47
MODBUS Mode When this mode is selected, the 1250-LTC will respond to
MODBUS commands via the serial port. This option requires the RS-485 (-M) or RS-232 (-S)
hardware option be installed. To select this mode use the menu command OP 51, RS232
command to choose mode “6”. (See Section 2.3, page 18 for programming instructions.)
Remote Display Driver Mode This mode must be used when the 1250-LTC is
connected to an INCON model RD4 Remote Display. It causes the 1250-LTC’s RS-232
output to transmit the proper protocol and timing for the RD4 to mimic what is on the
1250-LTC’s display. To select this mode use the menu command OP 51, RS232 command to
choose mode “7”.
3.4 Serial RS-485 Multi-Drop Option “-M”
The Serial RS-485 option on the Model 1250-LTC can be used to program the
instrument and to retrieve position data from the instrument, very much like the RS-232
option. It is a half-duplex configuration.
MODBUS is the most likely protocol to use for a 1250-LTC equipped with RS-485,
but the 1250-LTC with RS-485 will operate in all of the same serial modes listed above in
Section 3.3. Certain restrictions apply due to the nature of the half-duplex configuration: The
device communicating to the 1250-LTC must be able to turn off its transmitter very quickly
so that the 1250-LTC can respond over the same transmission lines. The Remote Display
Driver mode is not useful, since the model RD-4 will only accept an RS-232 signal.
Table 3.8 Wiring: Digital Connector Pin-Out
DB-25
Pin#
1250-LTC
Pin#
RS-232
Function
RS-485
Function
1 1 Chassis Gnd. Chassis Gnd.
14 2
2 3 Transmit (O) Data A(I/O) +
15 4
3 5 Receive (I) Data B(I/O) –
16 6
4 7 RTS (O) Not Used
17 8
5 9 CTS (I) Not Used
18 10
6 11
19 12
7 13 Signal Gnd. Signal Gnd.
20 14
8 15
21 16
9 17
22 18
10 19
23 20
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