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RS232 / RS485 / RS422 Questions
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1. |
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What are the
differences between industrial and commercial-grade converters?
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Just as there are many differences
between industrial- and commercial-grade computers, the industrial-
and commercial-grade converters differ in many ways, such as the
level of circuit protection, temperature range and noise tolerance.
Most importantly, true industrial-grade converters, such as
CommFront's, require rugged
components and reliability-engineering designs to withstand heat,
vibration, shock and other environmental factors, therefore, they
are much more durable
than commercial-grade
converters. Industrial-grade converters are designed to interface
with mission-critical equipment, such as computers, PLC, SCADA and HMI - whether in the field, server room, factory, control room, or
fire command center. Visit
the wikipedia's pages for more information on:
Temperature classifications and
Reliability engineering |
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2. |
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RS485/RS422
Topology: How to connect RS485/RS422 devices?
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RS485/RS422 is a
network in a linear, multi-drop configuration;
however, star-wiring is inevitable in some
places. Our RS485 Hub/Splitter
(Model:
HUB-485-4) splits an RS485 network into four, thus making star-wiring possible. |
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RS485/RS422
Cabling: What cables are recommended?
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Screened twisted-pair cable (e.g. CAT-5e) is recommended to prevent
external noises induced to the RS485/RS422 buses. |
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RS485/RS422
Distance: Maximum distance and how to extend it?
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A single RS485/RS422
loop can go up to 4000ft (1200m) by using standard copper wire (screened twisted pair
such as CAT-5e).
This
distance (using copper wire) can be extended to another 4000ft
(1200m) by using our RS485/RS422 repeater
(Model:
RPT-485_422-2 and
RPT-485_422-4). Note:
A maximum of two repeaters can be used in
series. The RS485/RS422 distance can be further
extended to up to 24.8 miles (40km) by using Single-Mode Fiber Optic
Cables and our Single-Mode RS232/RS485/RS422 to Fiber Optic Media
converter (Model:
FBR-Serial-2). |
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RS485/RS422
Termination (120-Ohm): Why is it important and when to use it?
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RS485/RS422 network is normally used over a long distance and in
an electrically noisy environment, noises, reflections, and data
errors might occur if there is no termination on the RS485/RS422
buses. Both ends of the transmission
lines should be terminated to reduce noises and reflections that can
cause data errors. There are several methods of terminating data
lines, and we recommend parallel termination. Instead of using pull-up or pull-down resistors, the
parallel 120-Ohm end-of-line terminators provide a
hassle-free and effective way of termination. However, since the Terminator
will increase the load on the data circuit, we recommend to turn ON
the Terminator only when the data rate is over 19.2kbps or the distance exceeds 660 feet (200m).
Note: CommFront's built-in 120 Ohm terminators can be easily turned ON or OFF through a simple jumper
setting, thus making our converters ideal solutions for all types of
installations. |
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How to turn ON or
OFF the 120-Ohm terminator?
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If you are using
our port-powered converters with the provided terminal blocks, the
120-ohm terminator is set through a simple jumper setting
(marked as "120" for CVT-485-x and "J4" for CVT-485_422-x or
CVT-422-x). Terminate the jumper to turn ON the terminator and
remove it to turn the terminator OFF.
If you are using
our port-powered converters without the provided terminal blocks,
short pin 6 & 7 for CVT-485-x or pin 4 & 8 for CVT-485_422-x /
CVT-422-x on the DB-9 connectors (RS-485/422 side) to turn ON the
terminator.
For
CVT-485_422-4, tie J3 & J4 to turn ON the terminator for RS485; tie
J1 & J2 to turn ON the terminator for RS422. |
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7. |
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RS232 Distance:
What are the maximum distances?
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RS232 can go up to fifty (50) feet
(about 15m). |
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RS232: What is DTE/DCE? How do I
know my RS232 device is a DTE or a DCE?
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DTE
stands for Data Terminal Equipment, a PC is a DTE. DCE stands for
Data Communication Equipment, a Modem is a DCE.
