User guide

Quantum Ethernet TCP/IP Modbus Application Protocol
80
840 USE 115 00 Version 1.0
C.2 Modbus Application Protocol PDU Analysis
The Modbus Application Protocol PDU is transmitted over a TCP/IP Ethernet stack.
Both Ethernet II and IEEE 802.3 framin
g
will be accommodated. Ethernet II
framin
g
is the default.
. . . from the wire in for IEEE 802.3 framin
g
. . .
. . . is IEEE 802.3 framin
g
if len
g
th <=1500 . . .
802.3_pdu ::= {dst_addr[6], src_addr[6], len
g
th[2], data=802.2_pdu}
*an IEEE 802.3 PDU has a maxFrameSize of 1518 octets
*an IEEE 802.3 PDU has a minFrameSize of 64 octets
802.2_pdu : {dsap[1], ssap[1], frm_cntrl[1], snap_hdr[5], data=ip_pdu}
*the snap_hdr is associated with a “well-known” 802.2 sap snap_hdr
::={or
g
_code[3], ethert
y
pe[2] }
*the snap hdr
(
sub network access protocol
)
allows the older st
y
le
Ethernet protocols to run on the newer IEEE 802.2 interface. The
ethert
y
pe parameter indicates the service, ex. ip or arp. IP has a value
0x800.
. . . from the wire in for Ethernet II framin
g
. . .
. . . is Ethernet II framin
g
if len
g
th >1500 . . .
802.3_pdu ::= {dst_addr[6], src_addr[6], len
g
th[2], data=ip_pdu}
. . . the common part of the packet be
g
ins here . . .
ip_pdu ::= {ip_hdr[20], data=tcp_pdu}
tcp_pdu ::= {tcp_hdr[24], data=appl_pdu=mbap_pdu}
The mbap_pdu is the Modbus Application Protocol whose messa
g
es are received
at a well-known port. The current maximum size of the mbap_pdu for this class of
services in 256 b
y
tes.
The structure and content of the mbap_pdu is defined to be:
mbap_pdu ::={ inv_id[2], proto_id[2], len[2], dst_idx[1], data=mb_pdu }