Datum Systems PSM-500L Manual de usuario descargar pdf (Pagina 4)

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

Rev 0.93 12/12/10

B-4

Data Byte Count Field

The Data Byte Count field includes the total number of Data Bytes only, and should be zero (00) for read

mode. Note that this determines if the command is a read (request for information) or write (command to

change parameters). The data byte count can only be one of two values, either “0” as is common for a read

request, or the actual fixed number of data bytes in that particular packet type. If the structure used in a

program to store the contents of a packet’s parameters is “packed” so that each element occupies the same

number of bytes as the protocol definition, then the data byte count is equal to the “sizeof(structure)” for

example in C.

Data Byte Field

Multiple data formats are used within the data field: 1 byte entries are a single character or unsigned byte

type or containers for bit flags; 2 byte numbers and 4 byte numbers in both signed and un-signed format.

Strings consist of multiple consecutive bytes or characters, and are terminated by a “0” value byte. An

extended 6 byte long integer is used for L-Band and RF Frequencies.

No floating point numbers are used, although the incremental value of an entry may allow a decimal point

value. For instance the transmit power level is entered as an integer in increments of 0.1 dB, so an entry of

-176 represents -17.6 dB. The incremental value (represented by 1 least significant bit change) is

determined from the Write Bytes section of the Command Tables by ignoring any decimal point and using

the number of displayed digits. Thus frequencies are entered in 1 Hz increments, data rates in 1 bps

increments, and times in increments as shown in the tables. No offsets are used in any of the number

entries.

Checksum Field

The checksum is computed as 256 minus the sum of all bytes excluding opening and closing pad bytes, and

the checksum itself. The checksum is modulo 256, that is the checksum never exceeds 255 in value but

rolls over at 256. The sum of all bytes (modulo 256) including the checksum itself is always zero.

Pad Bytes

The Pad Bytes are normally not used, and are not checked by the processor. Multiple pad bytes may be

used. Pad bytes are all 1’s or “FF” hex. The pad bytes serve several functions in an RS-485 configured

system, indicating clean transitions from idle to active states. The M500 modem does not send a starting

pad byte in RS-485 responses.

Binary Packet Response Message Format

The Binary Response Packet from the PSM-500 Modem to the controller adheres to the following message

format. The response from a modem will occur within approximately ½ second. Note that a modem set to

485 control port mode mutes its receive while sending the response message, so if the 485 bus is configured

as 4 wire or 2 wire a modem will not receive a message while responding to a previous message.

Byte 1

Opening Flag

5A hex

Byte 2

Destination

Address

8 bits

Byte 3

Source Address

8 bits

Byte 4

Binary

Command

8 bits

Byte 5

Status Byte

8 bits

Byte 6

Error Byte

Byte 7

Data Byte

Count

Byte 8 - (n-3)

Data Bytes

128 maximum

Byte n-2

Closing Flag

96 hex

Byte n -1

Checksum

The Destination address is taken from the Source address of the incoming packet to which this is a

response. The Status and Error Bytes are defined later in this Appendix. The Data Byte Count field includes

the total number of Data Bytes only, and should never be zero for a response message. The Pad Bytes are

normally not necessary and are not checked by the processor, and multiple pad bytes may be used. The

checksum is 256 minus the sum of all bytes excluding opening and closing pad bytes, and the checksum

itself. The sum of all bytes except pad bytes including the checksum itself is always zero.

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Datum Systems PSM-500L Manual de usuario Pagina 4

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