THEORY OF OPERATION
3-6
001-3282-101/102
3.6 RECEIVING DATA
NOTE: The levels in the following information are the levels at the RS-232 port. The levels on the pins of U230 are
the inverse of these levels because of the inversion produced by U240.
3.6.1 INTRODUCTION
The receive data signal is FSK-modulated tones from an RF receiver module. This signal is applied to the
RxA (Receive Audio) input of modem IC U230. This device filters the tones to attenuate out-of-band frequencies
and then convert the tones to digital data. The data is then filtered again and fed out of U230 on the RxD pin. U240
inverts the data and converts it to RS-232 levels. It is then applied to the RxD pin of the RS-232 port.
3.6.2 DTR SIGNAL
The DTR signal indicates that the data equipment connected to the port is ready to receive or transmit
data.
3.6.3 CARRIER DETECT (CD) SIGNAL
When a tone is detected on the RxA input of U230 for a time tCD (On), the CD output of the RS-232 port
goes high. This tells the data equipment that data is coming. FSK-modulated tones on the RxA input are then
converted to digital data and fed out on the RxD output of the port.
Table 3-1 MODEM TIMING PARAMETERS (BELL)
Symbol
Description
Bell 103 Orig
Bell 103
Answer
Bell 202
Units
tRC (On)
RTS to CTS on delay
30/60/180/240
30/60/180/240
30/60/180/240
ms
tRC (Off)
RTS to CTS off delay
0.5
0.5
0.5
ms
tCD (On)
Carrier Det on delay
8-20
8-20
6-12
ms
tCD (Off)
Carrier Det off Delay
6-20
6-20
3-8
ms
tSQ
Receiver squelch duration
60
60
33
ms
tSTO
Transmitter soft turn-off duration
-
-
25
ms