RS-485 SERIAL COMMUNICATIONS
RS-485 communications allows for transmitting and receiving of data over a
single pair of wires. This feature can be used for monitoring various values,
changing values, and resetting output(s), all from a remote location. Typical
devices that are connected to a C48T unit are a printer, a terminal, or a host
computer.
The RS-485 differential (balanced) design has good noise immunity and
allows for communication distances of up to 4000 feet. Up to 32 units can be
connected on a pair of wires and a common. The unit’s address can be
programmed from 00 to 99.
Communication Format
The half-duplex communication operation sends data by switching voltage
levels on the common pair of wires. Data is received by monitoring the levels
and interpreting the codes that were transmitted. After the unit receives a
Transmit Command or Print Request, it will wait 100 msec before it will begin
transmitting data. In order for data to be interpreted correctly, there must be
identical formats and baud rates between the communicating devices. The
formats available for the C48T unit are 1 start bit, 7 or 8 data bits, No parity or
1 parity bit (odd or even) and 1 stop bit. The available baud rates are 1200, 2400,
4800, or 9600 baud.
Before serial communication can take place, the unit must be programmed to
the same baud rate and parity as the connected equipment. In addition, the loop
address number and print options should be known. When used with a terminal
or host computer and only one unit is employed, an address of zero (00) may be
used to eliminate the requirement for the address specifier when sending a
command. If more than one unit is on the line, assignment of unique non-zero
addresses is recommended.
Sending Commands And Data
When sending commands to the C48T unit, a command string must be
constructed. The command string may consist of command codes, value
identifiers, and numerical data. Below is a list of commands and value
identifiers that are used when communicating with the C48T unit.
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DESCRIPTION
N(4EH)
P (50H)
S (53H)
Set value; Followed by one value identifier (1 or 2)
T (54H)
Transmit value; Followed by one value identifier (A, B, or E)
V (56H)
Unit Address; Followed by a one or two digit address number 1-99
Transmit Print Options; Transmits the options selected in the Print
Options section of the Programming Menu
Change value; Followed by one value identifier (A, B or E) then the
proper numerical data
COMMAND
R (52H)
Reset value; Followed by one value identifier (E, 1, or 2)
Figure 12, Data Format - 7 Data Bits
Figure 13, Data Format - 8 Data Bits
DATA FORMAT - 10 BIT FRAME
(Parity = none)
DATA FORMAT - 10 BIT FRAME
(Parity = odd or even)
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