Liquid Controls SP2900 - Manual (Page 17)

RS422 ELECTRICAL REQUIREMENTS
The input of the unit follows the standard E1A high imped-
ance minimum of 12K Ohms. When the 422 + (A) input is
more positive than the 422 - (B) input by .2V to 6V, a “1” or
“Mark” condition is recognized. When the 422+ input .2V to
6V, a “0” or “Space” is recognized. Data is recognized by
the popularity of the voltage difference between the two
lines. Noise picked up on the line will make little difference
since the noise is usually added to each line, and the
voltage differential remains the same. The output driver
drives the transmit lines to a differential of 2 to 6V. It is
designed to handle loads up to 60mA of sink or source
current and features positive and negative current limiting
for protection from line fault conditions. Since the RS422 is
more immune to noise, cable links up to 1000 feet or more
can be used. Because of the high input impedance of
RS422, line terminating loads are recommended. For hook
up to a single unit a 150 to 200 Ohm resistor across
Receive Data + or - at the unit and at the remote terminal is
often sufficient. For multiple hook-up, other standard
terminations should be used. Total loading should not be
greater than 90 Ohms.
RS232/RS422 SERIAL INPUT CODES
DXX(S) (Device and address number followed
by space) activates the unit that has
been assigned that number. That unit
comes on line and transmits “Device
XX:”. Unit is now ready to receive a code
or string of codes separated by a space.
A “Carriage Return” (Enter) code enters
the codes and processing of requests
begins.
CODES
DC Will transmit count.
DR Will transmit rate.
DT Will transmit grand total.
KC Will transmit counter K-Factor.
KC(S)XXX Will load counter K-Factor
number.
KR Will transmit rate K-Factor.
KR(S)XXX Will load rate K-Factor number.
PA Will transmit Preset A.
PA(S)XXX Will load preset A number.
PB Will transmit Preset B.
PB(S)XXX Will load Preset B number.
RC Will reset counter to zero if in
“RO” mode (adding) or set
counter to Preset A if in “SP”
mode (subtracting). Output is
reset.
RC(S)XXX Will set counter to number (no
other change is made).
RT Will reset grand total to zero.
RT(S)XXX Will reset grand total to number.
SERIAL INTERFACE OPERATION
Data is received and transmitted over standard EIA RS232
or RS422 levels. Each 10 bit character is made up of a start
bit, 7 bit ASCII code, parity bit and stop bit. Unit number,
baud rate and parity are entered in the “Outcard” set up
mode and remain in memory even if power is off.
Note that the input impedance of RS232 is 3K or 7K Ohm
worst case. The terminal addressing the unit must be
capable of driving all loads in the loop. RS422 input imped-
ance is much higher and there is usually no problem driving
15 units. Unit serial transmit line remains in a high imped-
ance “OFF” state until addressed. Insure that only one unit
is addressed at a time.
To address unit, transmit a “D” (device) followed by the 1 to
15 code number and a “Space”. Once the “Space” has
been received, the unit becomes active and responds back,
“Device XX:” (Device number). (Once active, the unit works
in a full duplex, echo back mode, so that data sent from the
terminal will be transmitted back for verification). Once the
unit is “on line”, use the proper serial transmit codes to
request data or set a new value. (See RS232/RS422 Serial
Input Codes). Up to 80 characters of data may be linked
together and transmitted to the unit in a string as long as
there is a space between the different codes. If an error is
made, a correction can be made by back spacing and
retyping correct data before the “Carriage Return” (Enter) is
sent, the unit starts processing the data and will transmit
the requested data on a non-priority basis over the data
transmit line. A unit keypad entry or incoming data will halt
the data communication cycle. Therefore, there should be a
pause after data is requested to insure that all data has
been transmitted before another unit is addressed and
brought on line. (If the unit is not busy, It should not require
more than 5 msec to process each request. To find the
cycle time to process and transmit a request, calculate the
bit transmit time by dividing 1 by the baud rate; multiply by
the number of requests made. Example: Typical time to
transmit 1 uninterrupted request at 300 baud rate is .272
sec. (1-300) x (80) + .005.
This time will be extended if the unit must service the front
keypad or one of the inputs. In practice if transmission has
not started within 2 seconds after data is requested, It can
be assumed that there is a problem).
When transmitting, the unit will precede each data value
with a “Carriage Return” and “Line Feed” code and answer
only with requested data in the order the requests were
made. After all requested data has been transmitted any
new communication must be started again by DXX (Device
number) and space.
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