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CH224ATF

Complete 2400 bps Modem Module

Cermetek Microelectronics 

Cermetek Microelectronics, Inc.

CH224ATF Complete 2400 bps Modem Module

Label

~RXDTTL

~TXDTTL

~RITTL

~DSRTTL

~CTSTTL

~DCDTTL

~DTRTTL

Table 9. Signal Designations

I/O

Signal Name Designation

The Serial Interface signals are all TTL-level.

OA

Received Data. Active low. The modem uses the ~RXD line to send data

received from the telephone line to the DTE and to send modem responses to the

DTE. During command mode, ~RXD data represents the modem responses to

the DTE. Modem responses take priority over incoming data when the two

signals are in competition for ~RXD.

IA

Transmitted Data. Active low. The DTE uses the ~TXD line to send data to the

modem for transmission over the telephone line or to transmit commands to the

modem. The DTE should hold this circuit in the mark state when no data is being

transmitted or during intervals between characters.

OA

Ring Indicate. Active low. ~RI output ON (low) indicates the presence of an ON

segment of a ring signal on the telephone line. The modem will not go off-hook

when ~RI is active; the modem waits for ~RI to go inactive before going off-hook.

For US models, ~RI will respond to ring signals in the frequency range of 15.3 Hz

to 68 Hz. The ring signal cycle is typically two seconds ON, four seconds OFF.

The OFF (high) condition of the ~RI input should be maintained during the OFF

segment of the ring cycle (between rings) and at all other times when ringing is

not being received.

OA

Data Set Ready. Active low. ~DSR indicates modem status to the DTE. ~DSR

OFF (high) indicates that the DTE is to disregard all signals appearing on the

interchange circuits except Ring indicator (~RI). ~DSR output is controlled by the

AT&Sn command.

If the AT&S1 option is selected, ~DSR will come ON in the handshaking state

when carrier is detected in the originate mode or when carrier is first sent in the

answer mode. In addition if a test mode is entered (AT&T1, AT&T3, AT&T6-

AT&T8), ~DSR will go off while the test is running. ~DSR goes OFF if ~DTR goes

OFF.

If AT&Q0 and AT&S0 are selected, ~DSR will remain on at all times regardless of

the modem’s current state.

OA

Clear To Send. Active low. ~CTS is controlled by the modem to indicate

whether or not the modem is ready to transmit data. CTS OFF indicates to the

DTE that it should not transfer data across the interface on TXD. In data modes,

the ~CTS output is always ON. In fax modes, ~CTS is optionally used for flow

control.

OA

Data Carrier Detect. Active low. When AT&C0 commands is not in effect, ~DCD

output is ON when a carrier is detected on the telephone line or OFF when carrier

is not detected.

~DCD can be strapped ON using AT&C0 command.

IA

Data Terminal Ready. Active low. The ~DTR input is turned ON (low) by the

DTE when the DTE is ready to transmit or receive data. ~DTR ON prepares the

modem to be connected to the telephone line, and maintains the connection

established by the DTE (manual answering) or internally (automatic answering).

~DTR OFF places the modem in the disconnect state under control of the &DN

and &Qn commands. The effect of ~DTR ON and ~DTR OFF depends on the

&Dn and &Qn commands. Automatic answer is enabled when ~DTR is ON if the

“Answer Ring count” selectable option is not set to 0. Regardless of which device

is driving ~DTR, the modem will respond to an incoming ring by going off-hook

and beginning the handshake sequence.

The response of the modem to the ~DTR signal is very slow (up to 10 ms) to

prevent noise from falsely causing the modem to disconnect from the telephone

line.

2001 Cermetek Microelectronics, Inc.

Page 8

Document No. 607-0005 Revision B1 (05/02)

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