STS 1656 - RS232 Serial Interface Operation; RS485 Serial Interface Operation; Formatted Packet of Data for RS485 Interface (ASCII Code); Table 8-3: RS485 Communication Packet Format

1656 & 1657 Battery Element Tester Owners’ Manual
STS Instruments Page 72 of 88
8.8 RS232 Serial Interface Operation
Operation over RS232 serial interface is similar to using USB but a serial port on the controller or
computer is required. Note that most PC’s no longer offer a standard COM port. In that case, it
is suggested to use the USB interface instead.
For serial operation, a straight through DB9 to DB9 male-female cable is needed to connect the
1656/1657 to a COM port. If the controller has a 25 pin RS232 connector, a DB9 to DB25
adapter is required.
Baud rate, start bits, data bits and stop bits setting on the PC or controller and the 1656/1657
must match in order for the serial interface to work. All command strings send must be
terminated by a LF or CR. The command syntax is the same SPCI syntax as for all other interface
types.
8.9 RS485 Serial Interface Operation
In contrast to RS232, RS485 is a multi-drop serial interface which means that multiple devices
can be connected to the same controller. The signal levels differ as well so do not attempt to use
an RS232 COM port on a PC to control the 1656/1657 over its RS485 interface.
Since RS485 is a multi-drop interface, device addressing is required so all commands must have
an address. It also means the 1656/1657 must have its RS486 device address set in the
INTERFACE SETUP menu in the UTILITIES menu.
Data sent over RS485 connections is packetized as an eight byte packet and includes a checksum
as its last byte to allow the receiving device to verify correct receipt of the packet content. This
requires more a correct CRC to be calculated and appended to every packet sent.
Formatted packet
8.9.1 Formatted Packet of data for RS485 interface (ASCII Code):
The required packet format for RS485 communication is as follows:
Order
Type
Content
data(0)=2
(RS485 Address)
data(1)=1
(Communication code)
data(2)=5
(Length of command, example
“*IDN?”=5)
data(3)=42
(*)
Command ASCII char
data(4)=73
(I)
Command ASCII char
data(5)=68
(D)
Command ASCII char
data(6)=78
(N)
Command ASCII char
data(7)=63
(?)
Command ASCII char
data(8)=180
(checksum of byte array of data(0) –
data(7))
Checksum
Table 8-3: RS485 Communication Packet Format