Changing The Default Failsafe Direction; And Analog Differences; Procedure To Establish Failsafe Operation - Honeywell SmartLine ST 800 User Manual

Pressure transmitters

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4.4 Changing the Default Failsafe Direction

Transmitters are shipped with a default failsafe direction of upscale. This means that the transmitter output

will set the current output to upscale failsafe (maximum output) upon detection of a critical status. You

can change the direction from upscale failsafe to downscale failsafe (minimum output) by moving the top

jumper located in the Electronics module.

4.4.1 DE and Analog Differences

Failsafe operation is somewhat different between DE and analog operation:

•

Analog operation – Upscale failsafe drives the transmitter output to 21.8 mA. Downscale

failsafe drives the transmitter output to 3.8 mA.

•

DE operation – Upscale failsafe causes the transmitter to generate a + infinity digital signal.

Downscale failsafe causes the transmitter to generate a – infinity digital signal.

The transmitter electronics module interprets either signal as not-a-number and initiates its own

configured failsafe action for the control system.

The failsafe direction display accessible via the HVC shows only the state of the jumper as

it correlates to analog transmitter operation. Failsafe action for the DE control system may be

configured to operate in a manner different from analog, as indicated by the state of the

transmitter jumper.

The integrated circuits in the transmitter PWA are vulnerable to damage by stray static

discharges when removed from the electronics housing. Minimize the possibility of static

discharge damage when handling the PWA as follows:

Do not touch terminals, connectors, component leads, or circuits when handling the PWA.

When removing or installing the PWA, handle it by its edges or bracket section only. If you

need to touch the PWA circuits, be sure you are grounded by staying in contact with a

grounded surface or by wearing a grounded wrist strap.

When the PWA is removed from the transmitter, put it in an electrically conductive bag, or wrap

it in aluminum foil to protect it.

The following procedure outlines the steps for positioning the write protect and failsafe jumpers on the

electronics module. See

Figure 18

for the locations of the failsafe and write protect jumpers.

ST 800 Pressure Transmitter User's Manual

Revision 15

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