they should be performed by (insert your comapny
name) or an authorized representative of (insert your company company name). For information contact: (insert your
company address).
Document No. 607-1808 Rev. A (3/99)
Table 1
CH1808 Pin Descriptions
Table 2
CH1808 Electrical Specifications
VCC = +5v + 10%
P a ra me te r
Outp ut hi g h
Outp ut lo w
S ymb o l
C o nd i ti o ns
Mi n.
IOH = 40mA
IOL = 1.0mA
Li ne Re si sta nce
Ti p to Ri ng
Inp ut Re si sta nce
Ti p o r Ri ng to a ny o the r p i n
Typ .
Ma x.
Uni ts
LLM D e te cti o n Ti me
OHD Thre sho ld C urre nt
Lo o p C urre nt
Ri ng D e te ct Thre sho ld
Le a ka g e C urre nt
C urre nt
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o hms
C o nfo rms to a ll FC C P a rt 68 surg e ,
ha za rd o us vo lta g e , a nd le a ka g e
re q ui re me nts
S urg e P ro te cti o n
LLM Thre sho ld Vo lta g e
o hms
Vo lta g e a cro ss Ti p a nd Ri ng ,
C urre nt fro m Ti p to Ti p Out
15.3 to 68 Hz
Le a ka g e to Ti p to Ri ng
S up p ly Vo lta g e = +5V
C X =0.001 uF (Ti p o r Ri ng to GND )
V C X =50V (Ti p to Ri ng )
C urre nt fro m Ti p to Ti p Out
se c
V rms
Document No. 607-1808 Rev. A (3/99)
Figure 5. CH1808 Physical Dimensions and Pin Functions
406 TASMAN DRIVE · SUNNYVALE, CALIFORNIA 94089 · TEL: (408) 752-5000 FAX: (408) 752-5004
E-mail: [email protected]
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.
1998 Cermetek Microelectronics
Cermetek Microelectronics, Inc.
Printed in U.S.A.
Document No. 607-1808 Rev. A (3/99)