It is important to note that when the CPHA bit is 0, the /SS line must be de-asserted and re-asserted between each successive data byte exchange (68HC11 Reference Manual, Section 8.3.2). If the CPHA bit is 1, the /SS line may be tied low between successive transfers. This signal synchronizes the exchange of bytes between the QScreen and its peripherals. Chassis and signal grounds are connected together to the digital ground (DGND) signal. A typical RS485 network can operate properly in the presence of reasonable ground differential voltages, withstand driver contentious situations, provide reliable communications in electrically noisy environments (good common mode rejection using twisted pair cable, shielding provides additional protection), and support thirty-two or more (many IC manufacturers have 1/2, 1/4, 1/8 unit load devices) drivers and receivers on the line. Modbus is restricted to addressing 247 devices on one data link, which limits the number of field devices that may be connected to a parent station (again, what is rs485 cable Ethernet TCP/IP is an exception).

Unit identifier is used with Modbus TCP devices that are composites of several Modbus devices, e.g. Modbus TCP to Modbus RTU gateways. To ensure that no two devices drive the network at the same time, it is necessary that each slave device be able to disable it’s own RS-485 data transmitter. In general, all devices on a network should use the same phase, polarity, and baud rate clock signal. For higher data rates, Modbus RTU recommends to use the fixed values 750 μs for t1.5 and 1.750 ms for t3.5. A Modbus RTU message must be transmitted continuously without inter-character hesitations. Modbus TCP nomenclature is the same as for the Modbus over Serial line protocol, as any device which send out a Modbus command, is the 'client' and the response comes from a 'server'. Modbus over UDP - some have experimented with using Modbus over UDP on IP networks, which removes the overhead of TCP. In fact, the program works the same as it did before, but now it is using the secondary serial port instead of the primary port -- and you didn’t even have to recompile the code! The QScreen Controller has two serial communications ports: a primary serial port called Serial 1 that supports both RS232 and RS485 protocols, and a secondary serial port called Serial 2 that supports RS232.

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. The DMX standard is very strict: Each pair is unidirectional (like RS422), it uses longer mark and space times between datablocks than specified in computer Serial communication. The historical data serves to meet an American Petroleum Institute (API) industry standard for how data should be stored. LRC is specified for use only as a checksum: because it is calculated on the encoded data rather than the transmitted characters, its 'longitudinal' characteristic is not available for use with parity bits to locate single-bit errors. To ensure frame integrity during the transmission, the time interval between two frames must be at least the transmission time of 3.5 characters, and the time interval between two consecutive characters must be no more than the transmission time of 1.5 characters. For example, a value of 122 (7A16) is encoded as two ASCII characters, "7" and "A", and transmitted as two bytes, 55 (3716, ASCII value for "7") and 65 (4116, ASCII value for "A").

LRC is calculated as the sum of 8-bit values (excluding the start and end characters), negated (two's complement) and encoded as an 8-bit value. Address, Function, Data, and LRC are ASCII hexadecimal encoded values, whereby 8-bit values (0-255) are encoded as two human-readable ASCII characters from the ranges 0-9 and A-F. Modbus ASCII messages are framed by a leading colon (":") and trailing newline (CR/LF). Pemex Modbus - an extension of standard Modbus with support for historical and flow data. Data types are mapped using standard addresses. Do play around with transfer speed on your own and give a try to other ethernet cable types too. Since Modbus was designed in the late 1970s to communicate to programmable logic controllers, the number of data types is limited to those understood by PLCs at the time. Modbus over TCP/IP, Modbus over TCP, or Modbus RTU/IP - a variant that differs from Modbus TCP in that a checksum is included in the payload, as with Modbus RTU.