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Digitax HD M75X Series Installation and Technical Guide
Issue Number: 5
4.13.2
Digitax HD M751
EIA 485 serial
communications
The EIA-485 interface provides two parallel RJ45 connectors allowing
easy daisy chaining, refer to Figure 4-29
on page 81. The drive supports Modbus RTU protocol. See
Table 4-23 for the connection details.
Standard Ethernet cables are not recommended for use when
connecting drives on a EIA-485 network as they do not have the correct
twisted pairs for the pinout of the serial comms port.
Table 4-23 Serial communication port pin-outs
Minimum number of connections are 2, 3, 7 and shield.
4.13.3
Digitax HD M751
Isolation of the EIA-485
serial communications port
The serial communications port is double insulated from the high voltage
drive circuits and meets the requirements for PELV (Protective Extra
Low Voltage) according to IEC61800-5-1. The communications ports
remain referenced to other PELV rated circuits within the drive (including
the control, feedback and digital I/O). Where further isolation from these
PELV rated circuits is required and additional external isolation barrier
will be required.
An isolated serial communications lead has been designed to connect
the drive to IT equipment (such as laptop computers), and is available
from the supplier of the drive. See below for details:
Table 4-24 Isolated serial comms lead details
The “isolated serial communications” lead has reinforced insulation as
defined in IEC60950 for altitudes up to 3,000 m.
4.13.4 Communication networks and cabling
Any isolated signal circuit has the capability to become live through
accidental contact with other conductors; as such they should always be
double-insulated from live parts. The routing of network and signal wires
should be done so as to avoid close proximity to mains voltage cabling.
4.14 Safe Torque Off (STO)
The Safe Torque Off function provides a means for preventing the drive
from generating torque in the motor, with a very high level of integrity. It
is suitable for incorporation into a safety system for a machine. It is also
suitable for use as a conventional drive enable input.
The safety function is active when the STO input is in the logic-low state
as specified in the control terminal specification. The function is defined
according to EN 61800-5-2 and IEC 61800-5-2 as follows. (In these
standards a drive offering safety-related functions is referred to as a
PDS(SR)):
'Power that can cause rotation (or motion in the case of a linear motor) is
not applied to the motor. The PDS(SR) will not provide energy to the
motor which can generate torque (or force in the case of a linear motor)'
This safety function corresponds to an uncontrolled stop in accordance
with stop category 0 of IEC 60204-1.
The Safe Torque Off function makes use of the special property of an
inverter drive with an induction motor, which is that torque cannot be
generated without the continuous correct active behaviour of the inverter
circuit. All credible faults in the inverter power circuit cause a loss of
torque generation.
Note on the use of servo motors, other permanent-magnet motors,
reluctance motors and salient-pole induction motors:
When the drive is disabled through Safe Torque Off, a possible
(although highly unlikely) failure mode is for two power devices in the
inverter circuit to conduct incorrectly.
This fault cannot produce a steady rotating torque in any AC motor. It
produces no torque in a conventional induction motor with a cage rotor.
If the rotor has permanent magnets and/or saliency, then a transient
alignment torque may occur. The motor may briefly try to rotate by up to
180° electrical, for a permanent magnet motor, or 90° electrical, for a
salient pole induction motor or reluctance motor. This possible failure
mode must be allowed for in the machine design.
The Safe Torque Off function is fail-safe, so when the Safe Torque Off
input is disconnected the drive will not operate the motor, even if a
combination of components within the drive has failed. Most component
failures are revealed by the drive failing to operate. Safe Torque Off is
also independent of the drive firmware. This meets the requirements of
the following standards, for the prevention of operation of the motor.
Machinery Applications
The Safe Torque Off function is suitable for use as a safety component
of a machine:
Safety Parameters
According to IEC 61508-1 to 7 / EN 61800-5-2 / EN 62061
If an Ethernet network adaptor is inadvertently connected to
a
Digitax HD M751
EIA-485 drive, a low impedance load
across the EIA-485 24 V is applied and if connected for a
significant period of time can introduce the potential risk of
damage.
Pin
Function
1
120
Termination resistor
2
RX TX
3
Isolated 0 V
4
+24 V (100 mA)
5
Isolated 0 V
6
TX enable
7
RX\ TX\
8
RX\ TX\ (if termination resistors are required, link to pin 1)
Shell
Isolated 0 V
In order to meet the requirements for SELV in IEC60950 (IT
equipment) it is necessary for the control computer to be
grounded. Alternatively, when a lap-top or similar device is
used which has no provision for grounding, an isolation
device must be incorporated in the communications lead.
Part number
Description
4500-0096
USB Comms cable
NOTE
CAUTION
WARNING
Type
Value
Percentage of SIL
3 allowance
Proof test interval
20 years
High demand or a continuous mode of operation
PFH (1/h)
4.21 x 10
-11
1/h
<1 %
Low demand mode of operation (not EN 61800-5-2)
PFDavg
3.68 x 10
-6
< 1 %
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