Abstract: After having analyzed the RS-485 main line and CAN hesitates similarities and differences not definitely, take the PCA82C250 interface circuit as the example proposed substitutes in the RS-485 bus interface with the CAN main line correspondence physics level interface circuit the electric circuit and the monolithic integrated circuit direct connection carries on the communication network design, thus may form a high performance, the low price and the data communication security, the reliable distributional observation system.
Key word: Correspondence physical interface RS-485 main line CAN main line MCU
1 outline
Uses in many machine data communications the correspondence physics level connection is the distributional observation system data sharing work foundation, the tradition mostly uses the electric current link or the RS-485/RS-422 main line take the monolithic integrated circuit as the core distributional many machine observation system as the simplified correspondence physics level.
Because electric current ring-like -like compares the RS-485/RS-422 main line form segment complex and so on reasons to withdraw from the historical arena basically. But the RS-485 main line wiring form compared the RS-422 main line wiring form to be short the diode order wire and the antijamming ability strengthens, thus made the wiring form to be simpler, the construction cost to be lower, therefore, the RS-485 main line has basically lorded over take the monolithic integrated circuit as the core distributional many machine observation system communication physics level application. But along with the technical development, the RS-485 main line’s main line efficiency is low, system timeliness is bad, the communication reliability is low, the later period maintains the cost to be high, the network project debugging is complex, the transmitting range ideal, the single main line cannot hang meet the node to be few, the application not nimble and so on shortcomings expose slowly, therefore, urgent needs to seek for one kind new, the simple effective correspondence physics level connection chip to substitute the RS-485 main line physics level interface circuit to carry on the network service, this to enhances many machine interconnections the distributional observation system’s reliability to have the great significance.
Compares with other field bus, CAN hesitates not definitely in aspects and so on traffic capacity, reliability, timeliness, flexibility, usability, transmitting range and cost has the obvious superiority, becomes the control and so on one of domain most promising field buses. Regarding the CAN main line’s physical level connection, existing mostly is the CAN main line physics level interface circuit and CAN the bus control unit connection constitution CAN main line communication network. Proved after author’s thorough analysis and the practice: The CAN main line physics level interface circuit (conforms to the ISO11898 standard) also to be possible to constitute one high with the monolithic integrated circuit direct connection reliable, the low cost, simple practical, many machine interconnection distributional observation systems.
2 CAN and the RS-485 physics level characteristic compares
The CAN main line has the specific interface electric circuit in physical level, this kind of interface circuit has the characteristic.
The CAN main line and the RS-485 main line physics level characteristic same spot includes:
· two-wire system, half-duplex serial communication;
· difference transmission, balanced receive;
· the transmission medium is a twisted pair line;
· needs the reflexless terminal resistor;
· the communication circuit may work under the 5V power source condition.
The CAN main line and the RS-485 main line compare, the CAN main line correspondence physics level interface circuit (take PCA82C250 as example) has the following merit:
· conforms to the ISO11898 international standard completely;
· the data transmission is away from long (is 10km/5kb/s);
· the data transfer rate high (reaches as high as 1Mb/s/40m);
· In the CAN main line’s main line value is two supplementary logic “the dominance” or “recessiveness” the figure value;
· may realize the multi-host -like communication network design, the signal passes “the line on the main line with” to be possible to realize the non-destructive bus arbitration;
· does not have the transmission, the receive switching control pin;
· has the anti-instantaneous pulse interference (- 150V<V pulse < 100V) main line protection ability;
· has compares the RS-485 main line high main line voltage (- 8V~ 18V) bearing capacity;
· has the transmission pulse slope control, may reduce the radio-frequency interference;
· the difference receiver may resist the wide scope the syntype disturbance;
· has between the main line and the power source and the place short circuit protection;
· has the anti-main line short circuit protection ability;
· still might protect the main line normal work under the specific single track condition;
· has the low electric current to wait for an opportunity the pattern;
· the main line interior has the resistor network, exterior does not need on, the lower position resistor;
· on the electricity node has not had the influence to the main line;
· may connect 110 nodes at least.
Thus it can be seen, uses the CAN main line’s physical level specific interface electric circuit to substitute the RS-485 bus interface electric circuit to form a mix pattern many machine interconnection distributional observation system’s communication network, may overcome the RS-485 main line’s inherent flaw, uses the CAN main line physics level fully the superiority, can by the simple form, the low price, the high performance construct has the competitive power extremely the distributional observation system.
Figure 2
3 PCA82C250 synopses
Conforms to ISO11898 the international standard CAN main line physics level specific interface electric circuit to have many kinds of completely, here only as the example does take CAN main line general interface circuit PCA82C250 to this kind of connection chip explained. PCA82C250 pin chart as shown in Figure 1. Various functions pin is as follows:
1 foot: Wants to transmit the data the input end;
2 feet: The power source carries;
3 feet: Power source end;
4 feet: Receive data out-port;
5 feet: Reference voltage out-port;
6 feet: Low level CAN main line input/out-port;
7 feet: High level CAN main line input/out-port;
8 feet: Main line pulse slope control resistance coupling end.
