Yes. Use one pair for Tx, Rx and a conductor from another pair for the ground reference signal. 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. Many terminals and PCs, however, do rely on hardware handshaking to determine when the other party (in this case the QScreen Controller) is ready to accept data. However, we have found that a large number of devices don't display this information. Each search involves sending a message and waiting for a response or a timeout (if the devices isn't there). After configuring the SPI system to communicate on a properly connected network of devices, sending and receiving data is as simple as writing and reading a register. 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.

So it can't initiate any messages because it never gets the token. A BACnet slave can be read/written but never gets the token. How many of the devices will be BACnet slaves? Token passing and looking for new devices on the MSTP trunk consumes a fair amount of bandwidth. 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. Interoperability of even similar devices from different manufacturers is not assured by compliance with the signal levels alone. It also defines three generator interface points (signal lines); A, B and C. The data is transmitted on A and B. C is a ground reference. Modbus defines client which is an entity which initiates a transaction to request any specific task from its "request receiver". This means the data client doesn't have to poll for data continuously but can wait passively to be notified of the change.

COV stands for Change of Value. When a device supports COV another device/application can subscribe to receive notifications when an object property changes. Can you use BACNets COV mechanism? Practically speaking you can go up to 4000 feet at baud rates up to 76800 baud. For example, at 115k baud your cable should not be much longer than 2500 feet. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). The capacitance is close to 12.5 pF/foot but there is a small impedance mismatch (100 ohms for Cat 5, 120 ohms for RS-485). You can see there are potential problems here. Timeouts cost time. The higher the number of Max Master (default is 127) the more potential devices must be searched for. 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. This reduces the number of messages on a network dramatically.

Modbus ASCII messages are framed by a leading colon (":") and trailing newline (CR/LF). Data rates of up to 10M bps are supported over short distances (40ft.). At the four-thousand foot distance limit, data rates of up to 100K bps are allowable. Differential signals and twisting allows RS485 to communicate over much longer communication distances than achievable with RS232. The RS485 standard is based on 32 devices. Some devices are slower than others.BACnet allows up to 15 msec for a device to use the token. Here are some typical numbers assuming the device doesn't support the multiple services (see below). 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. 6. How do you put more than 32 devices on a single RS485 trunk? 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.

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