24
Remote Display Driver Mode This mode must be used when the 1250B is connected to a
Model RD4 Remote Display. It causes the 1250B’s RS-232 output to transmit the proper protocol
and timing for the RD4 to mimic what is on the 1250B’s display. To select this mode, use the
menu command
OP 51, RS232
command to choose mode
“7”
.
The communication port settings: baud rate, word length, parity, stop bits, and address are
programmable using the
OP 80, Port, PORT
command. (See Table 2.1 and 2.2 for
command protocol and choices.)
NOTE: When the port is programmed for 2 Stop Bits, the Parity must be “NONE”.
Table 3.2 Wiring: Digital Communication Connector Pin-Out
1250B
Pin#
DB-9
Pin#
RS-232
Function
RS-485 Comm
Port Adapter Pin #
RS-485
Function
3
2
Transmit (O)
5
Data A(I/O)
+
5
3
Receive (I)
4
Data B(I/O)
-
7
N/C
RTS (Jumped to CTS)
3
Not Used
9
N/C
CTS (Jumped to RTS)
2
Not Used
13
5
Signal Gnd.
1
Signal Gnd.
3.5 Serial RS-485 Multi-Drop Option “-M”
The Serial RS-485 (
Half-Duplex
) option on the Model 1250B can be used to program
the instrument and to retrieve position data from the instrument. The MODBUS protocol is a
master/slave packet based protocol with the 1250B operating as a RTU slave. The MODBUS
function commands recognized by the 1250B are “
3
” (read multiple registers) and “
16
” (write
multiple registers). By supporting these two commands the 1250B is in level 0 compliance.
Using these two commands it is possible to configure the 1250B as well as monitor it for
current position. MODBUS RTU command and response packets are formatted as follows:
3.5.1 RS-485 Packet Format - Read
Reading from Holding Registers:
GAP
= A gap in transmission of 3.5 character frames indicates to the slaves that a new packet
is to follow. No transmission gaps within a packet may exceed 1.5 character frames.
Byte 1
= Device Address: Address 0 is a broadcast address that all units respond to regardless
of programmed address. All other addresses can be programmed and used in this mode.
Byte 2
= Function Code: When reading holding registers, this byte is “
03h
”