Red Lion GEMINI 42 - Manual (Page 32)

20 MA CURRENT LOOP COMMUNICATIONS
The Gemini 4000’s 20 mA Current Loop Communications Option allows a
“two-way” serial communications link to be established in order to monitor the
count, rate, Presets and Scale Factors from a remote location. Some typical
devices that can be connected with the Gemini are: a printer, terminal,
programmable controller, or host computer. For devices that use RS232, a GCM
Serial Converter Module is available to convert the 20 mA Current Loop signals
to RS232 and vice-versa.
There are two loops that must be established. One for sending commands to
the Gemini and one for receiving the data values from the Gemini. Up to sixteen
Geminis can be connected together in the “loop”. These units are assigned unit
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 count information or
as involved as using a host computer to automatically set up the Presets and Scale
Factors of a number of Gemini units.
With the Serial Loop Option, the following functions can be performed:
1. Interrogation of the Count, Rate, Presets, and Scale Factors.
2. Changing of the Presets and Scale Factors.
3. Resetting of the Count values.
4. Automatic print-out when using a printer and the Print Request Terminal.
5. Change viewed display channel.
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 will accept is: 1 start bit, 7
data bits, 1 odd parity bit, and 1 stop bit. The Baud Rates that are available are:
300, 600, 1200 and 2400.
The selection of the Baud Rate is accomplished by setting DIP switches. Refer
to the Current Loop Installation section, for set-up instructions.
SENDING COMMANDS & DATA TO THE GEMINI
When sending commands to the Gemini, 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 Gemini.
COMMAND DESCRIPTION
N (4EH) Address command; followed by a unit address number 1-15
and one of the following commands.
P (50H) Transmit per Print Options command.
R (52H) Reset command; operates on Count values and output
T (54H) Transmit Value command; operates on Rate or Count values,
Preset and Scale Factors.
V (56H) Change Value command; operates on Scale Factors, and the
Preset.
D (44H) Change Display command; operates on Display values E & F.
VALUE
IDENTIFIER
DESCRIPTION MNEMONIC
A (41H) Preset 1 (PS1)
B (42H) Preset 2 (PS2)
C (43H) Scale Factor A (SFA)
D (44H) Scale Factor B (SFB)
E (45H) Rate or Count A (CTA)
F (46H) Count B (CTB)
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FIG. 1: DATA FORMAT-10 BIT FRAME [300, 600, 1200, 2400 Baud]