Pioneer 2 / PeopleBot manual Packet Data Types, Packet Checksum, Packet Errors

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.

Table 3. P2OS Communication Packet Data Types

Data Type	Bytes	Order
integer	2	b0	low byte; b1	high byte
word	4	b0	low byte; b3	high byte
string	up to ~200,	b0	length of string;
	length-prefixed	b1	first byte of string

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.

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, b0 is the high byte and b1 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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