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PD57/60/86-x-1260 Hardware Manual • Hardware Version V1.10 | Document Revision V1.20 • 2019-03-22

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8 Communication

8.1 RS485

For remote control and communication with a host system the PD57/60/86-x-1260 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

Host

Slave Slave Slave

RS485

termination

resistor

(120 Ohm)

termination

resistor

(120 Ohm)

}

keep distance as

short as possible

Figure 9: RS485 bus structure with termination resistors

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 PD57/60/86-x-1260 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 stadard (EIA-485) allows up to 32 nodes to be connected to a single

bus. The bus transceiver used on the PD57/60/86-x-1260 units (SN65HVD1781D) oﬀers a signiﬁcantly

reduced bus load compared to the standard and allows a maximum of 255 units to be connected

to a single RS485 bus using standard 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.

4. COMMUNICATION SPEED:

The maximum RS485 communication speed supported by the PD57/60/86-x-1260 hardware is

1Mbit/s. Factory default is 9600 bit/s. Please see separate PD57/60/86-x-1260 TMCL ﬁrmware manual

for information regarding other possible communication speeds below the upper hardware limit.

5. NO FLOATING BUS LINES:

Avoid ﬂoating bus lines while neither the host/master nor one of the slaves along the bus line

is transmitting data (all bus nodes switched to receive mode). Floating bus lines may lead to

communication errors. In order to ensure valid signals on the bus it is recommended to use a resistor

network connecting both bus lines to well deﬁned logic levels.

There are actually two options which can be recommended: Add resistor (bias) network on one side

of the bus, only (120R termination resistor still at both ends):

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