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It is specifically developed for industrialmulti-drop systems that should be able to transmit and receive data athigh rates or over long distances.The specifications of the EIA RS-485 protocol are as follows:• Maximum line length per segment: 1200 meters (4000 feet)• Throughput of 10 Mbaud and beyond -Differential transmission(balanced lines) with high resistance against noise• Maximum 32 nodes per segment• Bi-directional master-slave communication over a single set oftwisted-pair cables• Parallel connected nodes, true multi-dropADAM-5000 systems are fully isolated and use just a single set oftwisted pair wires to send and receive! Since the nodes are connectedin parallel they can be freely disconnected from the host withoutaffecting the functioning of the remaining nodes. An industry standard, shielded twisted pair is preferable due to the high noise ratio ofthe environment.When nodes communicate through the network, no sending conflictscan occur since a simple command/response sequence is used.
Thereis always one initiator (with no address) and many slaves (withaddresses). In this case, the master is a personal computer that isconnected with its serial, RS-232, port to an ADAM RS-232/RS-485converter. The slaves are the ADAM-5000 systems. When systems arenot transmitting data, they are in listen mode. The host computerinitiates a command/response sequence with one of the systems.Commands normally contain the address of the module the host wantsto communicate with. The system with the matching address carriesout the command and sends its response to the host.C-2ADAM-5000Appendix CC.1Basic Network LayoutMulti-drop RS-485 implies that there are two main wires in a segment.The connected systems tap from these two lines with so called dropcables.
Thus all connections are parallel and connecting or disconnecting of a node doesn’t affect the network as a whole. SinceADAM-5000 systems use the RS-485 standard and an ASCII-basedcommands set, they can connect and communicate with all ASCIIbased computers and terminals. The basic layouts that can be used foran RS-485 network are:DaisychainThe last module of a segment is a repeater. It is directly connected tothe main-wires thereby ending the first segment and starting the nextsegment.
Up to 32 addressable systems can be daisychained . Thislimitation is a physical one. When using more systems per segment theIC driver current rapidly decreases, causing communication errors. Intotal, the network can hold up to 256 addressable systems. Thelimitation on this number is the two-character hexadecimal addresscode that can address 256 combinations. The ADAM converter,ADAM repeaters and the host computer are non addressable unitsand therefore are not included in these numbers.Setgment 1Setgment 2CoverterRepeaterRS232/RS4851RS-485RS-23212N1Figure C-1 DaisychainingADAM-5000C-3RS-485 NetworkStar LayoutIn this scheme the repeaters are connected to drop-down cables fromthe main wires of the first segment.
A tree structure is the result. Thisscheme is not recommended when using long lines since it will cause aserious amount of signal distortion due to signal reflections in severalline-endings.Figure C-2 Star structureC-4ADAM-5000Appendix CRandomThis is a combination of daisychain and hierarchical structure.Figure C-3 Random structureADAM-5000C-5RS-485 NetworkCombination of an ADAM-4000 and an ADAM-5000 in aRS-485 NetworkThe following figure shows how to integrate ADAM-4000 andADAM-5000 systems in a network.Figure C-4 ADAM-4000 and ADAM-5000 in a networkNote:C.2The speed of ADAM-4000 and ADAM-5000 in a RS485 network should be the same.Line TerminationEach discontinuity in impedance causes reflections and distortion.When a impedance discontinuity occurs in the transmission line theimmediate effect is signal reflection.
This will lead to signal distortion.Specially at line ends this mismatch causes problems. To eliminate thisdiscontinuity, terminate the line with a resistor.C-6ADAM-5000Appendix CFigure C-5 Signal distortionThe value of the resistor should be a close as possible to the characteristic impedance of the line. Although receiver devices add someresistance to the whole of the transmission line, normally it is sufficientto the resistor impedance should equal the characteristic impedance ofthe line.Example:Each input of the receivers has a nominal input impedance of 18 kΩfeeding into a diode transistor- resistor biasing network that isequivalent to an 18 kΩ input resistor tied to a common mode voltage of2.4 V.
