in reply to: Re: Hart Questions posted by Romilly Bowden on 09 August 2002 at 14:45:01.
(sorry, overshot my own space limit!)
3) Extended response time.
Up till now, the slave must respond within the allowed 256 ms. If it can't deal with the command, it could respond saying "busy". But that's not a good idea unless it's just a temporary situation, due to the slave doing some extended inner activity which cannot be interrupted. If a slave didn't reply at all, the master would probably do a retry (see 2 above) but you should not rely on this for satisfactory system operation - at the very least it throws out the scan timing.
I understand that HART rev. 6 will allow a properly-defined "delayed response" mechanism for use when a slave needs more time to reply properly. For field devices, it will only be allowed for "write" commands. For bridge devices running a higher communication speed to the master, it may apply to any command. The master remains responsible for making one or more further requests after a short time, to get the full reply.
Hope this helps a bit ...