DMCP User Guide Page 2 of 9 October 23, 2001
Because of the above statistics, DMCP can be significantly faster than Modbus/TCP, as
demonstrated by the following benchmark data:
Benchmarks DMCP Modbus/TCP
Read 125 words
TCP: 6.27 ms
UDP: 5.21 ms
6.46 ms
Read 512 words
TCP: 12.0 ms
UDP: 10.4 ms
31.2 ms (5 packets)
Read 2048 words
TCP: 35.9 ms
UDP: 42.4 ms (4 packets)
109 ms (17 packets)
Although DMCP is up to three times faster for large packets, it is worth noting that the
majority of transfers for most applications are to the status and command areas, which are
both under 125 words. On those transfers the speed advantage is smallest. These
statistics were taken from a Windows application requesting data from an RMC.
From the above information, the selection criteria can be simplified to the following:
• Choose Modbus/TCP if you want to be able to reuse your code with non-Delta
Ethernet products in the future.
• Choose DMCP if you need the additional performance, especially sending large
amounts of data.
The rest of this document is dedicated to defining DMCP. For more information on
Modbus/TCP, see Schneider Electric’s web site at www.modicon.com.
DMCP Characteristics
DMCP satisfies the above requirements, which restated and slightly appended, comes to
this basic list of characteristics:
• DMCP servers will listen on registered TCP and/or UDP ports 1324. The RMC
listens on both. It also listens on TCP and UDP private ports 50000 because this was
the port used before we had a registered port. This private port is being phased out.
NOTE: Prior to RMC ENET firmware 20001108, the RMC only listened on the
private ports. However, these ports are being phased out, so we
recommend updating to 20001108 or newer firmware rather than using the
private ports.
• All values are binary encoded. Values larger than a byte will be word-aligned and be
in little-Endian (LSB first) format. The exception to this is that the data contents of
reads and writes can be issued in either big- or little-Endian, depending on the
function used.
• No checksum is included in this application layer protocol since it this data is already
checked in the TCP, UDP (optionally), and Ethernet protocols.
• No source and destination node addresses will be added to the protocol, as the source
and destination IP addresses and port numbers are already available in the IP, TCP,
and UDP layers.
(34 pages)
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