Tech Thread
(Issue 3, 2005)
A
User's Guide to Configuring Serial Ports for DirectLOGIC PLCs:
A Two Part Series
by Robert Thornton
Product Engineer,
AutomationDirect
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Part One
The capabilities of Port 2 of the DL06, DL250-1 and DL260 CPUs
are often overlooked when designing a process control network.
Like Port 1, Port 2 can be used for programming the CPU or connecting
other RS-232 devices, such as an operator interface panel, to
the PLC. Unlike Port 1, Port 2 supports different types of network
interfaces and protocols and has more flexible communications.
While Port 1 is limited to 9600 baud, odd parity, and RS-232
serial communications, Port 2 supports RS232C, RS422/485 4-wire,
and RS-485 2-wire (DL06 and DL260 only) networks. It supports
a variety of protocols: K-Sequence, DirectNET, MODBUS
RTU, Non-Sequence (ASCII IN/OUT) and Remote I/O on the DL205
CPUs. The baud rate can be set from 300-38.4K baud. Parity checking
and stop bits are user selectable.
With this flexibility, common uses for Port 2 include PLC to
PLC communications, operator interfaces, bar code readers/printers,
load-cells, serial radio and telephone modems and DirectLOGIC
Remote I/O.
In order to fully understand the capabilities of Port 2, let's
take a look at the advantages of the different network interfaces
and protocols.
Network Interfaces
In the following, each of the three networking options available
on Port 2 will be discussed. Additional information can be found
on the Technical Support page at
www.automationdirect.com.
RS-232C
RS-232C Configuration Port 2 DL06,
DL250-1 and DL260
RS-232C is a simple way to connect two devices that are less
than 45 feet apart, such as connecting a host computer or an
operator interface to a DirectLOGIC CPU.
RS-232C Configuration
RS-232C is capable of operating at data rates up to 38.4 Kbps.
The data rate may need to be reduced depending on the type and
length of cable. RS-232C is a single ended interface, meaning
that a single electrical signal is compared to a common signal
(ground) to determine the binary logic states. As a result,
RS-232C is fairly susceptible to electrical noise.
RS-422/485 4-wire
RS-422 Configuration Port 2 DL06,
DL250-1 and DL260 as slave
RS-422/485 4-wire is a differential interface, which means that
logic levels are defined by the difference in voltage between
a pair of wires, like TXD+ and TXD-, instead of a single signal
wire to ground. The advantage of a differential interface is
that it is typically more immune to noise or voltage spikes
that may occur on the communication lines. It also has greater
drive capabilities that allow for longer cable lengths.
RS-422/485 is rated for a maximum cable length of 4000 feet,
and with the DirectLOGIC PLCs, a maximum data rate
of 38.4 Kbps. Again, the data rate is highly dependent upon
the cable type and length. AutomationDirect recommends using
a Belden 9729 or equivalent cable.
The advantage of a 4-wire network is that it uses a separate
pair of wires for transmit and receive signals. As a result,
timing issues associated with 2-wire networks are eliminated.
RS-485 2-wire
RS-485 2-wire is available on the DL06 and DL260 CPUs (MODBUS
RTU only). Like RS-422/485 4-wire, RS485 2-wire is a differential
interface and is rated for a maximum cable length of 4000 feet
and data rate of 38.4 Kbps. Since it is a 2-wire network, the
transmit and receive signals are exchanged on the same pair
of wires. As a result, protocol timing can be difficult with
some devices.
To determine which type of serial network to use, consider these
factors: How many devices will be on the network? What is the
maximum distance? What are the requirements of the other devices
on the network? Are you comfortable with handling timing issues
that may arise with a 2-wire multi-drop network?
Protocols
All of the protocols available have a maximum data rate of 38.4
Kbps. These protocols support half duplex communications with
DirectLOGIC PLCs.
K-Sequence
K-Sequence protocol can be used with RS-232C or RS-422 networks.
K-Sequence is typically used to communicate to a DirectLOGIC
PLC from a host computer running DirectSoft. It may also be
used to communicate between an operator interface acting as
the master to a PLC network. When using K-sequence with DirectLOGIC
PLCs, the PLCs cannot be the master.
One advantage that K-Sequence has is its ability to perform
write operations on individual bits such as I/O points or control
relays.
DirectNET
RS-485 2-wire Configuration
Like K-sequence, DirectNET protocol can be used with
RS-232C or RS-422 networks. DirectNET is an open protocol.
You can learn the specifics of DirectNET in the user
manual, DA-DNET-M, which can be found at www.automationdirect.com.
DirectNET is an easy-to-use protocol when used with the
DirectLOGIC family of products. It is suitable for
those applications requiring data to be shared between PLCs
or between PLCs and a host computer. A network utilizing the
DirectNET protocol requires one master station that
issues commands to transfer data between the master and slave.
Examples of master stations are operator interfaces, host computers
or even another PLC.
The advantage of DirectNET is that a DirectLOGIC
PLC can be the master station. DirectNET is available
on a number of DirectLOGIC PLCs where other protocols
may not be available. One disadvantage is that data manipulation
cannot occur at the bit level. Also, DirectNET is not
an industry-standard protocol.
MODBUS RTU
When using DirectLOGIC PLCs, MODBUS RTU protocol can
be used with RS-232C, RS-422/485 4-wire or RS-485 2-wire networks.
The are numerous advantages to using MODBUS RTU. First, it is
well known and widely used in all industries. Because of its
simple way of transferring data, it is up to three times faster
than DirectNET. It also utilizes CRC error checking,
making it more robust than DirectNET, which uses LRC
error checking.
Master PLCs
There can only be one master on a network with DirectLOGIC
PLCs. For example, a DL06 cannot be the master on a network
along with an EZTouch panel on the same network, because the
EZTouch panel must be the master. One solution to this, if it
is desired to keep Port 1 free, is to connect the EZTouch panel
to the PLC though a DCM option module or by using available
Ethernet options.
Non-Sequence
Non-sequence (a non-structured protocol) allows the CPU to use
Port 2 to either read or write raw ASCII strings using the ASCII
(DL06 and DL260 only) and PRINT instructions. It can be used
with RS-232C, RS-422 or RS-485 networks. (Note: there is no
provision for multiple stations with the Non-sequence protocol).
Remote I/O
DirectLOGIC Remote I/O is not just a protocol, but
also a network specification. Remote I/O is used to communicate
with remote I/O racks up to 1000 meters away. Using Remote I/O
allows the placement of sensors and other field devices a long
distance from the CPU. The built-in Remote I/O master channel
has the same capabilities as the RM-Net Remote I/O master module.
It can be used with DirectLOGIC Remote I/O Slaves such
as the T1K-RSSS, D2-RSSS and D4-RS. (The user might also consider
an Ethernet solution of ERMs and EBCs as a more efficient approach
when remote I/O is needed.)
In part 2 of this series, we'll be looking at how to set up
Port 2 using the DirectSoft PLC programming software.
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