red lion GEMINI 1000, Instruction Manual, Page: 24 / 60

GEMINI 2000 20 mA CURRENT LOOP COMMUNICATIONS
The Gemini 2000 20 mA Current Loop Communications Option allows a
“two-way” serial communications link to be established in order to monitor or
change the count value, Presets and Scale Factor from a remote location. Some
typical devices that can be connected with the Gemini 2000 are: a printer, terminal,
programmable controller, or host computer. For devices that use RS232, a
GCM232 Serial Converter Module is available to convert the 20 mA Current Loop
signals to RS232 and vice-versa. Also, a BCD converter is available to convert the
20 mA current loop signals, from the Gemini, to full parallel BCD, enabling the
Gemini to operate with Programmable Logic Controllers, etc.
There are two loops that must be established. One for sending commands to the
Gemini 2000 and one for receiving the data values from the Gemini 2000. Up to
sixteen Geminis or other RLC units with 20 mA serial communication capability,
can be connected together in the “loop” if a 24 V external current source is utilized.
A maximum of seven units can be installed in the loop if the Gemini’s 20 mA
current source is used. The units are assigned addresses by setting the Serial DIP
Switches on each unit. The applications can be as simple as attaching a printer to
obtain hard copy of the display information or as involved as using a host computer
to automatically set up Presets and Scale Factors on a number of Geminis.
With the Communications Option, the following functions can be performed:
1. Interrogation of the Count Value, Presets, and Scale Factor.
2. Changing of the Presets and Scale Factor.
3. Resetting of the Count Value and Outputs.
4. Automatic print-out when using a printer and the “Print Request” Terminal.
COMMUNICATION FORMAT
Data is sent by switching off and on the current in the 20 mA current loop. Data
is received by monitoring the switching action and interpreting the codes that are
transmitted. In order for data to be interpreted correctly, there must be identical
formats and Baud Rates.
The format that the Gemini 2000 will accept is: 1 start bit, 7 data bits, 1 odd parity
bit,and1stopbit.TheBaudRatesthatareavailableare:300,600,1200and2400.
The selection of the Baud Rate is done by setting DIP switches. Refer to the
“Current Loop Installation” section, for set-up instructions.
SENDING COMMANDS & DATA TO THE GEMINI 2000
When sending commands to the Gemini 2000, a command string must be
constructed. The command string may consist of command codes, value
identifiers, and numerical data. Following is a list of commands and value
identifiers that are used when communicating with the Gemini 2000.
COMMAND DESCRIPTION
N (4EH) Address command; followed by a one or 2 digit unit address
number 1-15 and one of the following commands.
P (50H) Transmit per Print Options command.
R (52H) Reset command
T (54H) Transmit Value command; followed by a value identifier
(A-D); operates on count value, Presets and Scale Factor.
V (56H) Change Value command; followed by a value identifier (A, B,
D); operates on Scale Factor and Presets.
X (58H) Transmit and reset count command.
VALUE
IDENTIFIER
DESCRIPTION MNEMONIC
A (41H) Preset 1 (PS1)
B (42H) Preset 2 (PS2)
C (43H) Count Value (CNT)
D (44H) Scale Factor (SCF)
–22–
FIG. 1: DATA FORMAT-10 BIT FRAME [300, 600, 1200, 2400 Baud]