IEC 104 vs IEC 101: differences and when to use each

IEC 60870-5-101 and IEC 60870-5-104 are two transport profiles of the same telecontrol standard. They share the application layer: the same ASDUs, type IDs, causes of transmission and quality bits. What differs is how the ASDUs travel between control centre and outstation.

What are the main differences?

How does IEC 101 work on a serial line?

IEC 101 uses the FT1.2 frame format: 8 data bits, even parity and 1 stop bit per character, a checksum per frame, and three frame types (fixed length starting with 0x10, variable length starting with 0x68 and repeating the length, and the single character 0xE5 used as a short acknowledgement). The link layer runs in one of two modes:

• Unbalanced: the master polls one or more outstations (multi-drop is possible). Outstations only answer; the master fetches class 1 data (high priority, typically events) and class 2 data (low priority, typically cyclic values) with request frames.
• Balanced: point-to-point, both stations may start a transmission. Spontaneous events are sent as they happen, which gives shorter reaction times; it is typically used on full-duplex point-to-point links.

Field sizes are project settings in IEC 101: link address 0, 1 or 2 octets, COT 1 or 2 octets, common address 1 or 2 octets and IOA 1, 2 or 3 octets. Both ends must be configured identically, which is the most frequent cause of "no data" on a new 101 link.

How does IEC 104 work on TCP/IP?

IEC 104 puts each ASDU behind a 6-octet APCI header on a TCP connection, normally to port 2404 of the outstation. The APCI has three formats: I-frames carry ASDUs and send/receive sequence numbers, S-frames only acknowledge, and U-frames control the link (STARTDT, STOPDT and TESTFR). Flow control uses the k and w windows and four timers; see the timer guide. Field sizes are fixed: COT 2 octets, common address 2 octets, IOA 3 octets. There is no link address, because TCP/IP handles addressing.

An outstation can accept several TCP connections. In a redundant setup, one connection is active (after STARTDT) and the others wait in standby until the master switches over.

Do both support time tags?

IEC 101 supports both the 3-octet CP24Time2a (minutes, seconds, milliseconds) and the 7-octet CP56Time2a (full date and time). IEC 104 only allows CP56Time2a types, because a TCP network can delay messages by more than the hour that CP24Time2a can express.

Which one should you use?

• Use IEC 104 when there is an IP network to the control centre: substations and plants with fibre or VPN links, most new SCADA projects, and whenever several masters or redundant connections are needed.
• Use IEC 101 for existing serial telecontrol links (leased lines, radio modems, RS-485 buses) or where the control centre only has serial front-ends.
• Neither protocol encrypts or authenticates by itself. Protect IEC 104 with network segmentation, firewalls and VPNs, or with TLS according to IEC 62351 where both ends support it.

Can one gateway serve both?

Yes. Because the application layer is shared, a gateway can build one point list and publish it over both profiles. Max IEC Gateway runs an IEC 104 server (port 2404, up to 10 client connections, adjustable k, w and t0–t3) and an IEC 101 outstation on a COM port (balanced or unbalanced, 8E1, configurable baud rate, link address and field sizes) at the same time, both fed from the same Modbus mappings.

Frequently asked questions

Can an IEC 101 master talk to an IEC 104 outstation?

Not directly. The application data is the same, but the transport differs, so a protocol converter or a gateway that offers both profiles is needed.

Is IEC 104 faster than IEC 101?

Usually yes. A 9,600 bit/s serial line carries only a few dozen short ASDUs per second, while IEC 104 over Ethernet is limited mainly by the k window and network latency.