DTE normally comes with a Male Connector, while DCE comes with a
Female Connector, but that is not always the case. Use the guide
below as a simple way to confirm:
Measure Pin 3 and Pin 5 (DB9
Connector) with a multi-meter. If you get a voltage of -3V to -15V,
it is a DTE device. If the voltage is on Pin 2, then it is a DCE
device. Note: The result for the DB-25 Connector is reversed, and
Ground for the DB-25 is on Pin 7.
To learn more about
RS232, check out:
RS232 basics: Three easy steps to understand and
control your RS232 devices |
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RS232: What are
Handshake Lines?
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Handshake Lines are used to control the
direction of data flow between DTE and DCE. They are:
RTS (Request to Send) - Asserted by DTE to prepare DCE to receive
data.
CTS (Clear to Send) - RTS Acknowledgment by DCE to allow DTE to
transmit.
DTR (Data Terminal Ready) - Asserted by DTE to indicate that it is
ready to be connected.
DSR (Data Set Ready) - Indicates an
active connection by DCE.
DCD (Data Carrier Ready) - Used by modem
when a connection has been established with remote equipment.
RI (Ring Indicator) - Used by the modem to indicate a ring signal from
telephone line.
To learn more about
RS232, check out:
RS232 basics: Three easy steps to understand and
control your RS232 devices |
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RS232: Do I need to use
Handshake
Lines to control the data direction?
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This is not necessary. The converters
come with the Auto-Turnaround feature, the RS-485 transceiver
circuit will change the data direction automatically upon sensing a
"start bit" (a logic high "Mark" to low "Space" transition) from its
RS232 data transmit line (TxD) so that the RS-485 side is in "listening" mode, thus reducing the chance of data collusion.
Note: Auto-turnaround feature is not
applicable for full-duplex communications such as RS232, RS422, and
TTL, because full-duplex communications consist of two channels: TX
(Transmitting) and RX (Receiving), and both channels can operate
simultaneously. |
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RS232: My control software
requires handshake lines to be tied together (e.g. DTR & DCR, RTS &
CTS). Can I still use your converters?
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Yes, to satisfy the requirements of some
software, the following handshake lines are tied together in the
converters.
- Pin 1 (DCD) is connected to Pin 4 (DTR) and Pin 6 (DCR).
- Pin 7 (RTS) is connected to Pin 8 (CTS).
Note: Above pin assignments are for DTE
devices. |
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12: |
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How do the converters
derive power? Do I need to turn ON DTR or RTS in order to power the
converters?
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Our port-powered converters derive power
from the RS232 port. The converters extract power from the TX line
using a capacitor charge pump, and do NOT require any handshake
signal (e.g. DTR or RTS). Also, it is not necessary for there to be
continuous data to power the converter. |
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The converters are
port-powered. Why is there a +5V sign on the converters? Can I
connect external power to this terminal?
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The main purpose for the +5V sign on the termination board is for
port power measurement, if you doubt that the
converter is not functioning, measure this pin and the Gnd with a
Volt meter. If you get a signal of more than +5V, your converters
are powered correctly, otherwise please check your connection or use
an external 5VDC regulated power supply if the port power is
insufficient. Note: If the converter is connected to is a DCE
device, you need to swap Pin 2 and 3 (or use a
Null-modem converter to connect between the DCE device and the
converter). |
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14: |
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How many serial
devices can I connect with the port-powered converters?
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You can connect a maximum
of 128 serial devices (RS485 or RS422) to our converters. However, this depends a great deal on a
few factors such as the RS485/RS422 devices connected to the
converter, data baud rate, cable distance, etc. Please refer to the
datasheets of the converters for more information. |
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What is the maximum
distance the converters can communicate with remote devices?
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Depending on the power from RS232 port,
the RS232/RS485/RS422 converters can go up to 4000 feet (1200 m) for
the maximum baud rate of 19200 bps. Distance can be extended by
using RS485/RS422 repeaters or fiber optic converters. Fiber Optic
converters can go up to 24.8 miles (40km). |
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How do I extend the RS232 data line distance?
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The RS232 data line has a distance
limitation of fifty (50) feet (about 15 m), while the RS485/RS422
can go up to 4000 feet (1200 m). It is a common practice to use the
converters to extend the RS-232 data line.
