20 mA CURRENT LOOP SERIAL COMMUNICATIONS (Optional)
GENERAL DESCRIPTION
The serial communication option is a half-duplex, two-way, 20 mA loop
that can connect to a variety of printers, computers, terminals and controllers
to suit many data-polling or automatic operation applications. The indicator
responds to a host of commands, including change alarm value, reset totalizer
and transmit input value. Two loops are required for all hook-ups; a transmit
(out-going data)
loop and a receive
(in-coming data)
loop. Since the indicator
monitors the receive loop for a busy signal
(current interrupted)
while
transmitting, the receive loop must be connected even if the indicator is
transmitting only, such as to a printer. A built-in 20 mA source can be used in
the transmit loop (only) by connecting the current return wire to -20 mA
SRC., instead of SO+. To bypass the built-in current source, make transmit
loop connections to SO+ and SO-. Additionally, multiple units and other Red
Lion Controls instruments can be serially addressed, up to a maximum of 99
units.
(The actual number in a single loop is limited by the Serial Hardware
Specifications).
To eliminate problems with ground loops, the serial circuitry
is isolated from both signal common and earth common. Optional 20 mA to
RS232C and 20 mA to RS422 converter modules expand the unit’s flexibility.
The IMS has a built-in feature that can be used to test the internal Serial
Communications hardware. This is accomplished by performing a simple
“loop-back” test, which verifies that the serial input, serial output, and 20 mA
source are all functioning properly. Refer to the
“Programming Module #9”
section for the procedure.
Note: When operating the unit with a printer, the receive loop of the indicator
must have current flowing into it before transmission can take place.
COMMUNICATION FORMAT
Data is sent by switching current on and off in the loop and is received by
monitoring the switching action and interpreting the codes that are transmitted.
In order for data to be correctly interpreted, there must be identical formats and
baud rates among the communicating equipment. The only format available with
this indicator is 1 start bit, 7 data bits, 1 odd parity bit and 1 stop bit. The baud
rates are programmable and the choices are: 300, 600, 1200 and 2400.
Before serial communication can take place, the indicator must be
programmed to the same baud rate as the connected equipment. In addition,
the loop address number, print options and full or abbreviated transmission
must be programmed. If only one indicator is to be used, then a loop address
number of “0” may by used to eliminate the requirement for the address
specifier when sending a command. If more than one indicator is on the loop,
assignment of unique addresses, other than zero, for each indicator is
recommended. Valid addresses of 0 to 99 may be assigned, but the built-in
current source, if used, is capable of driving up to 7 units. Additional drive
capability may be afforded by an external current source with a higher
compliance voltage. Refer to the “Programming Module #7” section to
program the serial option.
-29-
DATA FORMAT-10 BIT FRAME [300, 600, 1200, 2400 Baud]