5
3.4 SERIAL WIRING
The serial connections are made via terminal block TBE located inside the
unit on the left side for the LD2 and on the right side for the LD4.
RS485 Communications
The RS485 communication standard allows the connection of up to 32 devices
on a single pair of wires, distances up to 4,000 ft. and data rates as high as 19.2K
baud. The same pair of wires is used to both transmit and receive data. RS485
is therefore always half-duplex, that is, data cannot be received and transmitted
simultaneously.
RS232 Communications
RS232 is intended to allow two devices to communicate over distances up to
50 feet. Data Terminal Equipment (DTE) transmits data on the Transmitted Data
(TXD) line and receives data on the Received Data (RXD) line. Data Computer
Equipment (DCE) receives data on the TXD line and transmits data on the RXD
line. The LD emulates a DTE. If the other device connected to the meter also
emulates a DTE, the TXD and RXD lines must be interchanged for
communications to take place. This is known as a null modem connection. Most
printers emulate a DCE device while most computers emulate a DTE device.
Some devices cannot accept more than two or three characters in succession
without a pause in between. In these cases, the meter employs a busy function.
As the meter begins to transmit data, the RXD line (RS232) is monitored to
determine if the receiving device is “busy”. The receiving device asserts that it
is busy by setting the RXD line to a space condition (logic 0). The meter then
suspends transmission until the RXD line is released by the receiving device.
5
LD METER RECEIVING DEVICE
+3.3V
4
47K
47K
3
B (-)
A (+)
COMM.*
Transmit
Enable
* OPTIONAL
Terminal Block Connection Figure (RS485)
1
2
3
Terminal Block Connection Figure (RS232)
TBE
A
COMM
RXD
TXD
4
2
3
1
B
5
232
485
4.0 revIeWIng the Front Buttons and dIsplay
DSP
PAR
F1
F1
RST
MIN
MAX
TOT
1
2
KEY DISPLAY MODE OPERATION PROGRAMMING MODE OPERATION
DSP Index display through max/min/total/input readouts* Quit programming and return to display mode
PAR Access parameter list Store selected parameter and index to next parameter
F1 Function key 1; hold for 3 seconds for Second Function 1** Increment selected parameter value
F2
Function key 2; hold for 3 seconds for Second Function 2** Decrement selected parameter value
RST Reset (Function key)** Hold with F1, F2
to scroll value by x1000
* Display Readout Legends may be locked out in Factory Settings.
** Factory setting for the F1, F2, and RST keys is NO mode.
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