RS485 bus troubleshooter

Select the type of fault.

Detectors not found on scan
High bus error rate
Detectors cannot be programmed

Select the symptom.

Slow Command Module response

Detectors not found on scan

If no detectors are found on a bus scan then it is very likely that the cabling is causing the problem. Carefully check the cabling as described in 'High bus error rate' before proceding further.

If one or more detectors are missing then check their addresses as this is the most likely reason for them not being seen. If two or more detectors are set to the same address then only a single detector will be seen at that address. See also Detectors cannot be programmed .

Detectors cannot be programmed

If detectors cannot be programmed by the Remote Software when using a Command Module check that 'CTS enable' is checked in the Remote Software communications settings.

Command Module systems must be programmed from the Remote Software by connecting the RS232 cable into the Command Module socket. Plugging directly into a detector will allow the programming of that detector only and will cause the Command Module to show a communications fault while the detector is connected to the PC.

If two detectors are set to the same address they will usually interfere with each other and make programming the detectors at this address difficult or impossible, even though they will be detected properly with a bus scan.

High bus error rate

The bus error rate may be viewed in the diagnostics menu under 'loop errors'. Normally the bus error rate should be 0% and a non-zero error rate could indicate bus problems.

First inspect the cabling and ensure that the RS485 screen is connected to the SCRN terminal at each detector.Screen connectivity may be checked with a meter measuring resistance between the two screens at the Command Module if the RS485 is looped. The Command Module should be powered off to make this measurement.

The cable must be a twisted pair screened cable suitable for RS485. See the SenseNET manual or detector Installers Handbook for recommended cable types.

Check the cable length and ensure that cabling between Stratos-HSSD 2 detectors is a maximum of 1.2km and that the total length of cabling on a system using Stratos-Micras is 1.2km unless the Stratos-Micras have repeater/isolator units installed. Stratos-Micras with isolators can have a maximum of 1.2km of cable between detectors.

Ensure that the cable does not run near any high Voltage equipment or other equipment that may cause EMC interference. If this is the case then the cable may need to be re-routed.

If this does not cure the problem then there may be a double-addressing problem or a detector may be faulty.

Slow Command Module response

Slow Command Module response is normally caused by a high number of errors on the RS485 loop. This may be due to a number of reasons.

If the Command Module is responding slowly first check the cabling as described in the section 'High bus error rate' as this could be the cause of this sort of problem. The bus error rate may be viewed in the diagnostics menu under 'loop errors'.

Command Module software 1.7 or later handles bus errors more efficiently so check that this version or later is installed in the Command Module. Please note that this will not fix the underlying cause of the bus errors but it will help to minimise the symptoms.