
Section 8. Serial Communications
The Model 4200 is equipped with two serial interface ports. Port 1 is a duplex
(bi-directional) port that is usually used to communicate with a computer or terminal, and
port 2 is a simplex (printer) port that is usually used to send data to a printer or data logger.
The 4200 comes standard with both ports configured for RS232 operation and ready to
use. Other configurations are available.
PORT 1 - DUPLEX
Port 1 is a duplex (two-way) port, meaning that it can both transmit and receive data. It is a
half-duplex port, so it cannot send and receive simultaneously. Port 1 is usually connected
to a computer or terminal to issue commands to the scale, send data to the scale, or
request data from the scale. The 4200 command set allows the operator to perform any of
the front panel push-button functions, and some others that are not accessible from the front
panel. Data that can be sent to, or received from, the scale includes Overs, Unders, Tares,
Setpoints, and Labels. Port 1 makes it possible to easily load values for various products
or recipe ingredients. It also makes it easy to request data from the scale for purposes of
data collection.
Port 1 is available for operation in any of 3 configurations: RS232, RS485, or 20 mA
current loop. The 4200 is configured for RS232 operation unless one of the other options
is ordered. RS232 is the most commonly used communications standard in the computer
industry, so almost all computers come with a standard RS232 port. It is used to transfer
data between two pieces of equipment, in this case the 4200 and a computer or terminal.
RS232 cable length is limited to approximately 50 ft. (longer if a low capacitance cable is
used).
RS485 is necessary when a long cable is to be used, or when more than one scale is to be
linked to the same computer. In this case, one end of the cable is connected to the
computer, and each scale is connected to a tap or "drop" on the main cable. Each scale
must then have a unique scale address so that the scales will not conflict with one another
(see Section 6, parameter 21). A special serial card usually needs to be installed in the
computer to use RS485 communications.
The 20 mA current loop is a fairly common standard for industrial equipment. It is normally
used when long cables are needed, or in cases where previously existing equipment is
configured for 20 mA service.
Any of these configurations can be ordered with a data cable and DB9 or DB25 connector
(the most common computer serial connectors). The RS232 connector usually needs to
wired in the "null modem" configuration, which is shown in Section 12. The main board
connections are also shown in Section 12. Note that a shielded cable must be used to
avoid interference problems from other equipment.
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