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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 output common. Optional 20 mA to

RS232C and 20 mA to RS422 converter modules expand the unit’s flexibility.

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

programming section “Pro 7” to program the serial option.

-27-

DATA FORMAT-10 BIT FRAME [300, 600, 1200, 2400 Baud]

1 2 ... 24 25 26 27 28 29 30 31 32 33 34 ... 47 48

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Apollo Intelligent Meter Series IMY Manuale Utente Pagina 29