Distributed Systems and Computer Networks. Simple analog and digital data transmission technologies Comparison of Fieldbuses and Local Area Networks

Transcription

1 Distributed Systems and Computer Networks Simple analog and digital data transmission technologies Comparison of Fieldbuses and Local Area Networks

2 Simple Analog Technologies Instantaneous voltage or current value is used to carry information Voltage transmission instantaneous output voltage value linearily depends on transmitted signal value typical ranges 0 10 V, -5 V + 5 V, 0 24 V Analog current loop instantaneous current value depends on transmitted signal value typical ranges 0 20 ma, 4 20 ma second type allows for loop break detection and possibly the power supply of node Analog transmission technologies are susceptible to external disturbances but widely used (especially the current loop)

3 EIA/TIA 232 (RS 232) probably most widely used point-to-point technology synchronous and asynchronous communication many different versions most often used version available at IBM PC compatible computers originally designed to interconnect terminal and modem for data communication over arbitrary channel (e.g. PSTN) data terminal is called DTE (Data Terminal Equipment) modem is called DCE (Data Circuit-terminating Equipment)

4 EIA/TIA 232 (RS 232) DTE1 TALK / DATA TALK RS CS TR RD TD CD PSTN TALK / DATA TALK RS CS TR RD TD CD DTE2 DCE1 DCE2 Data transmission circuit can be based on different technologies radio-modems GSM network cable modems

5 EIA/TIA 232 (RS 232) SG DTR TD RD DCD RI CTS RTS DSR přijímá receives DTE vysílá transmits DTE DTE particular interface signals have the same name on both types of devices it means e.g. TD signal is transmitted by the DTE and received by the DCE Be careful!!! Often not followed!!!

6 EIA/TIA 232 (RS 232) TD (Transmitted Data) data transmitted by the DTE RD (Received Data) data received by DTE SG (Signal Ground) signal ground RTS (Request To Send) DTE ready to receive CTS (Clear To Send) DCE ready to receive DTR (DTE Ready) DTE ready DST (DCE Ready) DCE ready DCD (Data Carrier Detect) carrier received indication new name is RLSD (Received Line Signal Detect) RI (Ring Indicator) indication of ringing

7 EIA/TIA 232 (RS 232) often some signals are not used, especially in case of direct connection of two DTE devices e.g. null modem interconnection of DTR on one side and DSR + DCD on other side informs that devices are ready and connection is available RTS and CTS signals are used for data flow control

8 EIA/TIA 232 (RS 232) often some signals are not used, especially in case of direct connection of two DTE devices e.g. three-wire cable only the data signals are used HW data flow control is not possible (SW data flow control using X ON and X OFF characters is still possible)

9 EIA/TIA 232 (RS 232) unbalanced, single-ended duplex channel inverse logic is used -3 V -25 V voltage range represents log V +25 V voltage range represents log. 0 maximum recommended communication speed is 20 kbit/s PC implementation provides up to 115,2 kbit/s other implementations support even higher rates maximum recommended cable length is 15 m actual possible length depends on: communication speed influence of external environment (level of electromagnetic disturbances)

10 EIA/TIA 423 (RS 423) compared to EIA/TIA 232 standard it defines only physical layer pameters unbalanced line inverse logic -3 V -6 V voltage range represents log V +6 V voltage range represents log. 0 single transmitter can drive up to 10 receivers point to point or multi-drop configurations maximum recommended transmission speed is 100 kbit/s depends on cable length and level of external disturbances maximum recommended cable length is 1200 m

11 EIA/TIA 422 (RS 422) balanced (differential) transmission line -2 V -6 V voltage difference represents log V +6 V voltage difference represents log. 0 highly immune to common mode disturbance intended for point to point or multi-drop configurations maximum recommended communication speed 10 Mbit/s depends on cable length, its quality and level of external disturbances maximum recommended cable length is 1200 m 120 line impedance correctly terminated duplex channel is often built using two EIA422 pairs often used in industrial data acquisition systems

12 EIA/TIA 485 (RS 485) very similar to EIA/TIA 422 standard besides point to point and multi-drop configurations a multipoint (bus arrangement) configuration is possible (tristate driver output) balanced (differential) transmission line -2 V -6 V voltage difference represents log V +6 V represents log. 0 highly immune to common mode disturbances maximum recommended communication speed 10 Mbit/s depends on cable length, its quality and level of external disturbances maximum recommended cable length is 1200 m 120 line impedance terminated on ends

13 EIA/TIA 485 (RS 485) both the half and full duplex channels are possible often used in industrial applications driver can drive up to 32 so called unit loads source of many industrial standards limits in number of nodes within a single network segment I i 1 ma -7V -3V 5V -0,8 ma 12V U AB

14 Cable Length (m) Simple Digital Technologies Comparison of selected technologies cable 24AWG, 50 pf/m EIA-423 EIA-422 EIA EIA Data Rate (kb/s)

15 Comparison of Fieldbuses and Local Areal Networks Fieldbus designed for industrial environment industrial automation and data acquisition optimized to transfer small data blocks short data link layer frames usually low speed (tens up to hundreds kbit/s) low overhead error recovery implemented in data link layer in HW fast LAN designed for office environment for resource sharing applications optimized to transfer large data blocks long data link layer frames usually high speed (hundreds of Mbit/s up to Gbit/s) high overhead especially to transfer small pieces of data error recovery implemented in higher layers In SW slow

16 Comparison of Fieldbuses and Local Areal Networks Fieldbus mostly deterministic MAC algorithm required for real-time applications provides defined and limited data latency and response times often power supply over the data cable some can be used in explosive environment some can pass the certification for safety related systems LAN most widely used standard (Ethernet) uses CSMA/CD no limit for data delivery no defined maximum latencies and response times there are possibilities how to overcome this issue power supply over data cable also possible for some LANs high spread of LANs and low cost network elements support penetration to application that where reserved for fieldbuses in past