M
IDIAN
E
LECTRONICS
I
NCORPORATED
P
AGE
5
TDM-1 v 1.0
5-12-00
1. OPERATION
1.1.
General
The TDM-1 is a 1200 baud FSK modem that employs the
CCITT V.23 signaling frequencies of 1302 and 2097 Hz. It is
capable of simplex, half, or full duplex operation.
The TDM-1 can serve as a transparent data modem or an
addressable transparent data modem. There are 4 registers
that can be programmed with a unique 4-character address ID
so that the modem will not pass any data sent to it without the
proper header ID.
The unit is available in a DB25 connector, which is designed
to plug into the back of a PC or any device with a standard
RS-232 serial port (also know as Data Terminal Equipment, or
DTE
). In portable PC units it may be necessary to use a 9-pin
to 25-pin adapter between the PC and TDM-1 modem.
The radio side of the modem employs an RJ-11 connector
with a flying lead cable that can usually be connected to the
mic jack of most mobile radios. If possible it is best to utilize a
radio with a flat modulation response where the pre-emphasis
and de-emphasis can be turned off.
The TDM-1 employs a modem chip, an RS232 converter chip,
and a microprocessor buffer to control flow between them.
Most simple modems have no flow control or data buffer.
They simply apply a request to send line to key the radio’s
PTT and instantly begin sending data. The TDM-1 buffers
approximately 500 milliseconds of data (up to 63 characters).
This gives the radio time to key-up and for CTCSS to open
the receiving unit’s decoder or repeater before sending the
data. Upon receipt of the 61
st
character from the DTE, the
TDM-1 will drop the CTS (Clear to Send) line to the DTE,
halting further data. This is known as hardware flow control. If
the programmed key-up delay exceeds 500 milliseconds,
hardware flow control must be enabled in the DTE or data
may be lost.
2. PROGRAMMABLE FEATURES
MENU SYSTEM
The following describes the TDM-1 menu system used for
setting the modem parameters. In order to use the menu
system, the modem must be connected some sort of terminal.
A personal computer running a dumb terminal program is
typically
used.
See
topic
Terminal
Software
in
the
Troubleshooting
section of this manual.
To enter the menu system, stop transmitting data. Type ###
quickly, (within about 1 second). The menu appears after a
short delay, provided no other characters are typed. The
menu will appear as follows. The values shown are the factory
defaults.
There are 3 types of fields, binary fields, 2 digit
fields and 4 character fields. Binary fields are either
‘0’ or ‘1’. A ‘0’ means a feature is OFF or a polarity
is NEGATIVE (active LOW). A ‘1’ means a feature
is ON or a polarity is POSITIVE (active HIGH).
A 2-digit field is for timers such as key-up delay.
All
timers can be set for 0.1 to 5.9 seconds.
Four
character fields are for addresses (see below). To
change a particular item, simply press the letter key
corresponding to the item to be changed.
If for
example, B is pressed, the following will be
displayed:
PTT Hang Time 1.5s >
If you do not wish to change the setting, press
ENTER. Otherwise, type both digits of the new hang
time.
The decimal point will be placed automati-
cally. There is no need to press the ENTER key.
The entire menu will automatically be re-displayed
reflecting
the
changed
parameter.
Only
legal
characters will be accepted, others will be ignored
until 2 legal characters are typed (or ENTER is
pressed). Binary and address fields operate in a
similar
manner.
Parameter
changes
are
automatically saved to EEPROM as they are typed.
When all changes are complete, select option
L
to
return the modem to normal operation.
2.1
P
ROGRAMMABLE
F
IELDS
A) Keyup Delay (0.4 seconds)
The amount of time from when data is received
from the DTE to the time data begins being sent out
by the modem to the radio. PTT is asserted and
data carrier tone (1302 Hz) is turned on immediately
upon receiving the start bit of the first character.
The key-up delay is programmable from 0.1 to 5.9
seconds, which allows for a front porch key-up delay
Midian TDM-1 V1.0
A) Keyup delay 0.4s
B) PTT Hang Time 1.5s
C) COR Hold Time 0.4s
D) COR Polarity 1
E) PTT Polarity 0
F) Busy Lockout 0
G) Cont. Tone 0
H) Address #1 ----
I) Address #2 ----
J) Address #3 ----
K) Address #4 ----
L) Exit menu
Select >
