Serial Communication; Universal Serial Bus; Eia(Rs)485 Bus - GE P24DM Technical Manual

P24xM
3

SERIAL COMMUNICATION

The physical layer standards that are used for serial communications for SCADA purposes are:
Universal Serial Bus (USB)
●
EIA(RS)485 (often abbreviated to RS485)
●
● K-Bus (a proprietary customization of RS485)
USB is a relatively new standard, which replaces EIA(RS232) for local communication with the IED (for transferring
settings and downloading firmware updates)
RS485 is similar to RS232 but for longer distances and it allows daisy-chaining and multi-dropping of IEDs.
K-Bus is a proprietary protocol quite similar to RS485, but it cannot be mixed on the same link as RS485. Unlike
RS485, K-Bus signals applied across two terminals are not polarised.
It is important to note that these are not data protocols. They only describe the physical characteristics required
for two devices to communicate with each other.
For a description of the K-Bus standard see
reference R6509.
A full description of the RS485 is available in the published standard.
3.1

UNIVERSAL SERIAL BUS

The USB port is used for connecting computers locally for the purposes of transferring settings, measurements
and records to/from the computer to the IED and to download firmware updates from a local computer to the IED.
3.2

EIA(RS)485 BUS

The RS485 two-wire connection provides a half-duplex, fully isolated serial connection to the IED. The connection is
polarized but there is no agreed definition of which terminal is which. If the master is unable to communicate with
the product, and the communication parameters match, then it is possible that the two-wire connection is
reversed.
The RS485 bus must be terminated at each end with 120 Ω 0.5 W terminating resistors between the signal wires.
The RS485 standard requires that each device be directly connected to the actual bus. Stubs and tees are
forbidden. Loop bus and Star topologies are not part of the RS485 standard and are also forbidden.
Two-core screened twisted pair cable should be used. The final cable specification is dependent on the application,
although a multi-strand 0.5 mm
is important to avoid circulating currents, which can cause noise and interference, especially when the cable runs
between buildings. For this reason, the screen should be continuous and connected to ground at one end only,
normally at the master connection point.
The RS485 signal is a differential signal and there is no signal ground connection. If a signal ground connection is
present in the bus cable then it must be ignored. At no stage should this be connected to the cable's screen or to
the product's chassis. This is for both safety and noise reasons.
It may be necessary to bias the signal wires to prevent jabber. Jabber occurs when the signal level has an
indeterminate state because the bus is not being actively driven. This can occur when all the slaves are in receive
mode and the master is slow to turn from receive mode to transmit mode. This may be because the master is
waiting in receive mode, in a high impedance state, until it has something to transmit. Jabber causes the receiving
device(s) to miss the first bits of the first character in the packet, which results in the slave rejecting the message
and consequently not responding. Symptoms of this are; poor response times (due to retries), increasing message
error counts, erratic communications, and in the worst case, complete failure to communicate.
P24xM-TM-EN-2.1
K-Bus (on page308) and General Electric's K-Bus interface guide
2
per core is normally adequate. The total cable length must not exceed 1000 m. It
Chapter 16 - Communications
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