T2000-A75 Modem
Operation Manual
November 2001
M2256-000-00-051
© Tait Electronics Limited
Corporate HQ: PO Box 1645, Christchurch,
New Zealand.
Tel: +64 3 358 3399
Fax: +64 3 358 0340
Web: www.taitworld.com/
Contents
Section Title Page
1
Overview
RS232 Interface
Compatibility
A75 Configuration
Command mode
Transparent mode
2
3
Changing Modes
Transparent Mode Operation
Transparent Mode effective data rate calculation
Transparent Mode RTS/CTS Signalling
4
Command Mode Operation
Message Format
Calculating the [CHECKSUM]
Sending and Receiving Short Data Messages (SDM)
5
Commands
5
6
6
7
8
8
9
11
11
13
17
17
18
19
23
1 Overview
The CCDI (Computer Controlled Data Interface) is a command protocol embedded in the
T2000-A75 (A75) modem firmware to control the T201X radio.
T201X T201X
DB9
T2000-A75
(A75)
or
Tait Orca
CCDI
capable
Modem
PC
DTE
CCDI CCDI
COM
Port
RS232
RS232
COM
Port
PCPC
PC
The A75 is fitted to a T201X radio and is connected to a personal computer (PC) by a serial line
using five standard RS232 signals (TXD, RXD, CTS, RTS and GND).
The A75 can operate in two modes: Command mode and Transparent mode. In Command
mode, the A75 is controlled by the PC sending command sequences and receiving responses.
In Transparent mode there is a direct link between the PC and the FFSK modem on the A75
Modem board and the PC sends and receives data without having to pass messages using
CCDI commands. Transparent mode can be set to either 9600, 4800, 2400 or 1200 baud
between the PC and the A75. The over-air data rate is either 1200 or 2400 baud.
The T2000-A75 modem described in this manual is CCDI version 2.0 compatible. When an
A75 is retrofitted to a T2000 radio, some hardware modifications are needed. Refer to the A75
fitting instructions for details.
November 2001 M2256-000-00-051 5
RS232 Interface
The RS232 communication between the PC and the A75 has the following parameters which
are fixed in the A75 firmware.
For every byte sent, there are 10 bits sent including the start and stop bits:
• Number of data bits = 8
• Parity = none
• Number of start bits = 1 (set to ‘1’)
• Number of stop bit = 1 (set to ‘0’)
The RS232 D-Range socket at the rear of the T201X radio has the following configuration:
Connections Function Connections Function
1 Not used 6 Not used
2RXD7 RTS
3TXD8 CTS
4 Not used 9 Not used
5GND
Note: The start and stop bits are removed by the A75 for the over-air transmission of data. The FFSK
data sent is pure binary 8 bit data only.
Compatibility
T201X Radio Firmware
When programming the T201X radio firmware for use with the A75 and CCDI, use version 3.01
or greater.
T2000 PGM
When the A75 modem board is fitted, the T201X PGM program settings have the following
constraints:
• ‘BCD Channel Selection’ should be enabled and polarity set to Normal so that the A75
can change the channel with the GO_TO_CHANNEL command.
• Economy mode reduces the radio’s power consumption when it is idle. When economy
mode is active and there has been no valid activity on a channel for the duration of the
economy mode timer, the radio begins economy cycling. This means that the beginning
of a transmission may be lost if it is received after a period of inactivity.
If ‘Economy mode’ is enabled in a T201X with an A75 fitted, set the default Transparent
mode Lead In Delay to 200 ms.
If a faster data transfer rate is required, then the Lead In Delay can be reprogrammed as
low as 40 ms, but the Economy mode must be disabled.
6 M2256-000-00-051 November 2001
A75 Configuration
The A75 is configured for both Command and Transparent mode communication using the
PROGRAM command. It contains 18 ASCII hex characters and sets up the following parame-
ters:
• Transparent mode baud to either 1200, 2400, 4800 or 9600 baud
• Power-up default to Command or Transparent mode
• Command mode baud to either 9600 or the same as Transparent mode
• CTS and RTS signalling attributes
• whether the A75 sends an SDM Auto ACK after receiving an SDM; and waits for an
ACK after sending an SDM
• the Delay Time between receiving an SDM and returning an SDM Auto ACK
• the time that the A75 will wait for an SDM Auto ACK before sending a PROGRESS
message
• The Transparent mode Lead In Delay time. When the T201X radio receives serial data it
keys-up the transmitter but only starts to send the FFSK data after the Lead In Delay.
• The T201X Power-up channel
• The eight character Data ID assigned to the radio for receiving SDM data.
Refer to the PROGRAM command for detailed information.
November 2001 M2256-000-00-051 7
Command mode
In Command mode, the PC sends command sequences to the A75 and waits for a prompt before
beginning the next transaction. The A75 sends a prompt character ‘.’ to the PC to indicate that
it is ready to accept a new command. Some commands require the A75 to send a CCDI message
in response, before it sends the prompt.
Messages directed to the A75 will always be responded to, either by a return message, or by the
presence of the prompt.
Messages from the A75 are either solicited or unsolicited. Solicited messages are sent in response to commands from the PC. Unsolicited messages such as PROGRESS or ERROR messages are sent by the A75 if there is a significant change in the state of the T201X that the PC
should be aware of. When errors are detected, an unsolicited ERROR message is sent by the
A75 to the PC. The A75 does not send messages that require a reply.
