26
EM1500 Specifics
9-Wire Option (SER3 only)
The 9-wire option is only available at TTL levels.
To configure SER3 as a 9-wire DTE go to the AUX I/O tab in the configuration program. Check the
checkbox, “SER3 uses PF0-3.” (This option is also available on the Modem/SER3 tab.) JP2 and JP3 must
be jumpered for TTL levels if you want to configure SER3 as a 9-wire DTE.
NOTE:
It is possible to use SER3 in 9-wire mode with “pseudo-RS232” signal levels. See
Modem/SER3 panel, check “RS232 levels.” Drive levels are 0-3 V TTL, which is below
specification, but may work for short connections.
4.3.4 Serial Port 5 (RS485)
The EM1500 comes with an RS-485 port. Its connector is located on the front of the unit underneath the
Ethernet connector. Compared to RS-232, RS-485 supports higher speeds (up to 250 kbps), longer dis-
tances, and may be used in a multidrop configuration.
The EM1500 can be used in an RS-485 multidrop network spanning up to 1200 m (4000 ft), and there can
be as many as 32 attached devices. Connect the 485+ to 485+ and 485– to 485– using single twisted-pair
wires as shown in the figure below. Note that a common signal ground is recommended.
Figure 4.6 Multidrop Network
For best performance in a multidrop network, termination resistors are enabled only on the end nodes and
are disabled on intervening nodes. The EM1500 termination resistors are enabled by default. See
Appendix A.4, “EM1500 Jumpers,”
for information on how to disable them.
The RS485 port has some unique properties compared with the other four serial ports. The main difference
is that it is half duplex i.e. only one direction, transmit or receive, can be active at one time. Another differ-
ence is that there are no modem control lines such as CTS, RTS or DCD. The last difference is that RS485
supports multi-dropping i.e. more than one device using the RS485 cable as a “party line.” This is also
known as “daisy chaining.”
Because of these differences, the RS485 port has special considerations when configuring it.
The normal flow control disciplines associated with RS232 serial ports (i.e., XON/XOFF or modem hand-
shake lines) do not apply. Instead, the RS485 port requires a transmitter enable discipline. In general, there
must be one device or node that is configured to be a “master,” with every other device connected to the
RS485 cable configured as a “slave.”
485+
GND
485-
485+
GND
485-
485+
GND
485-
Summary of Contents for EM1500
Page 14: ...10 www rabbit com Introduction...
Page 22: ...18 www rabbit com Getting Started...
Page 76: ...72 www rabbit com EM1500 Configuration...
Page 90: ...86 www rabbit com EM1500 Specifications...
Page 104: ...100 www rabbit com Serial and TCP Protocols...
Page 118: ...114 www rabbit com EM1500 FAQ...