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CAN Primer

DeviceNet is based on the CAN protocol. A

fundamental understanding of CAN will help you

take full advantage of the NetMeter’s features and

significantly improve your ability to diagnose

network problems quickly.
CAN messages are transmitted as a difference in

voltage between two separate wires, CANH

(white) and CANL (blue). Differential transmission

helps CAN and DeviceNet to operate well even

with high levels of external interference (ie from

sources like motors, welders, etc.) Here’s what

you might see if you captured CAN signals on an

oscilloscope:

Differential receiver cancels

noise and extracts data signal

CAN signals have two states, dominant (0) and

recessive (1). The transceiver in each DeviceNet

node determines whether a signal is a 1 or a 0

based on the differential voltage between CANH

and CANL.

CANH

CANL

Dominant

( D )

Recessive

( R )

CANH

CANL

Dominant

Differential

Recessive

Differential

14

Individual wires

include data + noise

Differential Signal

(CANH - CANL)

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Содержание NetMeter

Страница 1: ...TM TM NetMeter for Patent Pending DeviceNet User s Guide P N 715 0016 V1 2 ...

Страница 2: ...elated measurements NetMeter requires 2 AA Alkaline batteries for viewing measurements offline NetMeter must be plugged into a powered network to get most measurements NetMeter will run off DeviceNet network power even without batteries installed Remember to install batteries if you plan to use the Lock and offline viewing features How to Use AutoSearch 1 Make sure the Lock switch is in the Run po...

Страница 3: ...on the network since the NetMeter was plugged in or reset LCD display of times to push the advance button after moving the selector switch to display this measurement Description Push the Advance button You can view this measurement for each active device by pressing the or buttons Pressing and at the same time displays the overall network measurement again The measured value exceeds the positive ...

Страница 4: ... Run position You may reset stored measurements by either turning the meter off and on again or by moving the Lock switch to and back to Run Each selector switch position accesses a different bus measurement and each supports several different measurement types Pressing cycles the display through the different measurements available at each switch position Some measurements allow a detailed view f...

Страница 5: ... activity detected 10 Measurement unit is bandwidth 11 Measurement unit is errors messages per second 12 Measurement unit is volts 13 Measurement displayed is in thousands kilo 14 Measurement displayed is a maximum value 15 Measurement displayed is a minimum value 17 Displayed when viewing measurements for a particular MAC ID Not displayed in AutoSearch mode when the value shown in the top left co...

Страница 6: ...curs Display What it means S 250 S 250 MIN 250 Real time error rate of 14 errors second Incremental error count on the entire network since the NetMeter was connected or reset Maximum bus error rate on the whole network since NetMeter was connected or reset Minimum bus error rate on whole network since NetMeter was connected to the network or reset 2 6 x1 x2 x3 S 250 MAX Node error measurements on...

Страница 7: ...Press or to identify the device s with higher error rates than other nodes Calculate the ratio of error rate to frame rate of suspect nodes and check for above average ratios Devices with above average error ratios should be investigated further Bus Errors deal with these two wires CANL CANH Replace the device and or cabling Temporarily remove the device from the network to see of the errors cease...

Страница 8: ...r node bandwidth recorded since NetMeter was connected or reset Minimum network or node bandwidth recorded since the NetMeter was plugged into the network or reset x1 x2 x3 x4 x5 Miinimum frame rate S on the network or node since the NetMeter was plugged in or reset Maximum frame rate S on the network or node since the NetMeter was plugged in or reset 250 S 250 MIN K S 250 MAX K K S 250 250 MAX 25...

Страница 9: ...one or more nodes with excessive bandwidth or a MAX bandwidth much higher than average Note about frame rate If you know the input and output size in bytes for a polled or strobed device see your configuration or the device documentation you can determine the scan rate per second as follows Bus Traffic deals with these two wires CANL CANH Scan rate Frame Rate from NetMeter Input Frames Output Fram...

Страница 10: ...ipple is 0 1Vp p Maximum bus voltage recorded since NetMeter was plugged in or reset is 17 4V x1 x4 x3 x2 V 250 V 250 MIN V 250 MAX V 250 P P V 250 MAX P P What s a transient A transient is a short temporary deviation of the bus voltage level 4 Every DeviceNet network has some level of bus power transients which is perfectly acceptable Transients in excess of 2V P P can contribute to node failures...

Страница 11: ...t be Check for network cables routed too close to strong sources of interference Check for aging network power supply with output ripple increasing over time For p p noise levels we suggest Bus Power Examples of transients 25V 13V Thresholds High Voltage Fault Low Voltage Warning Low Voltage Fault High P P Voltage Fault High P P Voltage Warning 25 0V 14 0V 13 0V 10 0V 2 0V 4 BUS POWER 11 If you su...

Страница 12: ... Shield Votlage The DeviceNet specification requires that the shield and V be connected together and both to earth ground at the power supply This has the following effects A correctly connected shield has no current flow in it and is at the same voltage grounded power supply V throughout the system Current flow in V causes a voltage rise in V Rising V voltage causes an apparent negative shift in ...

