As the Master Transmits its Data

Most computers conform to IBM PC AT-compatible RS232 interfaces which use 9-pin D-Type connectors, consequently the QVGA Controller brings out its serial ports to two female 9-pin D-Type connectors. MODBUS over Serial Line has two transmission modes RTU and ASCII which are corresponded to two versions of the protocol, known as Modbus RTU and Modbus ASCII. Modbus supports communication to and from multiple devices connected to the same cable or Ethernet network. It must not be allowed to float at any voltage level so devices have pull up/down resistors to pull the line to an allowable floating voltage. A number of vendors have a range of pull up/down resistors installed and allow you to change the selection using software or jumpers. Most vendors implement their devices as masters (ie token passing devices). Since most messages on a MSTP network are token passes a device that uses the token in 5 msec will consume much less bandwidth than one that takes 15msec. (A number of vendors relax this requirement to allow for other vendors implementations. Some devices are slower than others.BACnet allows up to 15 msec for a device to use the token. How many of the devices will be BACnet slaves?

70 as a best case and 100 as a worst case (obviously reading descriptions will take more) multiply by the number of objects and properties you are going to poll on a regular basis. However, the higher the baud rate the more sensitive the cable is to the quality of installation issues like how much twisted pair is unwound at each termination start to become very very important. Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. This has been done to avoid reflections of signals. While handshaking is still with us, it is usually best to disable it in software (if possible) and/or "loop-back" the pairs of signals (RTS to CTS, DTR to DSR, etc.) While RS232 was rumored to be on the "way out" with the advent of many of the new communications standards, it is still alive and well today. The simplest way of knowing if this is a factor Does the device work properly when it is the only device on the network ? When you install it in the full network other devices or this device stops working properly. 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.

When a device supports COV another device/application can subscribe to receive notifications when an object property changes. COV stands for Change of Value. Can you use BACNets COV mechanism? Another major advantage is that there can be more than one slave in the connection. Based on information from the Wikipedia page you posted, it looks like there are a limited number of communication standards transmitted over 485, with one being pointed out as the most frequently used. RS485 is made for serial communication with high speeds, support for long distances, and support for multiple devices (slaves). Its not possible to provide a calculator to work out how many devices to install on a single network but the following list provides some help in assessing bandwidth considerations. The load presented by other devices on the network affects this floating so the resistor values may need to be changed depending on the number of devices installed and the values of the pull up/down resistors they are using. If the vendor did a poor job, the default is to have the resistor active and they must be disabled unless they are the terminating devices on the network. The rest is used to maintain the network passing the token around and looking for new devices.

This reduces the number of messages on a network dramatically. So it can't initiate any messages because it never gets the token. A token is 8 bytes as is a Poll for master. It takes approx 30 bytes to poll for a single property. The APDUs are application layer message that poll and respond with property values they do work for us as data consumers. The primary channel’s UART translates the bit-by-bit data on the serial cable into bytes of data that can be interpreted by the QED-Forth Kernel or by your application program. It takes about 40 bytes to reply. Divide the baud rate by 10 to get bytes per seconds. 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.

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