Pioneer 2 / PeopleBot Manuel d'utilisateur télécharger pdf (Page 36)

Pioneer 2 Operating System

server to client. Both are bit streams consisting of four main elements: a two-byte

header, a one-byte count of the number of subsequent packet bytes, the client

command and its arguments or the server information data bytes, and, finally, a two-

byte checksum.

Packet Data Types

Client commands and server information packets contain several data types, as defined

in Table 3.

Packet Checksum

Calculate the PSOS/P2OS client-server packet checksum by successively adding data

byte pairs (high byte first) to the running checksum (initially zero), disregarding sign and

overflow. If there is an odd number of data bytes, the last byte is XORed to the low-

order byte of the checksum.

Table 3. P2OS Communication Packet Data Types

Data Type Bytes Order

integer 2 b

low byte; b

high byte

word 4 b

low byte; b

high byte

string up to ~200,

length-prefixed

length of string;

first byte of string

int calc_chksum(unsigned char *ptr) // ptr is array of bytes

{ // first is data count

int n;

int c = 0;

n = (ptr++); /* Step over byte count */

n -= 2; /* don't include checksum word */

while (n > 1)

{

c += (*(ptr)<<8) | *(ptr+1);

c = c & 0xffff;

n -= 2;

ptr += 2;

}

if (n > 0)

c = c ^ (int)*(ptr++);

return(c);

}

NOTE: In P2OS, the checksum word is placed at the end of the packet, with its bytes in

the reverse order of that used for arguments and data; that is, b

is the high byte and b

is the low byte.

Packet Errors

Currently, P2OS ignores a client command packet whose Byte Count exceeds 200 or has

an erroneous checksum. The client should similarly ignore erroneous server information

packets.

P2OS does not acknowledge receipt of a command packet nor does it have any facility

to handle client acknowledgment of a server information packet. Consequently, Pioneer

client/server communications are as reliable as the physical communication link. A

cable tether between the robot and client computer, such as a piggyback laptop,

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Pioneer 2 / PeopleBot Manuel d'utilisateur Page 36

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