There
are 3 ways to extend the distance of your RS232 devices: Convert to
2-wire RS485, convert to 4-wire RS422 or use our isolated RS232
repeaters.
To protect your RS232 devices from surge voltages while extending the
RS232 distance, we would recommend the RPT-232-3 - an optically
isolated RS232 data repeater (extender) (TX, RX, and GND), which
works in pairs to extend the full-duplex RS232 distance up to 1.8
miles (3 km) @ 300bps baud or 1600 feet (500m) @ 28.8kbps baud. This
product features opto-isolation circuitry, which effectively protect
your RS-232 devices from
ground
loops, transient surges, lightning and spikes. |
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17: |
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How do I extend the RS485/RS422 distance?
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A single RS485/RS422
loop can go up to 4000ft (1200m) by using standard copper wire (screened twisted pair
such as CAT-5e).
This
distance (using copper wire) can be extended to another 4000ft
(1200m) by using our RS485/RS422 repeater
(Model:
RPT-485_422-2 and
RPT-485_422-4). Note:
A maximum of two repeaters can be used in
series.
The
RS485/RS422 distance can be further extended to up to 24.8 miles
(40km) by using Single-Mode Fiber Optic Cables and our Single-Mode
RS232/RS485/RS422 to Fiber Optic Media converter (Model:
FBR-Serial-2). |
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18: |
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What is the maximum
number of supported nodes in a RS485/RS422 network if the repeater is used?
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The maximum number of supported nodes in
a RS485/RS422 network depends on the RS485/RS422 master, for
example, if the RS485 master supports only 32 nodes, the maximum
number of supported nodes in the RS485 network is 32. The use of
RPT-485_422-2 only extends the RS485/RS422 distance, it will not
affect the total number of nodes that can be used in the network.
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19: |
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What is
Opto-Isolation? What are the benefits of using RS232 Isolators or
Opto-Isolated Converters?
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Opto-isolation uses optical isolators to
isolate two serial devices. Optical isolator is a common element
used for RS232, RS485 and
RS422 systems. With an optical isolator, only light passes between
two serial devices, this is performed with an LED and a
photosensitive transistor.
There are many benefits of using opto-isolated converters, most
importantly, it protects your RS232 devices from
transient
surges, lightning and spikes.
Optical isolation also eliminates ground loop and noise problems
because the ground of two connected devices are separated. |
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20: |
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What is a Static
Protection Circuit? Is it an Opto-Isolator?
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The Static Protection Circuit is also
called an ESD (Electro-Static Discharge). The charged capacitance of
a human body enables a person to destroy integrated circuits with a
single touch. Such a contact can easily occur during the
installation of an interface cable. To prevent ESD damage, our
converters come with the protection circuit to protect against ESD levels for up to ±15kV.
Static Protection Circuit is NOT an
Opto-Isolator. To prevent your
PC or serial devices from being damaged by
ground
loops, transient surges, lightning and spikes,
opto-isolated converters (i.e. CVT-485-3, CVT-485_422-3) or
isolators (i.e. CVT-232A-3 or CVT-232B-3) are required. |
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What is the
difference between 3-wire and 7-wire RS232 Isolators?
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The major difference between 3-wire and
7-wire RS232 Isolators is: 3-wire isolator optically isolates three
(3) RS-232 lines (TX, RX and GND), while 7-wire isolator optically isolates
seven (7) RS-232 lines (TX,
RX, RTS, CTS, DTR, DSR and GND).
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How do I use
CVT-485_422-1 / CVT-485_422-3 as a RS232/RS422 converter?
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CVT-485_422-1 and CVT-485_422-3 are
dual-function converters, so they can be used to convert any
standard RS232 port into a two-wire balanced half-duplex RS485 port, or a
four-wire balanced full-duplex RS485/RS422 port and vice versa. This is done
through the Jumper settings on the terminal block. For example:
Jumper 1, 2 & 3 are ON (shorted) by default, which is used for
RS232/RS485 conversion. When all three jumpers are removed
(opened), it becomes a RS232/RS422 converter. |
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23: |
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How do I perform a
Loopback Test for my RS232/RS485 converter?
