Tất cả những nhà sản xuất cung cấp các đơn vị thiết kế cho truyền thông kỹ thuật số nối tiếp. Phần lớn các ứng dụng sẽ được tìm thấy yêu cầu tốc độ và hiệu quả truyền dữ liệu nối tiếp. Vì lý do này, các nhà sản xuất cung cấp các dữ liệu nối tiếp truyền thông như là một phần của hệ thống cơ sở của nó. Mỗi của các hệ thống chìa khóa trao tay tiêu chuẩn kết hợp truyền thông đa điểm RS-485 được thiết kế cho giao diện máy tính trực tiếp. Thực tế. | CHAPTER 19 Serial Communications All of the manufacturers provide units designed for digital serial communications. The great majority of applications will be found to require the speed and efficiency of serial data transfer. For this reason each of the manufacturers provides serial data communications as an integral part of its base system. Each of the standard turnkey systems incorporate RS-485 multidrop communications designed for direct computer interface. This fact alone should be enough to convince those still wed to 4 to 20 mA interfaces of the inefficiencies of such an approach. If it were cheaper or easier to follow that path at least some of the manufacturers would be doing so. A. INTERFACE All of the units provide a standard RS-485 interface. This allows for multiple units to be connected to a single-twisted shielded-pair cable. The number of counters that can be connected to the system depends on the number of instrument addresses available and the limitations of the data collection software. All of the manufacturers allow for up to 32 units. More units may be added to each system but will have to be placed on a separate RS-485 network. ARTI Chemtrac Met One and IBR provide RS-232 access to the particle counter. This allows individual parameters to be set without connecting to the RS485 network. Only one unit may be connected at a time on an RS-232 network. Chemtrac provides a separate connector while Met One and IBR require moving a couple of jumpers to switch from RS-485 to RS-232. RS-485 communications can be conducted over two- or four-wire cabling. Two-wire communications involves transmitting and receiving data over the same pair of wires while the four-wire arrangement keeps transmission and receiving lines separate. There is no difference in speed as the units do not transmit and receive at the same time. ARTI Met One and Chemtrac are designed to operate on a two-wire arrangement although the Chemtrac unit may also be operated in four-wire 163 .
