I have been reading this list for a few weeks now with great interest. I'm
in the final stages of designing a system for the school I work at that
requires a multi-drop RS485 network and while I understand the hardware side
of the standard, I could use some pointers on protocols.

I understand the basics of packet and Ethernet but all I want to implement
in this system is (hopefully) a simple master/slave arrangement with small
amounts of data (10 bytes every few seconds). There is no requirement for
slave/slave transmissions. I am using a 16C74 at 4Mhz as I need the I/O and
therefore have the UART available.

What is common practice for RS485, to use ASCII for data to distinguish it
from commands,  unique preambles that are unlikely (or disallowed) from
normal data, fixed length packets? Any tips on building a reliable protocol
would be much appreciated.

Thanks for any help,


Phil Tipping   G8RCP
IT Systems Manager
phil@alperton-school.demon.co.uk