Pioneer 3TM, 2TM manual Packet Checksum, Packet Errors

ActivMedia Robotics Operating System

always first. Some data are strings of up to a maximum 200 bytes, prefaced by a length byte. Unlike common data integers, the two-byte checksum appears with its most- significant byte first (opposite order).

Packet Checksum

Calculate the PSOS/P2OS/AROS client-server packet checksum by successively adding data byte pairs (high byte first) to a running checksum (initially zero), disregarding sign and overflow. If there are 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: The checksum integer is placed at the end of the packet, with its bytes in the reverse order of that used for data integers; that is, b0 is the high byte and b1 is the low byte.

Packet Errors

AROS ignores a client command packet whose byte count exceeds 204 (total packet size of 206 bytes) or has an erroneous checksum. The client should similarly ignore erroneous SIPs.

AROS does not acknowledge receipt of a command packet nor does it have any facility to handle client acknowledgment of a SIP. Accordingly, when designing client applications, keep in mind serial communication limitations, particularly data rates and physical linkage. Communication between an onboard PC client connected with the server via a signal cable is much more reliable than over radios, for example. And don’t expect to send a client command every millisecond if the HOST serial port’s baud rate is set to 9,600 kbps.

Because of the real-time nature of client-server mobile-robotics interactions, we made a conscious decision to provide an unacknowledged communication packet interface. Retransmitting server information or command packets would serve no useful purpose, because old data would be virtually useless in maintaining responsive robot behaviors.

Nonetheless, the client-server interface provides a simple means for dealing with ignored command packets: Most of the client commands alter state variables in the server. By examining those values in respective SIPs, client software may detect ignored commands and re-issue them until achieving the correct state.

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