Exception. Moxa MGate MB3180/MB3280/MB3480 Series, MGate MB3170/MB3270 Series

Exception

The master sends a request to the slave. The slave may not support the command or an error is detected, so it sends an exception to the master.

Broadcast

The master sends a broadcast command, such as a reset command. Every slave on the network complies with the command, and no response is sent to the master.

Requests Need a Time Limit

The original Modbus protocol was not designed for simultaneous requests or simultaneous masters, so only one request on the network can be handled at a time. When a master sends a request to a slave, no other communication may be initiated until after the slave responds. The Modbus protocol specifies that masters use a response time-out function to identify when a slave is nonresponsive due to device or line failure. This function allows a master to give up on a request if no response is received within a certain amount of time.

This is illustrated as follows:

Response Time-Out

The master sends a request. The slave is unresponsive for the amount of time specified by the response time-out function. The master gives up on the request and resumes operation, allowing another request to be initiated.

To allow for a wide range of devices, baudrates, and line conditions, actual response time-out values are left open for manufacturers to determine. This allows the Modbus protocol to accommodate a wide range of devices and systems. However, this also makes it difficult for system integrators to know what response time-out value to use during configuration, especially with older or proprietary devices.

The MGate MB3000 provides a patent-protected function that tests all attached devices and recommends a response time-out value. This function saves considerable time and effort for system integrators, and it results in more accurate time-out settings.

MGate MB3000 Modbus Gateway User Manual 87

Modbus Ethernet vs. Modbus Serial

Although Modbus is intended as an application-layer messaging protocol, the data format and communication rules for Ethernet-based Modbus TCP are different from serial-based Modbus ASCII and RTU.

The major difference between the Ethernet and serial Modbus protocols is the behavior of the communication model. Modbus ASCII and RTU allow only one request on the network at a time. Once a request is sent, no other communication on the bus is allowed until the slave sends a response, or until the request times out. However, Modbus TCP allows simultaneous requests on the network, from multiple masters to multiple slaves. TCP masters cannot send more than one request at a time to a slave, but they can send requests to other slaves before a response is received. The Modbus TCP standard recommends that slaves be able to queue up to 16 requests at a time. The MGate MB3000 will queue up to 32 requests from each TCP master, for up to 16 or 32 TCP masters.

Integrate Modbus Serial and Ethernet with

Gateways

Ordinarily, Modbus TCP and Modbus ASCII/RTU are unable to communicate with each other. However, with a Modbus gateway in between the Modbus serial network and the Modbus Ethernet network, TCP masters are able to communicate with serial slaves, and serial masters are able to communicate with TCP slaves.

MGate MB3000 Modbus Gateway User Manual 88

B.

SNMP Agents with MIB II and RS-232-

Like Groups

The MGate MB3000 Series has built-in Simple Network Management Protocol (SNMP) agent software that supports SNMP Trap, RFC1317 and RS-232-like groups, and RFC 1213 MIB-II.

RFC1213 MIB-II Supported SNMP Variables

System MIB sysDescr sysObjectID sysUpTime sysContact sysName sysLocation sysServices

Interfaces MIB ifNumber ifIndex ifDescr ifType ifMtu ifSpeed ifPhysAddress ifAdminStatus ifOperStatus ifLastChange ifInOctets ifInUcastPkts ifInNUcastPkts ifInDiscards ifInErrors ifInUnknownProtos ifOutOctets ifOutUcastPkts ifOutNUcastPkts ifOutDiscards ifOutErrors ifOutQLen ifSpecific

IP MIB ipForwarding ipDefaultTTL ipInReceives ipInHdrErrors ipInAddrErrors ipForwDatagrams ipInUnknownProtos ipInDiscards ipInDelivers ipOutRequests ipOutDiscards ipOutNoRoutes ipReasmTimeout ipReasmReqds ipReasmOKs ipReasmFails ipFragOKs ipFragFails ipFragCreates ipAdEntAddr ipAdEntIfIndex ipAdEntNetMask ipAdEntBcastAddr ipAdEntReasmMaxSize ipRouteDest ipRouteIfIndex ipRouteMetric1 ipRouteMetric2 ipRouteMetric3 ipRouteMetric4 ipRouteNextHop ipRouteType ipRouteProto ipRouteAge ipRouteMask ipRouteMetric5 ipRouteInfo ipNetToMediaIfIndex ipNetToMediaPhysAddress ipNetToMediaNetAddress ipNetToMediaType ipRoutingDiscards

ICMP MIB icmpInMsgs icmpInErrors icmpInDestUnreachs icmpInTimeExcds icmpInParmProbs icmpInSrcQuenchs icmpInRedirects icmpInEchos icmpInEchoReps icmpInTimestamps icmpTimestampReps icmpInAddrMasks icmpInAddrMaskReps icmpOutMsgs icmpOutErrors icmpOutDestUnreachs icmpOutTimeExcds icmpOutParmProbs icmpOutSrcQuenchs icmpOutRedirects icmpOutEchos icmpOutEchoReps icmpOutTimestamps icmpOutTimestampReps icmpOutAddrMasks icmpOutAddrMaskReps

89 MGate MB3000 Modbus Gateway User Manual

Address

Translation MIB atIfIndex atPhysAddress atNetAddress

TCP MIB tcpRtoAlgorithm tcpRtoMin tcpRtoMax tcpMaxConn tcpActiveOpens tcpPassiveOpens tcpAttemptFails tcpEstabResets tcpCurrEstab tcpInSegs tcpOutSegs tcpRetransSegs tcpConnState tcpConnLocalAddress tcpConnLocalPort tcpConnRemAddress tcpConnRemPort tcpInErrs tcpOutRsts

UDP MIB udpInDatagrams udpNoPorts udpInErrors udpOutDatagrams udpLocalAddress udpLocalPort

RFC1317 RS-232-Like Groups

RS-232 MIB rs232Number rs232PortIndex rs232PortType rs232PortInSigNumber rs232PortOutSigNumber rs232PortInSpeed rs232PortOutSpeed

Async Port MIB rs232AsyncPortIndex rs232AsyncPortBits rs232AsyncPortStopBits rs232AsyncPortParity

Input Signal MIB rs232InSigPortIndex rs232InSigName rs232InSigState

Output Signal MIB rs232OutSigPortIndex rs232OutSigName rs232OutSigState

SNMP MIB snmpInPkts snmpOutPkts snmpInBadVersions snmpInBadCommunityNames snmpInBadCommunityUses snmpInASNParseErrs snmpInTooBigs snmpInNoSuchNames snmpInBadValues snmpInReadOnlys snmpInGenErrs snmpInTotalReqVars snmpInTotalSetVars snmpInGetRequests snmpInGetNexts snmpInSetRequests snmpInGetResponses snmpInTraps snmpOutTooBigs snmpOutNoSuchNames snmpOutBadValues snmpOutGenErrs snmpOutGetRequests snmpOutGetNexts snmpOutSetRequests snmpOutGetResponses snmpOutTraps snmpEnableAuthenTraps snmpSilentDrops snmpProxyDrops

MGate MB3000 Modbus Gateway User Manual 90