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Datum Systems, Inc. PSM-500 Modem Remote Control Protocol

Rev 0.93 12/12/10

B-5

Data Payload Format

There may be 0 data bytes in a simple controller request for information, but all other packets have a non-

zero data byte count and the corresponding number of bytes of information representing the data sent or

received. The data field contains a fairly regular format consisting of Records (Pascal) or Structures (C)

with multiple elements. One of the main purposes of this Appendix is to define those structures for each

possible data field. The particular data field structure to be used in a message packet is defined by the

Command number. A response packet therefore consists of the header and the data structure as defined in

the header’s command byte.

The standard data structure contains first a field of bit flags (usually 32) that are used in a response to

indicate values that have changed since the last read. These same change bit flags are used in a command to

change information as “write enable” bits to indicate which particular value is to be changed. A command

message must contain the full field of data, but the modem only pays any attention to the values with their

corresponding write enable flag bit set.

Each control source has its own set of change flags to determine if the changed data has been read or not.

i.e. the USB, front panel, remote control port J6 and the Modem Control Channel (MCC).

Next in the data structure are a field of bit flags used to specify the value of parameters that can only take

on a few possible values, for example 1 bit is assigned to indicate if the transmit output is enabled. The

parameter bit field is followed by the byte and multi-byte parameters that may be numbers or strings.

Example Binary Packet Command and Response Messages

As an example of using the remote control assume that the controller should check the transmit frequency,

and if not set correctly change it to 74.652 MHz. The modem address is set to 12 decimal and the controller

is address 200 decimal. Setting the transmit frequency in this example will consist of three operation; 1)

Checking the current frequency, 2) Setting the transmit frequency and 3) Checking the response to insure

that the setting was correctly accomplished. What is not shown in any of these steps are the necessary

background work such as building and receiving packets, calculating checksums and testing for errors.

1. Check the transmit frequency:

To read the transmit IF frequency we send a command request to the modem with the Mod IF command

number (41 hex) and set the data byte count to zero.

Command packet from controller to read the transmit frequency

Byte 1

Opening Flag

“A5 hex”

Byte 2

Destination

Address- 8

bits “01 hex”

Byte 3

Source

Address- 8 bits

“CC hex”

Byte 4

Binary

Command

8 bits

“41 hex”

Byte 5

Mode Byte

8 bits

“05”

Byte 6

Data Byte

Count

“00 hex”

Byte 7

Closing Flag

“96 hex”

Byte 8

Checksum

“B2 hex”

Response packet from modem

Byte 1

Opening Flag

“5A hex”

Byte 2

Destination

Address- 8

bits “CC hex”

Byte 3

Source

Address- 8 bits

“01 hex”

Byte 4

Binary Command

8 bits

“41 hex”

Byte 5

Status Byte

8 bits

“76”

Byte 6

Error Byte

“00 hex

Byte 7

Data Byte

Count

“24 hex”

Byte 8 - (n-3)

Data Bytes

128 maximum

See Below

Byte n-2

Closing Flag

“96 hex”

Byte n -1

Checksum

“6A hex”

1 2 3 4 5 6 7 8 9 10 ... 56 57

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