User Manual

MDS 05-4670A01, Rev. E SD Series Reference Manual (x710 Mode) 47
Physical Interface
The physical interface for analog operation utilizes pins on the radio’s
COM1 (management) connector. This connector is multiplexed with the
serial data lines used for software commands. The applicable pins of the
DB-9 are as follows:
• Pin 4—Transmit Audio
(-20 dBm to 0 dBm, as set by TXLEVEL command)
• Pin 5—Ground (negative supply potential)
• Pin 6—Push-to-Talk (3 V TTL, 5 V tolerant)
• Pin 8—Receive Audio
(-20 dBm to 0 dBm, as set by RXLEVEL command)
Operational Characteristics
Analog operation is available in two general modes: Analog-only or
Mixed Analog/Digital.
Analog-only operation is typically useful when interfacing with external
analog modems. Operation is selected by specifying MODEM NONE . In
this mode, RX and TX Audio are both always active; Carrier Detect
output and the LINK LED are asserted based on the receiver
unsquelching. Key sources automatically assume the use of TX Audio
as the input source. In analog-only mode, carrier detect is based on
squelch status.
Mixed analog/digital operation is useful for orderwire application, or for
analog systems that need to make use of GE MDS Network-Wide Diag-
nostics. Operation is enabled by selecting a digital modem type (for
example, MODEM 9600 ) and selecting AUDIO ON . In this mode RX Audio
is always active, but TX Audio only becomes active when the
Push-to-Talk (PTT, Pin 6) input signal is asserted. All other key sources
use digital modulation automatically. In this mode, Carrier Detect
output and the LINK LED are asserted based only on detection of digital
modulation.
The PTT “asserted” condition is defined by the PTTSIG command.
Entering PTTSIG HI configures the system for an active high Push-to-Talk
signal. Entering PTTSIG LO configures the system for an active low
Push-to-Talk signal. Entering PTTSIG OFF disables the Push-to-Talk
input—useful for RX audio monitoring of the channel when no TX
analog transmission is desired.
Analog operating characteristics (selected either by PTT assertion or the
MODEM NONE command) vary based on channel type.
As a simple rule, SD characteristics associated with MODEM NONE are
driven by the last digital modem selection. For example, if MODEM 9600
is selected followed by MODEM NONE , the characteristics for analog oper-
ation match the corresponding 12.5 kHz FCC/IC channel constraints. If