Pass-through messages

HART pass-through is a mechanism for sending a HART message directly to a HART Field Device through the HART interface devices. These messages are usually sent from an instrument configuration package or asset management product such as

FactoryTalk AssetCentre

.

The default value of 15 seconds is adequate for most applications. The only reason to set the value shorter is if you require recovery from a failed pass-through client in less than 15 seconds. The only reason to set this value longer is if your pass-through client does not check for the reply too often. The disadvantage of setting the value very long, (for example, 255 seconds), is that if your ladder logic starts a HART pass-through operation using CIP MSG message service code 16#4E and then does not follow up with a CIP message to collect the reply 16#4F, then the buffers used to hold the reply remain busy until this time has expired, preventing the initiation of further HART pass-through messages. This applies to all channels on the device.

This diagram shows a simplified description of the device activities for the 1756-IF8H and 1756-OF8H devices.

This is an illustration of the once per device scan pass-through configuration for the 1756-IF8H and 1756-OF8H devices.

This illustration shows the location in the scan cycle where pass-through takes place for once per channel scans for the 1756-IF8H and 1756-OF8H devices.

This is an illustration of the once per device scan pass-through configuration for the 1756-IF16H and 1756-IF16IH devices.