2 Alphanumeric LCD, and MAX485 UART to RS485 converter IC connected to each end of an Ethernet Cat-6E cable via an RJ45 connector. An "Automatic Converter" (RS232 to RS485) will turn on the RS485 transmitter when data is detected on the RS232 port, and revert back into the receive mode after a character has been sent. Let's say you adopted all the best practices for installation of the network but you get intermittent or unacceptable performance because of packet loss, noise, collisions Then you should consider hiring an expert to resolve your problems because now you are in the Art part of RS485. The primary and secondary serial communications ports are accessible through the QScreen's 10 pin, dual row Serial Header (H5) which is typically not installed, the 24 pin, dual row Field Header (H3), and through the individual DB-9 Serial 1 and Serial 2 connectors. Table 11 5 shows the connection diagram for a standard 9-pin serial cable. The RS485 connections are not brought out to the Serial 1 Connector.

For example, an Automatic RS232 to RS485 converter, could be connected to a computer's RS232, full-duplex port, and transform it into an RS485 half-duplex, multi-drop network at distances up to 4000ft. Converters in most instances, pass data through the interface without changing the timing and/or protocol. For example, at 115k baud your cable should not be much longer than 2500 feet. Practically speaking you can go up to 4000 feet at baud rates up to 76800 baud. We also see that the maximum allowed voltage levels for all interfaces are in the same range, but that the signal level is lower for the faster interfaces.Because of this RS485 and the others can be used in situations with a severe ground level shift of several volts, where at the same time high bit rates are possible because the transition between logical 0 and logical 1 is only a few hundred millivolts.Interesting is, that RS232 is the only interface capable of full duplex communication. RS232 has a separate communication line for transmitting and receiving which-with a well written protocol-allows higher effective data rates at the same bit rate than the other interfaces.

Our advice: For longer networks with lots of devices, choose 38k400 baud over 76k800 baud and optimize using COV, separate networks and by setting the Max Master to a lower number. Do the devices support the Read/Write Property Multiple services or must each property be read in a separate message.Find the answer to this question by reading the BIBs statement for each device or you could explore the device object of the device, find the property called BACnetServicesSupported and then look at the 14th item in the array to see if Read Property Multiple is supported and the 16th for Write Property Multiple. The more you read and the greater the frequency the more bandwidth that will be consumed. Obviously, if you can read a chunk of properties in one message you will be better off than if you can only read a single one. Assume that 50% of your bandwidth will be used by overhead (token, poll for master). Token passing and looking for new devices on the MSTP trunk consumes a fair amount of bandwidth. Some devices are slower than others.BACnet allows up to 15 msec for a device to use the token. The APDUs are application layer message that poll and respond with property values they do work for us as data consumers.

When a device supports COV another device/application can subscribe to receive notifications when an object property changes. There are a variety of ways the MOSI, MISO, SCK and /SS pins on your QScreen Controller can be connected. 24 AWG solid wires, which are typically used in Cat 5 cables, can break easily when connected to a screw terminal block, which does not provide any strain relief. A BACnet slave can be read/written but never gets the token. How many of the devices will be BACnet slaves? The more slaves the fewer token passes. The rest is used to maintain the network passing the token around and looking for new devices. There are nonelectrical considerations to determine how many devices you put on an MSTP network. Up to 32-nodes (drivers and receivers) are allowed on one multi-drop, bi-directional network. Only one active master may control the network at a time; however, the device that assumes the role of master may change according to an appropriate protocol. However, we have found that a large number of devices don't display this information. For devices where you have to consider the devices at the far end of your cables to be unable to be modified, either hardware or software wise, you have to get clever regarding how you determine if the device is connected.

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