Tait T2000-A75, Operation Manual, Page: 19 / 38

November 2001 M2256-000-00-051 19
Sending and Receiving Short Data Messages (SDM)
A SDM can only be sent in Command mode from a PC but can be received in both Command
and Transparent modes.
Sending SDMs
In the examples shown, the first two bytes of the [PARAMETERS] field are set to FF which
translates to an
SDM Lead In Delay of 5.1 seconds. The next eight bytes (12345678) are the
SDM Data ID of the radio receiving the SDM. The SDM text is ‘Hi’.
If the SDM data includes a SDM [MESSSAGE] parameter, the SDM text (‘Hi) will update the
SDM text buffer in the A75 EEPROM. If there is no SDM [MESSSAGE] parameter attached,
the buffer will not be cleared but retains its current contents
When an SDM is sent (
➀
), the T201X radio keys its transmitter immediately, and the SDM is
sent by the A75 after the SDM Lead In Delay.
SDM Auto ACK is either enabled or disabled when the modem is configured. This is done in
the A75 with the PROGRAM command or in the PGM programming software for the Tait Orca.
If the A75 has SDM Auto ACK is set to ‘1’ (enabled) in bit 7 of the [ITEM1] parameter of the
PROGRAM command, then:
• when the A75 receives an SDM, it returns an ‘ACK’
• when the A75 sends an SDM, it starts the
SDM Wait for ACK timer
In the above example, the receiving modem A75(2) has the SDM Auto ACK disabled so no
‘ACK’ is returned. After the SDM Wait for ACK time, the A75(1) generates a PROGRESS
message (
➁
) of [PTYPE] SMD ACK (1D) with the flag set to ‘0’ (No ACK Received). Refer
to the PROGRESS command details for further information.
SDM
Wait for
ACK
Time
RS232 over air RS232
2
p 1D0
[SIZE] [CHECKSUM]
PC(1) A75(1)
A75(2)
or Tait Orca
PC(2)
1
Lead In
Delay
s FF12345678Hi
[SIZE] [CHECKSUM] SEND_SDM
PROGRESS
Auto ACK = 1
Auto ACK = 0
PROGRESS
Wait for ACK triggers
PROGRESS