ReflectionsWithout a scope and expertise, you won't know this is a factor. The simplest way of knowing if this is a factor Does the device work properly when it is the only device on the network ? The most common cable type is Cat 5e-UTP (unshielded twisted pair) which may work over shorter distances in less demanding applications with low EMI noise levels. How Does a Router Work? However, note that the functions that write to EEPROM disable interrupts for 20 msec. Some devices are slower than others.BACnet allows up to 15 msec for a device to use the token. What happens if one device waits too long after sending its last bit before releasing the line it's possible that the other devices will miss some bits of data. The QED-Forth kernel includes pre-coded drivers that configure the SPI for maximum speed data transfer using a format that is compatible with the on-board D/A and 12 bit A/D.

While the default baud rate of the primary serial port is 9600 baud, you can speed your communications and download times appreciably by switching to a faster baud rate. Timeouts cost time. The higher the number of Max Master (default is 127) the more potential devices must be searched for. When using the default RS485 receivers with an input resistance of 12 kΩ it is possible to connect 32 devices to the network. The RS485 standard allows the user to configure inexpensive local networks and multidrop communications links using twisted pair wire. So take advantage of that and let your systems MCUs implement heart beat handshaking directly though the RS485 communications layer using appropriately designed small packets. 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.

A number of vendors have a range of pull up/down resistors installed and allow you to change the selection using software or jumpers. 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. I think you may be able to add a few more devices and still have a fully operational network depending on the load presented by the terminating resistors. Think of RS485 as the younger, faster brother of RS232. The other devices will think that it is transmitting or receiving and thus blocking the line. When a device wants to send it must grab the line. Max Master determines the biggest address that must be searched for. The status of a device as master or slave determines how the various pins must be configured.

If more than one slave tried to drive the transmit line simultaneously, their serial drivers would fight with each other for control of the bus. A serial bus for Modbus over Serial Line can have a maximum of 247 slaves communicating with 1 master. Henceforth, we shall consider Slave devices to be measuring instruments with serial communication, even if the cabling is similar for all Modbus devices. The GROUND line serves as a common voltage reference for the master and slave. 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. At each end of the trunk install the resistors between the Tx and Rx terminals. If you don't have obvious ends of the trunk (because you created a star) then we recommend re-cabling to form a linear trunk or we wish you luck. The best topology is a single trunk that in-outs on the terminal blocks of each device it connects.

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