It is this configuration which provides the large common range ofthe receiver required for RS-485 systems! (See Figure C-6 below).ADAM-5000C-7RS-485 NetworkFigure C-6 Termination resistor locationsBecause each input is biased to 2.4 V, the nominal common modevoltage of balanced RS-485 systems, the 18 kΩ on the input can betaken as being in series across the input of each individual receiver.If thirty of these receivers are put closely together at the end of thetransmission line, they will tend to react as thirty 36kΩ resistors inparallel with the termination resistor. The overall effective resistancewill need to be close to the characteristics of the line. The effectiveparallel receiver resistance RP will therefore be equal to:RP = 36 x 103/30 = 1200 ΩWhile the termination receptor RT will equal:RT = RO / [1 - RO/RP]Thus for a line with a characteristic impedance of 100 Ω resistorRT = 100/[1 - 100/1200] = 110 ΩSince this value lies within 10% of the line characteristic impedance.Thus as already stated above the line termination resistor RT willnormally equal the characteristic impedance Zo.C-8ADAM-5000Appendix CThe star connection causes a multitude of these discontinuities sincethere are several transmission lines and is therefore not recommend.Note:C.3The recommend method wiring method, that causesa minimum amount of reflection, is daisy chainingwhere all receivers tapped from one transmission lineneeds only to be terminated twice.RS-485 Data Flow ControlThe RS-485 standard uses a single pair of wires to send and receivedata.
This line sharing requires some method to control the direction ofthe data flow. RTS (Request To Send) and CTS (Clear To Send) are themost commonly used methods.Figure C-7 RS-485 data flow control with RTSIntelligent RS-485 ControlADAM-4510 and ADAM-4520 are both equipped with an I/O circuitwhich can automatically sense the direction of the data flow. Nohandshaking with the host (like RTS, Request to Send) is necessary toreceive data and forward it in the correct direction. You can use anysoftware written for half-duplex RS-232 with an ADAM networkwithout modification.
The RS-485 control is completely transparent tothe user.ADAM-5000C-9RS-485 NetworkThis page intentionally left blankC-10ADAM-5000DHow to Use theChecksum FeatureHow to Use the Checksum FeatureA checksum helps you to detect errors in commands from the host tothe modules, and in responses from the modules to the host. Thefeature adds two extra checksum characters to the command orresponse string, which does reduce the throughput.D.1Checksum Enable/DisableTo enable configuration of a module’s checksum feature, its INIT*terminal should be shorted to its GND terminal, after which the moduleshould be rebooted. The checksum feature is enabled by setting bit 6of the data format/checksum parameter to 1.
To disable the checksum,set the parameter to 0. Remember that when using the checksumfeature, it should always be enabled for all connected devices including the host computer.The checksum is represented by a 2-character ASCII hexadecimalformat and is transmitted just prior to the carriage return. The checksum equals the modulo-256 (100h) sum of all the ASCII values in thecommand preceding the checksum. If the checksum in a command ismissing or incorrect the module will not respond.Example 1The following is an example of an Analog Data In command andresponse when the checksum is enabled:Command: #05S10C(cr)Response: +3.56719D(cr)The input value at the module in slot 1 of the ADAM-5000 systems ataddress 05h is +3.5671 V. (The date format is engineering units.) Thecommand checksum (0Ch) is the sum of the ASCII values of thefollowing characters: #, 0, 5, S and 1.
The response checksum (9Dh) isthe sum of the ASCII values of the following characters: “>” “+” “3”“4” “5” “6” “7” and “1”D-2ADAM-5000Appendix DExample 2This example explains how to calculate the checksum value of a ReadHigh alarm limit command string:Case 1. (If the Checksum feature is disabled)Command: $07S1RH(cr)Response: !07+2.0500(cr) when the command is valid.Case 2.
(If the Checksum feature is enabled)Command: $07S1RHA9(cr)Response: !07+2.0500D8(cr)where:A9 represents the checksum of this command, and<R>D8 representsthe checksum of the response.The checksum of the command string is derived as shown below:A9h = (24h+ 30h + 37h + 53h + 31h + 52h + 48h) MOD 100hThe hexadecimal ASCII codes for $, 0, 7, S, 1, R and H are 24h, 30h,37h, 53h, 31h, 52h and 48h respectively.
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