Technical data

SED2 Operation & Maintenance Manual
78 Siemens Building Technologies, Inc.
RS-232
Personal computers use the RS-232 standard for interfacing to a peripheral. When fully
implemented, it uses many interconnecting wires and protocols to exchange data. In its most
simple form, it consists of three wires: transmit, Tx; receive, Rx; and ground, GND. It allows
communications between two machines only over a short distance. The Tx line of one machine
connects to the Rx of another, and vice-versa. Voltage levels are typically +/- 12V.
RS-485
The RS-485 standard allows communications between many machines, has a high noise
immunity, and operates over long distances (up to 1000m, 3280 ft). It uses differential voltages,
switching between 0 and 5V. All Siemens drives use RS-485 hardware protocol and some offer
RS-232 interfaces as well.
Typical RS-485 Multi–drop Interface
Troubleshooting with RS-485
The following notes help understand hardware problems that occur with RS-485 systems and
Siemens drives.
• RS-485 is used extensively during the testing of the SED2 in production, and is fully tested
before the drive is shipped.
• Hardware problems with RS-485 are often associated with reversed polarity. It is essential to
connect P+ and N- correctly in all cases.
• Termination resistors are recommended in industrial environments. A value of 120 ohms
between the P+ and N- inputs is recommended, and should mount to the drive farthest from
the controller. Occasionally, additional biasing resistors may be placed between P+ and 24V,
N- and 0V, but these are not usually necessary as the drives include internal biasing
resistors.
• Always test an RS-485 system in the simplest configuration. For example, use a controller
with one drive and use the default address and baud rate.
• Look at the bus with an oscilloscope. The drive will always respond to a valid message. This
means that the drive listens to the bus at all times, and will reply to all messages with the
correct identifier and Cyclic Redundancy Check (CRC). The only exception is the broadcast
message, which none of the drives answers.
• Check the drive address. All drives on a bus must have unique addresses, even if they are in
local control. The drives will always reply to a valid message, even if serial control is not
enabled.
I/O, Point Database, and Parameters
Hardware Inputs and Outputs
For a complete list of SED2 hardware inputs and outputs, see other sections of this manual. The
field panel can access selected I/O to the SED2.