Liquid Controls SP2800 - Manual (Page 44)

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SECTION 8 SERIAL COMMUNICATIONS
This section applies to units which have the
Serial Communications interface option. Up
to 15 units can be linked together. Unit sta-
tus can be accessed and new set points and
K Factors can be entered through the serial
port. The unit cannot be taken out of the Run
Mode through the serial port. Menu changes
must always be made through the front key-
pad (except Preset, Prewarn, and K Factors).
Data is transmitted at selected baud rates us-
ing standard seven bit ASCII characters and
parity with two additional bits of "Start" and
"Stop" to make up the standard ten bit charac-
ter.
8-1 Unit Code
Each Batcher in the hookup must be assigned
a code number from 1 to 15, through the front
keypad, in the OUTCARD setup mode (see
Section 3-3, Menu Item 5). Number "00" is
reserved for a dedicated hookup to only one
terminal and its transmit output line remains in
an "on" active state. (Units assigned other
numbers have outputs that remain in the "off"
high impedance state until addressed by their
code number or brought on line by a positive
edge of the Strobe input). Once a unit is ad-
dressed, do not address another unit until the
data has been sent and any data requested
has been transmitted back.
8-2 Baud Rate
The Baud rate is the speed at which data is
transmitted, expressed in bits per second.
Baud rates of 300, 600, 1200, 2400, 4800, or
9600 are available. Use the front keypad to
call up the OUTCARD setup mode (see Sec-
tion 3-3, Menu Item 5) and select the desired
baud rate that is compatible with the remote
terminal.
8-3 Parity
Parity is a bit of information that is in-
serted before the stop bit and is used to
help check if the transmission is correct.
In the OUTCARD setup mode, select be-
tween:
a) ODD (Parity bit is logical zero if total
number of logical 1's in the first seven
bits is odd)
b) EVEN (Parity bit is logical zero if total
number of logical 1's in the first seven
data bits is even.)
c) MARK (Parity data bit is always logical
1.)
d) SPACE (Parity data bit is always logi-
cal 0.).
The Batcher does not check the parity but
does transmit the parity chosen.
Idea: Use the MARK parity for terminals that
need two stop bits and/or no parity
since these terminals ignore the parity
anyway.
Note: If the parity of the terminal is not
known, it is often practical to key in a different
parity until the correct one is found.
8-4 RS232 Electrical Requirements
The Batcher uses standard EIA specifications.
Standard inputs must present a load of 3000
to 7000 ohms. A voltage level of +3 V to +25
V (referenced to signal ground) is read as a
"Space" or "0" and indicates an active state
(asserts a control line). A voltage level of -3
V to -25 V (referenced to signal ground) is
read as a "Mark" or "1" and does not indicate
an active state (does not assert a control line).
Outputs must send a voltage of +5 V to +25
V (referenced to signal ground) for a "Space"
and a voltage level of -5 to -25 V for a "Mark"
when loaded with a 3000 ohm load to signal
ground. Outputs must be capable of being
shorted to other signal lines without burning
out.
Note: It is normally recommended. that cable
length be limited to 50 feet.