PCA82C250 may provide to the main line data differential motion transmission ability and to the correspondence main line data differential motion receptivity. Its pin 8 are more special, this pin uses in the selecting circuit own working; High speed, the slope controls and waits for an opportunity. When this foot earths, PCA82C250 work in high-speed service way; After meeting a certain resistance number the resistor, earths again, uses in controlling the transmission data pulse the rise and the drop slope (the slope in proportion to pin 8 on magnitudes of current), with reduces the radio-frequency interference; When this foot meets the high level, the electric circuit enters the low electric current readiness for action. Under this way, the transmitter is shut down, the receiver transfers to the low electric current work, but the receiver still might “the dominance” the position make to the CAN main line on.
If PCA82C250 is in the correspondence main line’s network termination, needs to add 120Ω about on the main line the matched resistance.
4 application examples
Take Atmal at89C55 monolithic integrated circuit as the example, AT89C55 and the RS-485 bus interface electric circuit and AT89C55 and CAN main line physics level specific interface electric circuit’s contrast connection diagram as shown in Figure 2.
Contrast may see by Figure in 2, PCA82C250 and at89C55 hardware connection must be simpler than MAX485 and at89C55 hardware connection, because, the PCA82C250 course of communications does not need to receive and the transmission hardware switching control, only controls by the software when meets floats, the CAN main line performance is “recessiveness” the figure value, namely CANH and CANL are the suspended state (VCAHN≈CANL≈VCC/2, is equal in closes main line), this to have “the dormancy” the function system to provide the network security safeguard; When TXD end input for low level, the CAN main line performance is “the dominance” the figure value (to bus transfer valid data position), namely CANH output high voltage (approximately 3.5V, when VCC is 5V), CANL outputs the low level (approximately 1.5V, when Vcc is 5V). Obviously, under the multi-main engine condition, “the dominance” the position and “recessiveness” the position introduction, may realize the non-destructive bus arbitration on the main line, which main equipment adjudicates to be next holds main line’s equipment. As a result of does not have usefully to the PCA82C250 reference voltage value of exports, therefore, the PCA82C250 5 feet may be hanging, but 8 feet meet resistance RS uses in controls the CAN main line’s output pulse rise, the drop along the slope, reduces main line’s radio-frequency interference. When on the RS resistance is bigger than 0.75CC, the PCA82C250 chip enters the low power loss readiness for action; When on the RS voltage is smaller than 0.3Vcc, PCA82C250 enters the high-speed service condition; When on the RS voltage is in 0.4Vcc to 0.6Vcc, PCA82C250 enters the CAN bus out pulse rise, the drop along the slope control correspondence condition, on its slope size and the RS voltage is proportional.
In Figure 2, two communications system’s software is nearly same. When uses PCA82C250 achievement bus interface substitution original MAX485, makes the change on the software to include: First, may cancel the RS-485 main line’s correspondence directional control command section, because the CA7402097N bus interface has not needed this function; Next, RS-485 main line when the main line transmits, because transmits, the receive control end to connect in together, namely automatic shut-off main line data receive function, but the CAN bus interface also in carries on the main line data while main line data transmission’s the receive (CAN bus interface not to provide correspondence receive, transmission data separation control function), therefore, should have the consideration regarding this in the software design. Certainly, this has provided the condition for many machine communications system’s in main line data conflict’s software recognition and the arbitration.
When needs MCU and between the communication network electrical isolation, may increases 2 electro-optics in MCU and between the CAN main line’s physical level specific interface electric circuit to isolate the component (for example 6N137 electro-optic isolating circuit), then realizes MCU and between the communication network electrical isolation.
5 conclusions
Proved after the practical application system’s examination that uses the CAN main line’s physical level specific interface electric circuit (for example PCA82C250 and so on) to substitute the RS-485 main line specific interface electric circuit to form a mix pattern many machine interconnection distributional observation system’s communication network, may overcome the RS-485 main line’s inherent flaw to a great extent, moreover only makes the little revision on the software, even does not revise the original RS-485 main line’s correspondence software to be able to adapt the new system work. When necessity then realizes the multi-host -like many machine data communications through the revision original RS-485 main line’s correspondence software, has used the CAN main line physics level superiority fully. In the hardware aspect, can by the simple form, the low price, the high performance construct has the competitive power extremely the distributional observation system, enables many machine interconnections the distributional observation system’s communication network performance to promote, to guarantee under the bad operating mode condition communications system’s security, the reliable work.