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Performing a Loopback Test is easy with
our 232Analyzer
software. Simply connect two RS232/RS485 converters to
your PC's COM ports (if your PC does not have enough COM ports, you
may use USB to RS232 converters or cables). Run two instances of 232Analyzer software
on your PC. When you send commands (i.e. "CommFront" in ASCII
format) from one COM port, you should receive the same commands on
the other, and vice versa. |
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By performing a simple Loopback Test
like this, you can test you RS485 transceiver, which is very
helpful when you are in doubt about the performance of your
converters or cables.
Note: RS232/RS485 loopback requires two RS485 converters to properly
perform the test. |
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Click
here
to download a FREE copy of 232Analyzer software. |
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24: |
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How do I perform a
Loopback Test for my RS232/RS422 converter?
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Performing a Loopback Test is easy with
our
232Analyzer software. Simply connect TX+ and TX- to RX+ and RX-
respectively and connect the RS232/RS422 converter to your PC's COM
port. Run 232Analyzer software and send commands (i.e. "CommFront"
in ASCII format) from the software, you should receive an echo of
the commands sent. By performing a simple Loopback Test like this,
you can test both the Transmitter and Receiver of your RS232/RS422
converter, which is very helpful when you are in doubt about the
performance of your converters or cables. |
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Click
here
to download a FREE copy of 232Analyzer software. |
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25: |
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How do I perform a
Loopback Test for my RS232 COM port?
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Loopback Test provides an effective way
to test your RS-232 COM ports or cables. By using our
232Analyzer software,
you can test both your RS-232 COM ports and cables: |
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If you are testing your local RS232 COM
port, simply connect Pin 2 to 3 of your RS232 port, then run the
232Analyzer software and send commands from the Send_Command_Pane,
you should get an echo of the commands sent.
If you are communicating with a remote
RS232 device, run the
232Analyzer software on both ends of the PC and turn ON/OFF the
line state or send commands from one end, and you should be able to
see the changes or receive the commands on the other end.
By performing a simple
Loopback Test like this, you can test both the RS232 COM port and
cable, which is very helpful when you have doubts about the
performance of your COM ports or cables.
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Click
here
to download a FREE copy of 232Analyzer software. |
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26: |
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How do I perform a
Loopback Test for my 3-wire RS232 Isolator?
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Performing a Loopback Test is easy with
our 232Analyzer
software. Simply connect the female
connector end of the isolator to your PC’s COM port and
connect the other end (male connector) to another of
your PC’s COM ports with a null-modem converter (or
connect pin 2 to 3, pin 3 to 2, and pin 5 to 5 between
the male connector and your PC's RS232 connector).
Run two instances of 232Analyzer software
on your PC. When you send commands (i.e. "CommFront" in ASCII
format) from one COM port, you should receive the same commands on
the other, and vice versa.
By performing a simple loopback test like this, you can
test both the transmitter and receiver of the isolator. This is very
helpful when you are in doubt about the performance of your
isolator.
Click
here
to download a FREE copy of 232Analyzer software. |
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27: |
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How do I perform a
Loopback Test for my 7-wire RS232 Isolator?
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Performing a Loopback Test is easy with
our 232Analyzer
software. Simply connect the female
connector end of the isolator to your PC’s COM port and
connect the other end (male connector) to another of
your PC’s COM ports with a null-modem converter (or
connect pin 2 to 3, pin 3 to 2, pin 4 to 6, pin 6 to 4,
pin 7 to 8, pin 8 to 7, and pin 5 to 5 between the male
connector and your PC's RS232 connector).
Run two instances of 232Analyzer software
on your PC. When you send commands (i.e. "CommFront" in ASCII
format) from one COM port, you should receive the same commands on
the other, and vice versa.
By performing a simple loopback test like this, you can
test both the transmitter and receiver of the isolator. This is very
helpful when you are in doubt about the performance of your
isolator.
Click
here
to download a FREE copy of 232Analyzer software. |
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