User Manual

PD28-1-1021 / PD28-3-1021 V1.4 Hardware Manual (Rev. 1.03 / 2018-JAN-25) 12
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In addition to power stabilization (buffer) and filtering this added capacitor will also reduce
any voltage spikes which might otherwise occur from a combination of high inductance
power supply wires and the ceramic capacitors. In addition it will limit slew-rate of power
supply voltage at the module. The low ESR of ceramic-only filter capacitors may cause
stability problems with some switching power supplies.
Keep the power supply voltage below the upper limit of 28V!
Otherwise the driver electronics will seriously be damaged! Especially, when the selected
operating voltage is near the upper limit a regulated power supply is highly recommended.
Please see also chapter7, operating values.
There is no reverse polarity protection!
The module will short any reversed supply voltage due to internal diodes of the driver
transistors.
3.4.1.2 RS485
For remote control and communication with a host system the PD28-1-1021 / PD28-3-1021 provides a two
wire RS485 bus interface. For proper operation the following items should be taken into account when
setting up an RS485 network:
1. BUS STRUCTURE:
The network topology should follow a bus structure as closely as possible. That is, the connection
between each node and the bus itself should be as short as possible. Basically, it should be short
compared to the length of the bus.
c:>
node
1
node
n
- 1
node
n
Host
Slave Slave Slave
RS485
termination
resistor
(120 Ohm)
termination
resistor
(120 Ohm)
}
keep distance as
short as possible
Figure 3.4: Bus structure
2. BUS TERMINATION:
Especially for longer busses and/or multiple nodes connected to the bus and/or high
communication speeds, the bus should be properly terminated at both ends. The PD28-1-1021 /
PD28-3-1021 does not integrate any termination resistor. Therefore, 120 Ohm termination resistors
at both ends of the bus have to be added externally.
3. NUMBER OF NODES:
The RS485 electrical interface standard (EIA-485) allows up to 32 nodes to be connected to a single
bus. The bus transceivers used on the PD28-1-1021 / PD28-3-1021 units (TMCM-1021 hardware V1.2:
SN65HVD3082ED, since hardware V1.4: SN65HVD1781D) have a significantly reduced bus load and
allow a maximum of 255 units to be connected to a single RS485 bus using TMCL firmware. Please
note: usually it cannot be expected to get reliable communication with the maximum number of
nodes connected to one bus and maximum supported communication speed at the same time.
Instead, a compromise has to be found between bus cable length, communication speed and
number of nodes.