Termination; Number Of Allowed Devices On The Rs-485; Slew Rate; Isolated (Or Common) Ground - ACL CSC400 Installation And Operation Manual

Some PC software (or other Masters) will work with Line Polarization off, while others may need the non-inverting signal to
be driven high during idle times on the RS485 bus. For example, the PC software Modnet for Modbus RTU will work with
Line Polarization off but it shows an extra "0x00" byte received at the beginning and end of a Modbus packet. However, the
Modbus Reader PC software shows a Frame Error received by the CSC400 Controller if no Line Polarization is turned on.

Termination

This type of termination refers to bus termination between the pairs, not the termination resistors used for Line Polarization.
This termination connects signal "Data A +" to "Data B –" through a 120 ohm resistor.
An RS-485 bus should only be terminated at each end of the cable (at each device at the end of the cable). No other devices in-
between the two devices at each end should have termination resistors installed or enabled.
The CSC400 Controller has a 4-pin DIP switch with the third switch from the top labeled "120ohm term". This can be used to
connect a built-in 120 ohm resistor. Simply push the third DIP switch to the right and the 120ohm termination resistor will be
connected.

Number of Allowed Devices on the RS-485

The number of devices allowed on an RS-485 bus depends on a variety of factors: the total length of the wire, the wire gauge,
the signaling characteristics or the "Unit Load" of each device on the bus (receiver input impedance, capacitance).
The CSC400 Controller uses newer RS485 transceivers with advanced fail-safe features. Due to these newer transceivers, the
theoretical maximum number of devices allowed on the bus is 256 because the receiver's input impedance is 96kohm which is
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1/8 the input impedance of older transceivers at 12kohm (1/8 of a "Unit Load"). The Modbus specification limits this
theoretical maximum further to 247 devices allowed on an RS-485 bus.
Any Modbus system allows a minimum of 32 devices on the RS-485 bus without use of a repeater. More devices may be
allowed depending on the characteristics of all devices on the RS-485 bus.
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The CSC400 Controller allows more than 32 devices to be present on the RS-485 bus due to each transceiver occupying 1/8
of a Unit Load on the bus. Since each installation is different, with different cable lengths and the potential for other devices to
be present on the bus, the user needs to test out the maximum number of devices that can be placed on each RS-485 bus.

Slew Rate

The CSC400 Controller incorporates RS-485 transceivers with slew rate limited drivers. Slew rate refers to the speed at which
a signal changes state from a 0 (Low) to a 1 (High) or from a High to a Low state. Slew rate limited drivers slow down the rise
and fall times of a signal which help with reducing signal reflections, reducing EMI emissions, and possibly allowing a bus to
work without termination resistors.
Unfortunately, with slower rise and fall times, the maximum communication speed (or baud rate) is reduced. The drivers on
the CSC400 Controller can operate at a maximum rate of 115kbps but the maximum setting allowed in the CSC400 firmware is
38.4kbps (38400 baud, or raw bits per second).

Isolated (or Common) Ground

The "Isolated Ground" terminal on each CSC400 Controller is isolated from the onboard CSC400 ground. This isolated ground
connection should be used to connect all common ground connections on all RS-485 devices on the bus. This common ground
should be connected to earth or protective ground at one end of the RS-485 cable only (preferably), usually at the master
device.
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