Idle Bus Voltage Levels - Metasys - LIT-12012443 - Software Application - MS-FIT100-0 Field Inspection Tool

MS-FIT100-0 Field Inspection Tool User Guide

Product name
MS-FIT100-0 Field Inspection Tool
Document type
User Guide
Document number
LIT-12012443
Revision date
2019-09-11

With proper EOL termination, the idle bus voltage should read approximately 2.7–2.3 VDC or 0.4V DC differential. The + communication line is biased at 2.7 VDC with respect to the COM line and the—communication line is biased at 2.3 VDC with respect to the COM line. The idle bus voltage can vary with wire length and bus loading but should not be lower than approximately 0.2 VDC differential or higher than approximately 0.9 VDC differential.

Note: It is also possible to read minor voltage fluctuations while reading the idle bus voltage. These minor changes are normal.

Reading the idle bus voltage is a good way to find common wiring mistakes. If you discover that the MS/TP voltages are outside the recommended ranges, see Table 6 for guidelines and possible solutions.

Table 1. Using Voltages and Issues to Troubleshoot
FIT Display Installation Problem Idle Bus Voltage Symptom FIT Issues Menu Resolution Process
None None None FIT is showing a correct reading
Missing all EOLs, Cut Cables Reading value greater than 3 VDC. 2nd Token Look at as-built drawings. Check EOLs. Use FIT to look for quantity of devices on bus. Check each controller’s EOL switch using the FIT.
EOL(s) ON but no power at controller + to - reading drops too low and other readings are dropping low in their range. None Look for quantity of devices on bus. Check each controller EOL switch using the FIT.
- and COM are swapped. + to COM and - to COM drops with the - to COM reading attempting to go to near 0 VDC. CHRFrame

BadFrame

OutOfSeq

Controllers near the problem are not reporting. Look for missing addresses.
+ and - are swapped The + to - reading is trying to go to 0 VDC and the + to COM and - to COM is trying to equalize including all readings resulting at 0 VDC. ChrFrame Zero or few controller addresses are reporting.Split the bus in half and test each segment individually. Continue this process with smaller segments until the issue is found.
+ and COM are swapped All the readings are trying to go to 0 VDC. ChrFrame Zero or few controller addresses are reporting.Split the bus in half and test each segment individually. Continue this process with smaller segments until the issue is found.

Keep in mind the following:

  • The further away from a problem the FIT is positioned, the reduced effect on a reading that problem may have.
  • Each bus segments Idle Bus Voltage must be read independently. It cannot be read across repeaters.