Страница 13: ...me industrial applications and some end users employ different shield connection techniques to meet specific requirements In these cases the shield voiltage will differ from that expected in typical DeviceNet installations Semiconductor Fab Applications Disregard or shield voltage indications if the shield is intentionally connected differently than defined in the DeviceNet Specification Flat Medi...

Страница 14: ... operate well even with high levels of external interference ie from sources like motors welders etc Here s what you might see if you captured CAN signals on an oscilloscope Differential receiver cancels noise and extracts data signal CAN signals have two states dominant 0 and recessive 1 The transceiver in each DeviceNet node determines whether a signal is a 1 or a 0 based on the differential vol...

Страница 15: ...C common mode voltage caused by voltage drop in the cable refer to page 16 and 17 is the primary cause of the voltage shift illustrated above Noise induced in the data wires also contributes to the voltage offset If the combination of DC common mode voltage and induced noise causes the signal voltages to exceed the transceivers capabilities bit errors are more likely to occur CANH D CANL D CANH R ...

Страница 16: ... from 0VDC at the power supply This offset results in each station having a slightly different reference point V which causes an apparent shift in signal voltages from each node s point of view the signal voltages don t really change but the difference in V makes it look that way Intermittent factors such as external interference variations in network load current and electrical noise internal to ...

Страница 17: ...he 5V recommended in the DeviceNet Specification to keep total CMV below 6 5V NetMeter s CMV measurements are not affected by where on the network you take the measurement DC CMV problems are typically caused when devices draw more current than expected or when cable resistance is higher than expected too much cable or a combination of both CMV problems can be intermittent since few devices have a...

Страница 18: ...olts A recessive differential voltage that is not sufficiently close to zero may be mis interpreted by a transceiver as a dominant bit resulting in communication errors Display What it means Current CANH L recessive voltage differential is 0 09V Minimum CANH L recessive voltage differential since the NetMeter was plugged in or reset is 0 08V Maximum CANH L recessive voltage differential since NetM...

Страница 19: ...ferential voltage between CANH and CANL is close to zero represents the logical value 1 opposite from what you might expect The bus is always in the recessive state except when a node is transmitting NetMeter measures the difference between CANH and CANL for recessive bits on the network and if the difference is not close to 0 volts NetMeter indicates a warning or a fault CANH L Diff V deals with ...

Страница 20: ...s voltage can not be measured because there is no bus activity Why is this measurement important See the description of recessive on page 18 The dominant differential voltage is usually around 2V A lesser differential voltage may be mis interpreted by a transceiver as a recessive bit causing bus errors At the beginning of each frame it is possible for several nodes to transmit simultaneously resul...

Страница 21: ...might expect The bus can only be in the dominant state when a node is actively transmitting NetMeter measures the difference between CANH and CANL for dominant bits on the network and if the difference is not within acceptable limits NetMeter indicates a warning or a fault Check for shorts and opens on the CANH and CANL wires Check for a missing bad or extra terminators should be two 120 ohm termi...

Страница 22: ...ault 8 50V 7 00V 2 00V 3 00V 9 11 DeviceNet messages consist of 1 s and 0 s which are represented as differences in voltage between the CANH and CANL wires However if the absolute voltage of the signal measured to the V wire at any node is too high or too low bits may not be received correctly NetMeter measures each of the key CANH CANL and Differential voltages and if the readings are too high or...

Страница 23: ... DeviceNet network For some devices this is as simple as setting a switch to select the MACID node address and baud rate Other devices require configuration from a PC based software tool Available from SST Most importantly configuration means that before you replace a device with another identical device you must configure the replacement device properly Determinism is a measure of the predictabil...

Страница 24: ... circuit typically a chip that converts digital bits to from the differential voltages on the network cable Transceiver is a contraction of mitter and re A temporary deviation from the mean voltage of a power or signal conductor trans ceiver The NetMeter has two configuration switches located in the battery compartment The Meter Power switch on left selects between bus battery power powered by bus...

Страница 25: ...nt This equipment generates uses and can radiate radio frequency energy and if not installed and used in accordance with the instruction manual may cause harmful interference to radio communications Operation of this equipment in a residential area is likely to cause harmful interference in which case the user will be required to correct the interference at his own expense Warning Changes or modif...

Страница 26: ... or exceeds the requirements of the following standard EN 61326 1997 including amendment A1 1998 Electrical equipment for measurement control and laboratory use EMC requirements This is a Class A product In a domestic environment this product may cause radio interference in which case the user may be required to take adequate measures This equipment is neither designed for nor intended for operati...

Страница 27: ...head Connectivity will repair or replace at our option all products returned freight prepaid which prove upon examination to be within the Warranty definitions and time period The Warranty does not cover costs of installation removal or damage to user property or any contingent expenses or consequential damages Maximum liability of SST Woodhead Connectivity is the cost of the product s If your Net...

Страница 28: ...ublication Date February 2002 SST part of Woodhead Connectivity 50 Northland Road Waterloo Ontario Canada N2V 1N3 Tel 519 725 5136 www myNetAlert com www mySST com www woodheadconnectivity com Document Info Specifications Environment Bus Power Voltage Measurement Maximum Limits Power Supply Shield Voltage Measurement CAN Voltage Measurement Storage Temperature Range Voltage between any two termina...

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