Doran 2200 Series, Instruction Manual

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RS485 Communications Option
The scale offers an RS485 data communications option. RS485 communications are
similar to RS232 except that RS485 provides better noise immunity, it is suited to longer
cable distances, and it will allow multiple scales to be attached to the same data line.
RS485 achieves its performance advantage over RS232 by utilizing a differential input
and output. In other words, the RS485 device sends two copies of its data; one in
positive logic and one in negative logic. The receiving device looks at the two outputs
and takes the difference between them. If the difference is positive the bit is a "1' if it is
negative, it is a "0."
The ability of RS485 to have multiple devices attached to the same line complicates the
serial communication. Because of the multiple device capability, each device must have
a unique dedicated address. Also, no device is permitted to communicate unless the
master serial bus controller has specifically activated it. Since a scale with RS485 may
not communicate unless it is activated by the master serial bus controller, a print buffer
is provided to store scale data until it is requested.
When the indicator is setup for RS485 communication, all print requests are redirected
to the print buffer rather than the printer port. All weight readings are stored in this buffer
until a buffer dump command is received from the serial port. Once the dump command
is received, the scale takes control of the serial bus and transmits the stored scale
readings. When the buffer is empty, the scale releases the bus and will not
communicate until the master serial bus controller again activates it.
Warning: If the scale data stored in the Print Buffer exceeds the buffer size, only the
latest data that will fit in the buffer will be saved. If the scale is set for the default print
string "
FO
", the buffer will store up to 130 print strings before the buffer stops saving
data.