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CH1787

Small Footprint Hardware Controllable 2400bps Modem

Cermetek Microelectronics 

Cermetek Microelectronics, Inc.

CH1787 Small Footprint Hardware Controllable 2400bps Modem

Table 1. CH1787 Pin Descriptions (Continuation).

13 CTS - CLEAR TO SEND. Active LOW. Output always low. Indicates CH1787 is ready to accept

data from DTE.

14 DSR O DATA SET READY. LOW indicates handshaking with a remote modem is in progress, and/or

the data carrier of a remote modem is detected.

15 ORA I ORA (active low). When LOW, CH1787 will dial one of two stored telephone numbers

depending on the status of the TST pin. The numbers are programmed using the AT&ZO

command. A pre-stored number is provided to allow testing. It is in permanent loop back

answering on the second ring. The CH1787 will make up 15 attempts to connect with the

stored number at 60 second intervals as long as ORA is low. DCD low indicates a successful

connection. If ORA goes high, no further attempts to connect will be made. If, after

disconnecting from a valid connection, ORA is low the CH1787 will be unresponsive until ORA

is first placed high then low. The FCC requires that automatic dialing attempts not exceed 15

to the same number.

16 TXD I TRANSMIT DATA. Serial receive data input. Marking or a binary 1 condition is transmitted

when a HIGH is asserted.

17 DIR O DIR indicates when the TXD and RXD lines are used for internal CH1787 connection. When

DIR is high, valid data is on RXD and TXD. When DIR is low, the user may ignore RXD data

and should not place any data on TXD since it will be ignored by the CH1787. The user should

monitor this pin or gate it as shown in Figure 4 to prevent data from begin erroneously

interpreted by the user’s host processor.

18 IRQ - 4.7KW resistor to 5V.

19 SLEEP O SLEEP. A LOW indicates modem is in low power idle mode. Used to control power to other

devices. See Figure 5. When the modem is inactive for a period of time specified by register

S24, the CH1787 will power down to about 50% of its normal operating power. I/O Lines will

become undefined. The factory set default for the CH1787 is sleep mode is inhibited

(S24=255).

20 GND I GROUND.

21 DCD O DATA CARRIER DETECT. LOW indicates that a data carrier from a remote modem has been

detected. DCD follows carrier is the default.

22 HS

O SPEED INDICATION. A LOW on this pin indicates the modem is operating at 2400bps.

23 ORG I ORIGINATE (active LOW) places CH1787 in off hook mode without dialing. Used to originate

a connection on dedicated leased lines (i.e.,”dry” lines). The remote modem must be in

answering mode.

24 N/C - No connection.

25 N/C - No connection.

26 N/C - No connection.

27 SPK O SPEAKER. Audio Output. See Figure 5.

28 TST I Test pin input used in conjunction with ORA to steer the dialing between a user stored number

(TST LOW) and a pre-stored Cermetek test number (TST HIGH). See ORA description.

29 CON O CON indicates a valid 2400bps connection. May be used in place of DCD and HS to start the

communications or to indicate the start of an exchange of data.

30 HNG I HNG (active LOW). Used to force the CH1787 to disconnect. The HNG pin is only functional

when DCD is active (LOW).

31 RXD O RECEIVE DATA. Serial receive data output. Received MARKING or a binary 1 condition is

indicated by a HIGH.

32 VCC I 5 volts ± 5%. Note: Noise should be less than 50mV

33 GND I GROUND.

34 AAR I AAR (active LOW). When asserted low then high, CH1787 will auto answer during a RING

cycle. AAR can be tied low primarily to enable Auto Answer on the first RING. AAR will not

override auto answer condition e.g., S0 ≥ 1.

Spare pins are available for custom functions. Contact Cermetek with your requirements.

Note: (1) If VCC has a slow power up ramp time, the internal reset may be ineffective.

2002 Cermetek Microelectronics, Inc.

Page 5

Document No. 607-0002 Revision B (03/02)

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