 Automatic
Serial Data Collection
As more and more devices come equipped
with some type of communication for data collection and monitoring,
the amount of data that can be collected by a PC in a system
is astounding. A problem arises, however, when multiple instruments
or devices need access to the PC through the serial port,
but only one port is available.
There are multiple options for expanding
the capability of a port that was designed to communicate
with only one device. You can add a separate serial port for
each RS-232 device that you need to talk to. By adding serial
ports, multiple devices can have their own dedicated port,
often concurrently and at high speeds. The drawback is that
separate serial ports may be expensive and require some effort
to configure. There is also a practical limit on resources
available inside the PC to allocate to Serial Ports.
Multiple devices can be tied together
by converting the signal at each point to a multi-drop bus,
like RS-485. This allows up to 32 devices on a single network
without repeaters. The devices can also be connected up to
4000 feet away from the PC. The disadvantage is that each
device must have its own unique address. Also, some protocol
needs to be established to prevent more than one device at
a time from sending data to the PC. Usually this is done by
having the PC poll each device individually. This does not
lend itself well to devices that send their data automatically,
without being requested.
Using the 232BSS4 buffered Smart Switch
is an excellent way to collect data from multiple devices
utilizing RS-232 serial ports. Up to four instruments, card
readers, or time clocks can be directly connected to the 232BSS4,
providing access through a single serial port on your PC.
If you have more than four devices that
are sending data, more than one 232BSS4 can be cascaded, allowing
more ports to be added. The 232BSS4 buffers data from all
ports at once, making it well suited for devices that will
send a fixed amount of data at sporadic intervals, such as
measurement instruments, security access card readers, barcode
scanners, or time clocks. Below is an explanation of how best
to utilize the 232BSS4 to connect these devices to your PC.>
Figure 1
>Figure 1 shows a typical setup, with
16 devices sending information at random times to the PC.
The switches at Level 2 receive the data and send it on to
the Level 1 switch. The Level 1 switch buffers the data and
sends it to the PC. Each of the switches will automatically
send the data when it is available. If another port has access
to the PC, the data will be buffered until a path is available.
Cascading in Automatic mode:
For best results in an automatic system,
the 232BSS4s in the system should be set up the following
way.
- Set the switches to turn on the AutoSelect
feature.
- Set the Port Timer to 0
- Set the Inactivity Timer on all of
the switches to somewhere between 50 and 500 milliseconds,
depending on the space between data packets and the response
time required to each connected device.
- If you need your software to identify
which port sent the data, set up the port preamble on the
Level 2 switches. Set each port to have a unique preamble.
The preamble can be 1, 2, 3, or 4 characters long, and any
characters can be used. The preamble will be added to the
beginning of each new data packet arriving at the PC. The
data collection software on the PC should be written to
recognize this preamble as an address or identifier of the
data source.
Cascading in Polled Mode
This configuration will also work with
systems where the devices have to be polled for information.
With the setup of Figure 1, the switches can be configured
so that you still only have to send one set of switching characters.
To do this, set the switches up in the following way:
- Set the switch on Level 1 to broadcast
to all slaves on startup.
- Set the switch on Level 1 to AutoSelect
mode, no preamble.
- Set the switch on Level 1 for 0 Port
Time
- Set the switch on Level 1 for a short
Inactivity Time. Somewhere around 5 milliseconds or so.
- Set the switches on Level 2 for no
AutoSelect.
- Set the preamble on the Level 2 switches
to be unique from each other. One suggestion is to use a
3 character command. Use the first character as a start
of command character like STX. The second character could
represent the address of the Level 2 switch you are going
to. The third character is the command character.
- Write the software to send the command
to select the port. By setting the Layer 1 switch to broadcast
and AutoSelect, only 1 set of command characters is needed.>
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