Transparent mode
Transparent mode creates a direct link between the PC and the FFSK modem located on the
A75. The transparent link allows the PC to send and receive data without having to pass messages using the CCDI Command mode.
The transparent link is administered by the A75 which reads FFSK data arriving from another
FFSK modem and sends it to the serial port. Any form of binary data is acceptable but the escape
sequence to switch back to Command mode should be avoided. The escape sequence character
is set to ‘+’ by default, or defined within the TRANSPARENT command.
PROGRESS messages are unsolicited status messages sent by the A75 but can be filtered out
in Transparent mode by enabling TMODE Filter control with the FUNCTION command. When
the TMODE filter is enabled, progress messages are not sent by the A75. When a PROGRESS
message is sent in Transparent mode, the escape character is attached to the front of the message, e.g. zzzp0207C7 where ‘z’ is the escape character.
In Transparent mode, the A75 does not generate or detect CRC checksum data. However if a
SDM message is transmitted or received then the CRC checksum data is used.
Transparent mode serial baud can be set to either 9600, 4800, 2400 or 1200 baud using the PRO-
GRAM command.
8 M2256-000-00-051 November 2001
2 Changing Modes
In order to change from Command mode to Transparent mode, the PC must send a TRANS-
PAR EN T command to the A75. Once acknowledged, any further communication is linked di-
rectly to the A75 modem in Transparent mode.
When the TRANSPARENT command is sent to the A75, the escape sequence character that is
used to return to Command mode is also sent.
The escape sequence consists of a 2 second idle time, followed by three escape characters (within 2 seconds), followed by a further 2 second idle time.
If the T201X defaults to Transparent mode at power-up, the escape character is set to the default
value of ‘+’.
Example
A70(2)
or Tait Orca
PC(2)
RS232
tz[SIZE] [CHECKSUM]
(2s idle) + zzz + (2s idle)
PC(1)
A70(1)
‘.’
Transparent
mode
‘.’
Command
mode
RS232 over air
1. t is the message [IDENT] for a TRANSPARENT command telling the A75 to go into Trans-
parent mode
2. The [SIZE] parameter in this example is 01 as there is one character of data to follow in the
[PARAMETERS] field.
3. z is the escape character in the [PARAMETERS] field. When 2 second idle + zzz +
2 second idle is detected in Transparent mode, the A75 is forced back to Command mode.
4. The [CHECKSUM] in this example is B1. Refer to the Calculating the [CHECKSUM] sec-
tion for details.
November 2001 M2256-000-00-051 9
10 M2256-000-00-051 November 2001
3 Transparent Mode Operation
The Transparent Mode transmission format is as follows:
Lead In
Delay
(200ms)
Preamble Sync Length data block
First block
(maximum 46
bytes)
Block Head
. . . . . . . . nth block
(maximum 46
bytes)
dummy CRC
(maximum
46 bytes)
(00)
Tail Tim e
( 20ms)
The default Transparent mode Lead In Delay is set to 200 ms. If a faster data rate is required the
Lead In Delay can be reprogrammed to a minimum of 40 ms using the PROGRAM command.
Note that if the Lead In Delay is set this low, Economy mode must be disabled.
The Tail Time of 20 ms is fixed in the A75 firmware.
Transparent Mode effective data rate calculation
The Transparent mode effective data rate is dependant on the amount of data sent in one transmission. The more data is sent, the better the data rate.
Example
If we assume that 1K bytes of data is sent with a Lead In Delay of 200 ms:
Time to send data message + block head for each block of data
= ((( 1000 + ( 22 x 6 )) x 8 /1200 ) x 1000 = 7546 ms
Where:
1000 = bytes of data (1K bytes)
22 = (1000 bytes/46 bytes per data block) + 1 block to round up to the nearest block
= number of block heads
6 = bytes per block head (2 bytes for PREAMBLE + 2 bytes for SYNC + 2 bytes
for block LENGTH)
therefore:
(22 x 6) = total bytes in the block heads
8 = bits per byte
1200 = bits per second (bps) over-air standard baud
1000 = to convert bps to ms
Lead In Delay and Tail Time = 200 + 20 = 220 ms
Total time = 7546 + 220 = 7766 ms
Effective data rate = 10000 (bits) / 7766 (ms) = 1287 bps
If 1M byte of data is sent, the effective data rate goes up to 1500 bps.
November 2001 M2256-000-00-051 11
Transparent mode Lead In Delay
The Lead In Delay time ensures that data is not lost from the front of the data block while the
transmitter keys-up. To achieve this, CTS inhibit is set to ‘0’ so that radio does not receive any
serial data from the PC during the Lead In Delay.
The Lead In Delay time can also be used to activate a series of repeaters in some common applications. When data is detected at the input buffer of the radio, the following chain of events
occurs:
1. the transmitter keys-up
2. a carrier is sent from the radio transmitter
3. if the receiving Base Station is set in Repeater Mode, the carrier is detected and Rx Gate
becomes active which in turn makes the PTT line active
4. the active PTT line keys-up the transmitter
This sequence is repeated with as many Base Stations as are in the chain.
The optimum length of the Lead In Delay depends on the number of Base Stations that need to
be activated before any data is sent.
12 M2256-000-00-051 November 2001
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