Negative values from PCD3.T760 are not interpreted correctly on the PCD

FAQ #100466

 

Symptom
The temperature values read from the PCD3.T760 aren't negative but very high (around 65000). Positive values read are interpreted correctly. All modules that do offer the possibility of reading temperature values are affected (PCD3.W220, PCD3.W340, PCD3.W350, PCD3.W360, PCD3.W745).

Reason
The reason for this phenomenon is the fact that the values transmitted over Profibus DP or Profi-S-I/O are signed 16-bit values. These values are copied directly into the signed 32-bit registers of the PCD. Due to the lack of a transfer function that converts the negative values (and copies the sign bit from the 16th to the 32th bit), the negative 16-bit values aren't correctly interpreted when copied into a 32-bit register.

Solution

An according transfer function has been implemented in the firmware of recent PCD systems. For configuring this transfer function, select the checkbox "Signed" in the Profi-S-I/O (or Profibus DP) Network Configurator as in the screenshot shown. The minimal PG5 version to use this functionality is PG5 1.4.120. The first pilot firmware version supporting the discussed transfer function is $27 for PCD3 and $29 for PCD2.M480.

Firmware versions supporting the transfer function (systems not listed in this table do not support the transfer function mentioned):

PCD System	minmal Firmware version
PCD1.M135	0E3
PCD2.M150	0E3
PCD2/4.M170	0E3
PCD2.M480	030
PCD2.M5xx0	1.08.19
PCD3.Mxxx0	031

On other systems than the ones listed above the following workarounds are possible:

• Reading the raw values from the W-modules (1:1) and then converting these values e.g. in Fupla by using the FBox "20-point-conversion". A table for this conversion accroding to the applied module and senser type can be requested from pcdsupport@saia-pcd.com
• Converting the read temperature values at the very beginning of the COB 0. Because the read values are copied to the PCD registers always right before the start of the execution of the COB 0, this will convert the values always just after the writing into registers. All following COBs created by the user will have the correct values read from the PCD3.T760. An example code for this method is attached to this FAQ. This file also contains guide how to introduce this function into an existing project.