OK, I see I wasn’t very clear, maybe because I wanted to explain it in brief,
In reply to Andy & Patrick :

The communication is parallel consisting of packets, each packet begins with a command byte in which the 9th bit is 1 and following the command there are optional data. Depend on a particular application you may define different commands, and each command should be defined for all 8051s(I forgot to mention in previous message that this idea is easily expandable to many more 8051s), so they know from the command byte, what they should expect (1,2 or more data phase) after reading all the data they calculate CRC with the same routine, if it had the same value with what was in command byte they send ACK otherwise possibly there was a write glitch and they send a NAK.

Patrick said Do you need the complexity of an ACK/NACK protocol?
The advantages of using ACK and NAK is that you first make sure that the data is received on the other side of bus correctly(you may want to resend the data) and second each 8051 makes sure that the other micro is conscious. And as I said you may exchange some status parameters too and in some cases it is very useful.
Patrick said have a look at the TCP/IP stack well, I think TCP/ IP is too much for a 8051, considering the fact that the 8051 has many other jobs to do, the advantage of this protocol is that you can transmit data at a very high speed( compare to serial protocols like I2C) this protocol is two way and 8051s do not need to be synchronized with each other they all can carry on their jobs and sometimes(before other 8051s assume that they have lost consciousness) they can read their messages and it is very simple so it doesn’t need much coding as TCP/IP. And as I said it has other advantages like cross checking other 8051s and it is reliable (it has a CRC check in it). Patrick said Is this bus going to be used over long distances, or only over short distances? If it's a way of communicating on the same PCB, then you would get away without error checking and ACK/NACK. Well I think, if the distance is so long that signals may be lost completely, this protocol is not suitable, but as I said it has a noise proof features (CRC check). As I said ACK and NAK have multiple purposes( I explained it above).
Patrick said If there is no supervisor chip, will you not spend to much time running your supervision-process? Well this feature is most useful for a heavy traffic, and if the traffic is very low and the 8051 is so busy that it cannot afford sending a few more check packets each second then you are right. Thank you.

Regards
Ebrahim Vakilpour