Networking the MAP450B Mini-Terminal
Technical Note 1011A
Applicable Model(s)
MAP450B
Title
Networking the MAP450B Mini-Terminal
Date
10/30/2023
Rev
01
P/N
0907-1011A
Summary
Control systems often require operator input from a number of different locations in the system.
Since it is not cost effective to run a separate communications line from the host to each and
every terminal location, some sort of local network is desirable when multiple terminals are
needed. The MAP450 Mini-Terminal can be configured so that it can be connected to a multidrop
RS-485 network.
Network Formats
There are several different forms of communications networks. The main formats are the star, ring, and multidrop (see Figure 1). Each has its own set of advantages and disadvantages. Since the MAP450 only supports the multidrop format of network, this is the one that will be further discussed.
MAP450 Multidrop Network
The MAP450 network consists of a host or master controller (located at one of the network ends) connected to multiple MAP450s via a single, continuous 5-wire data link. Each MAP450 is connected to the network by locally tapping into the data link with short stub cables. Since the network is controlled by a single host controller, each end of the data link must be properly terminated to reduce noise pickup and interference. The total length of the data link, from the host to the end terminator, can be up to 5,000 feet long. The number of MAP450s that can be connected to the network is 100 or less, dependent upon the transceiver loading, data rate, and cable capacitance. Network repeaters can be added to the data link if needed, allowing additional MAP450s to be connected for each repeater used (up to a maximum of 255 units).
Communications Wiring
The 5-wire network data link consists of 2 sets of twisted pairs and a signal ground. One of the twisted pair is connected from the hosts transmit lines to all of the MAP450s receive lines. The other twisted pair has all of the MAP450s transmit lines connected together and going into the hosts receive lines. The single wire is used as a communications signal ground.
For short networks (under 1,000 feet), low-capacitance 28-gage wire such as BELDEN #9805 or BELDEN #8133 can be used. For longer networks (over 1,000 feet) 24-gage wire such as BELDEN #9843 or BELDEN #8333 should be used. Each of these Belden cables consists of 3 sets of twisted pairs, so the signal ground is connected to one of the twisted pairs instead of a single wire.
Network Termination
The network data link must be properly terminated to reduce the amount of interference and noise pickup from the surrounding environment. This is accomplished by using a termination circuit at both ends of the network cable. Figure 4 shows a typical termination circuit. The host termination shown is only needed if the host’s RS-485 adapter does not contain any built-in terminator circuitry. The end termination must be done at the extreme end of the network cable. If the network cable ends at the last installed MAP450 terminal, the end termination can be connected using that MAP450’s terminal block.
Terminal Connection
Each of the MAP450s are connected to the network data link using the terminal blocks of the internal TB2 connector. If the MAP450 is panel mounted, the network cable is brought directly to the terminal block as shown in Figure 5 below. In this instance, each of the network wires are cut, stripped, and reconnected using the terminal block screws to capture each cable lead. If the MAP450 is pedestal mounted, a short twisted pair stub cable is spliced into the network wiring and then run to the MAP450’s terminal block through the enclosure’s cable nut. The cable shields must be connected together to maintain shield continuity.
Network Grounding
To communicate properly, serious attention must be paid to the grounding scheme of any device connected to the network data link. Improper grounding, termination, and faulty shielding of the data link is one of the most common causes of system failure in a multidrop network. For proper operation, the following general rules must be observed.
1.
The cable shield must not be used as the signal ground.
It is tempting to try and reduce the cost of the cabling by using a 4 wire cable with the shield used as the signal ground, DON’T DO IT. The initial cost savings are always exceeded by the maintenance costs once the system is operating under field conditions. Often, it is necessary to completely replace the network data link with the proper cable (5 wire plus shield) to eliminate noise problems in the system.
2.
The cable shield should be connected to the chassis ground only at one point. This should be
at the host connection for the best noise immunity.
The cable shield is often connected to the chassis ground at both ends of the cable connection. Since this makes an earth ground to earth ground connection between the two devices, it can cause what is called a “ground loop” problem. Since the earth grounds are probably at different voltage potentials, a large current (often several amps in magnitude) will flow between the earth ground points. The shield usually has a much lower impedance to the current flow than the normal earth ground current path. This can result in the cable shield carrying large amounts of unexpected current. For proper shielding to be accomplished, all of the cable shields should be connected together in a contiguous whole, and then grounded only at a single point (at the host’s chassis ground).
3.
The signal ground must not be connected to the chassis or earth ground.
The chassis ground is intended as a safety ground for power supplies, EMI filters, voltage spike protection circuits, 120 VAC neutral returns, and all manner of AC and DC driven devices. As a result, the chassis ground can carry large voltage potentials and currents. Connecting the signal ground to the chassis ground can damage the device connected to the network data link.
4.
Other Considerations
The most common error when installing the network is running power lines (especially 110 VAC) in the same wire channel as the network data link. This is often done to reduce the cost of the wire conduit. Including a 110 VAC, 60 hz line in the wire channel is sure to cause loss of communications on a regular basis. If the AC line is powering relays, motors, or even fluorescent lighting, the high voltage spikes and noise WILL BE induced on the network data link, destroying any or all attached network devices. 208 VAC three phase power is even worse, since the voltage spikes and noise is of greater magnitude and intensity.
Short runs of low voltage DC power can normally be included in a communications wiring channel or cable. This allows the pedestal mounted MAP450 to use a single stub cable to connect to the data link and its local power supply. However, DC power should not be run down the whole length of the network to power every device. Small clusters of MAP450s (two or three terminals) that are physically adjacent may be powered from a single supply if necessary.