M
IDIAN
E
LECTRONICS
I
NCORPORATED
P
AGE
6
TDM-1 v 1.0
5-12-00
time to open a repeater or mobile radio’s CTCSS decoder
before the data is transmitted. To compute the front porch
key-up delay time, EIA specs state that a CTCSS decoder
should open in 250 msec at 100 Hz. The time increases as
CTCSS frequencies decrease. Therefore, from the high-end
frequencies to the low frequency, the time could run 100 to
500 msec. In addition, transmitter synthesizer lock time can
also be a consideration and can vary from a few milliseconds
to 100 msec. Also, E & M lead on microwave systems that
can be tied to the repeater can add 50 to 100 msec delay.
Satellite voting systems can also introduce additional delays.
B) PTT Hang Time (1.5 seconds)
PTT remains asserted for this amount of time after the key-up
delay if no more data is being sent by the DTE. If additional
data is sent by the DTE, PTT will remain asserted for this
amount of time after the DTE stops sending data. This
minimizes keying and unkeying for slow typists. This time may
be reduced if data is being sent by automatic means. If the
key-up delay is 500 ms or less, program this time to be
greater than the key-up Delay. This will ensure that enough
time is available to transmit characters that were buffered
during the key-up Delay. If the key-up delay is 600 ms or
more, program this time for at least 600 ms.
C) COR Hold Time (0.4 seconds)
This applies to busy lockout only. The unit will not assert PTT
(if busy lockout is enabled) for this amount of time after carrier
drops. If the data buffer fills up during this time, the CTS line
will be dropped to prevent the DTE from sending more data.
Once this time and the front porch key-up delay time have
expired, the buffered data will be sent. CTS will once again be
asserted to allow data to flow.
If data is being sent over a channel employing voice, this
register should probably be programmed for 2 to 3 seconds to
prevent the TDM-1 from transmitting between short pauses in
someone’s conversation.
Note that this parameter is ignored if busy lockout is not
enabled in programmable field
F
.
D) COR Polarity (1)
COR is internally pulled up and the default polarity is positive.
COR is used to qualify characters coming into the modem
from the radio receiver. If COR is not in the active state, data
will not pass from the modem to the RS-232 port.
COR is also used for the busy lockout functio n (see above).
If using full duplex mode it may not be necessary to connect
the COR wire provided it is programmed for positive polarity.
E) PTT Polarity (0)
Normally, the open collector PTT output is pulled to
ground to assert PTT. Almost all radios use a PTT
switch to ground. If using a radio with positive PTT it
may be necessary to add a pull-up resistor to the
open collector transistor on the TDM-1. If there is a
pull-up resistor in the radio this will not be
necessary. The pull-up resistor should be pulled up
to 5V logic when connecting to a microprocessor
PTT input. Anything higher than 5V could damage
the radio’s microprocessor.
F) Busy Lockout (0)
Enables busy lockout feature described under
COR
Hold Time
(field
C
).
Government regulations require radio users to open
tone
squelch
and
monitor
a
channel
before
transmitting. If they do not, they can interfere with
other users already on the channel.
Busy Channel
Lockout
automatically checks the channel, and will
not allow the transmitter to key-up and send data
until the COR Hold Time programmed in field
C
has
expired.
G) Continuous Tone (0)
By default, the modem will generate a data carrier
tone only during PTT. If full duplex or wire-to-wire
operation is desired, the carrier tone may be
enabled continuously by setting this field to a 1.
(H, I, J, K) Addressability Feature
Normally, all data received from the modem is
passed to the RS-232 port. If the addressability
feature is enabled, the modem will not pass data to
the RS-232 port until one of the four, 4-character
decode addresses appears in the data stream.
By default, the addressability feature is disabled. If
any of the address fields have a non-dash character
as the first character, the addressability feature will
be enabled. Any four printable characters may be
used as a decode address. There are a total of four,
4-character decode addresses available (H -K).
If, for example, address field H is programmed to
*123
, the modem will not pass data until the
address string *123 appears in the incoming data
stream. All data following the *123 will be passed
from the modem to the RS-232 port. Data will
continue to pass until COR drops. The decode
system will be reset immediately after COR drops
and will have to be addressed again in order to pass
data. For this reason, COR must be connected in
order to use the adressability feature.
