What Is an M12 B-Coded Connector?

An M12 B-coded connector is the M12 coding reserved for PROFIBUS fieldbus, with a key on the opposite side to the standard A coding. It usually has 5 pins and is defined in IEC 61076-2-101, the same standard as the A, C and D codings.

B coding exists to keep the fieldbus separate. PROFIBUS devices on a machine sit alongside sensors that use A-coded M12, and the cables look much the same. Moving the key means a fieldbus cable can never be screwed onto a sensor port, or a sensor cable onto the bus, during installation or maintenance.

PROFIBUS itself is a fieldbus standardised in IEC 61158 and IEC 61784. Its DP version runs over an RS-485 physical layer at up to 12 Mbit/s, linking PLCs to drives, remote I/O and valve islands.

B coding marks the DP version specifically. PROFIBUS PA, the process automation version used on instruments in chemical and water plants, runs at 31.25 kbit/s over a two-wire bus that also powers the devices, and it commonly uses A-coded M12 instead. A site with both versions therefore has two connector codings for what is nominally one fieldbus, and the coding is a quick way to tell which segment a cable belongs to.

The PROFIBUS guidelines assign the five pins of a B-coded connector as shown. The data lines use the standard PROFIBUS cable colours, so a technician can check a termination by eye.

Assignments for pins 1 and 3 vary between device makers, so check the datasheet before relying on them to power a terminating resistor.

PROFIBUS DP cable is a shielded twisted pair with a characteristic impedance of 150 ohm, usually with a violet outer sheath. The higher the data rate, the shorter each segment can be:

Each PROFIBUS segment needs an active terminator at both ends. With M12, that is usually a B-coded terminating plug screwed onto the last device, rather than a switch on a 9-pin D-sub connector. T-pieces and Y-splitters keep the bus continuous when a device is removed, so the rest of the line keeps running during a swap.

One caution from our own catalogue: a few rugged computers use a B-coded socket for their DC power input, because the key stops a sensor cable being connected by mistake. Never assume a B-coded socket on a computer is a PROFIBUS port; read the datasheet first. Our overview of M12 codings shows how the other codings are used.

PROFIBUS is tolerant of noise when it is wired well and unforgiving when it is not. Most faults on an M12 segment trace back to a handful of causes, and checking them in order saves time on site.

First, confirm there are exactly two active terminators, one at each end, and none in the middle. A missing terminator causes reflections at higher data rates; an extra one loads the line and shrinks the signal. Second, check the A and B lines are not crossed at any connector, which is easy to do on a field-wired plug. Third, check the shield is bonded at every device and that the cable is genuine 150 ohm PROFIBUS cable rather than general-purpose twisted pair. Finally, check the segment length against the data rate. Dropping the bus from 12 Mbit/s to 1.5 Mbit/s often makes an over-long segment work while a repeater is fitted.

Many plants now move from PROFIBUS to PROFINET, which runs over Ethernet on D-coded or X-coded M12 instead. B-coded connectors remain common on existing lines and wherever fieldbus devices are extended or replaced, and a mixed site often runs both for years. An edge computer that collects data from such a line usually connects to the PROFINET side over Ethernet and reaches PROFIBUS devices through the PLC or a gateway.

Our IP67 computers use M12 Ethernet for that connection, and our guide to M12 waterproof connectors explains why sealed connectors suit the machine side.

We integrate edge computers with the Ethernet, serial and fieldbus gateway connections a line needs as standard, and we check the coding of every port against your cabling before the system ships.