MODEM - LINE CONNECTED

HARDWARE

COM24 modem

Serial Port 2 can conveniently be connected to a modem that accepts 'AT' commands. COM24 is a miniature type well matched to TDS2020F that takes little current. It can be switched off to save power when not in use, and is small, only 61 x 57 x 25mm in size including the 25-pin D-socket. The modem is capable of 2400 baud and 1200 baud; the baud rate is selected automatically from the first incoming signal from the TDS2020F. To avoid shared line disruption the modem (from version 3) prior to making a call will 'listen' on a shared line for voice or data transmission and will not seize the line if it is in use. The temperature specification is -30�C to +70�C.

The power supply needed is +6 to 16V. Typical current consumption is

Active	30mA (max. 40mA)
Idle	4ma (max 6mA)
Power-down	3�A (max. 10�A)

TDS2020F	Pin	Modem 25-pin socket	Function
*Connections for modem model COM24:*
SOUT2	c13	2	Data transmit
SIN2	c14	3	Data receive
GND	c32	7	Ground
VIN	a32 (+6 to 12V)	15	Modem power supply
*Connections for most PC modems:*
SOUT2	c13	2	Data transmit
SIN2	c14	3	Data receive
GND	c32	7	Ground
VIN	a32 (+6 to 16V)	4	Request to Send
VIN	a32 (+6 to 16V)	20	Data Terminal Ready

The COM24 has an internal voltage regulator with input on pin 15 and output on pin 14. To drive the unit from the TDS2020F's switched +5V output, connect pin c15 to both pins 14 & 15 on the COM24.

The telephone connection is by a 6-way socket. A UK telephone lead is supplied unless otherwise agreed.

MAKING A CALL

A TDS2020F can make or receive a call, file #MODEM.TDS is for the former. Use 1200 2BAUD or similar to initialise Port 2. Then, in order, use these words:

?MODEM	Establish that a modem is attached and responding.
DIAL	Call a telephone number fed as a parameter in string format.
HANG	Replace the 'receiver' on hook.

Between the second and third operations serial Port 2 can exchange data with the remote computer. To test operation of your system use library file #TIMEEUR.TDS or #TIMEUSA.TDS to automatically dial up a national time service, collect the current date and time, set the TDS2020F clock accurately then hang up-all automatically.

RECEIVING A CALL

If the TDS2020F is to receive a call no software apart from the #SERIAL2.TDS Port 2 driver is needed. Make the connections above and use *KEY? to let your software know when something is arriving on Port 2 from the modem.

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