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Joseph Doherty 399257377b Auto: ablegacy-13 — DH+ via 1756-DHRIO bridging validation

Closes #256

2026-04-26 08:56:23 -04:00

5.7 KiB

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AB Legacy — DH+ via 1756-DHRIO bridging

PR ablegacy-13 / #256. The AB Legacy driver can address a PLC-5 sitting on a DH+ link by routing CIP requests through a 1756-DHRIO module installed in a ControlLogix chassis. This is the canonical way to keep an installed-base PLC-5 fleet alive after the chassis- level migration to ControlLogix; the DHRIO module exposes a DH+ "side" that talks to the legacy PLC-5 / SLC-DH+ peers and a backplane "side" that the ControlLogix CPU + Ethernet bridge can route through.

Wire layout

OtOpcUa server  ──EtherNet/IP──►  1756-EN2T (slot 0)  ──backplane──►  1756-DHRIO (slot N)  ──DH+──►  PLC-5

Two CIP hops:

Backplane — port 1, slot <N> (the slot the DHRIO module lives in).
DH+ — port 2, station <S> (the DH+ node address of the target PLC-5, in octal).

Resulting CIP path: 1,<N>,2,<S>.

The first port 1 is always the backplane; port 2 is the DH+ side of the 1756-DHRIO module. This mirrors the convention Rockwell uses in RSLinx + RSLogix 5.

Octal station number

The DH+ network was specified with octal node addresses. Rockwell tooling displays them in octal too (RSLogix 5 → "DH+ Node Address" field on the controller properties dialog). The driver follows suit — the station segment of the CIP path must be parsed as octal (digits 0..7 only; 8, 9, and multi-byte garbage are rejected).

DH+ addresses run 0..77 octal == 0..63 decimal. Quick reference:

Octal	Decimal	Octal	Decimal	Octal	Decimal	Octal	Decimal
00	0	20	16	40	32	60	48
01	1	21	17	41	33	61	49
02	2	22	18	42	34	62	50
03	3	23	19	43	35	63	51
04	4	24	20	44	36	64	52
05	5	25	21	45	37	65	53
06	6	26	22	46	38	66	54
07	7	27	23	47	39	67	55
10	8	30	24	50	40	70	56
11	9	31	25	51	41	71	57
12	10	32	26	52	42	72	58
13	11	33	27	53	43	73	59
14	12	34	28	54	44	74	60
15	13	35	29	55	45	75	61
16	14	36	30	56	46	76	62
17	15	37	31	57	47	77	63

Anything past 77 octal (i.e. decimal > 63) is invalid on a real DH+ network and rejected by the parser.

PLC-5 only

DHRIO bridging is PLC-5-only. The driver enforces this at AbLegacyDriver.InitializeAsync time: a DH+ bridge path combined with PlcFamily=Slc500 / MicroLogix / LogixPccc throws InvalidOperationException("DHRIO bridging is PLC-5-only") immediately rather than letting reads silently fail with BadCommunicationError on the wire.

Background: the 1756-DHRIO module only speaks DH+ to PLC-5 / SLC-DH+ peers, and libplctag's PCCC stack only exposes the PLC-5 side. SLC 5/04 boxes on DH+ can be physically reached through a DHRIO module, but the protocol stack needed to drive them isn't exposed by libplctag — out of scope for this driver.

CLI worked example

PLC-5 at DH+ node 07 (octal == 7 decimal), DHRIO module in slot 3, gateway 192.168.1.10:

otopcua-ablegacy-cli probe `
    -g ab://192.168.1.10/1,3,2,07 `
    -P Plc5 `
    -a N7:0

# Read N7:10 from the PLC-5 across the DHRIO bridge
otopcua-ablegacy-cli read `
    -g ab://192.168.1.10/1,3,2,07 `
    -P Plc5 `
    -a N7:10 `
    -t Int

The driver surfaces the parsed bridge form on the host-address record: BackplaneSlot=3, DhPlusPort=2, DhPlusStation=7 (decimal-translated). Use those values when reading driver-diagnostics output to confirm the bridge was recognised — a non-bridge CIP path leaves all three fields null.

Manual smoke procedure

There is no automated end-to-end coverage for DH+ bridging because the only path to wire-level validation is real hardware (libplctag's ab_server Docker image doesn't simulate the DHRIO + DH+ + PLC-5 stack). The unit-test layer covers parser positive / negative cases.

Hardware smoke checklist:

Confirm the 1756-DHRIO module is present in the target ControlLogix chassis. RSLinx Classic should show DH+, 1 under the chassis tree with the PLC-5 nodes enumerated underneath.
Note the DHRIO module's slot number (the <N> in 1,<N>,2,<S>).
Note the target PLC-5's DH+ node address — read it off the front-panel switch bank, or the controller properties in RSLogix 5. Read it as octal.
From an OtOpcUa box that can reach the EtherNet/IP gateway:
```
otopcua-ablegacy-cli probe -g ab://<gateway>/1,<slot>,2,<station-octal> -P Plc5 -a S:0
```
S:0 (status file word 0) is non-destructive and present on every PLC-5.
If the probe succeeds, exercise an N file read against a known non-zero address. Compare against the value displayed in RSLogix 5 → Online → Data → N7.

If the probe fails with BadCommunicationError:

Wrong slot number — re-check via RSLinx.
Wrong octal node — convert from RSLogix 5's display value (already octal); a decimal-thinking conversion mistake is the most common smoke failure.
DHRIO module's DH+ baud rate doesn't match the PLC-5's switch setting (57.6k / 115.2k / 230.4k) — driver-side problem this can't paper over.
A scanner on the DHRIO is in scheduled-mode and starving unscheduled PCCC traffic — bump the DHRIO's unscheduled-message slice in RSLogix 5000.

Octal	Decimal	Octal	Decimal	Octal	Decimal	Octal	Decimal
00	0	20	16	40	32	60	48
01	1	21	17	41	33	61	49
02	2	22	18	42	34	62	50
03	3	23	19	43	35	63	51
04	4	24	20	44	36	64	52
05	5	25	21	45	37	65	53
06	6	26	22	46	38	66	54
07	7	27	23	47	39	67	55
10	8	30	24	50	40	70	56
11	9	31	25	51	41	71	57
12	10	32	26	52	42	72	58
13	11	33	27	53	43	73	59
14	12	34	28	54	44	74	60
15	13	35	29	55	45	75	61
16	14	36	30	56	46	76	62
17	15	37	31	57	47	77	63

Octal	Decimal	Octal	Decimal	Octal	Decimal	Octal	Decimal
00	0	20	16	40	32	60	48
01	1	21	17	41	33	61	49
02	2	22	18	42	34	62	50
03	3	23	19	43	35	63	51
04	4	24	20	44	36	64	52
05	5	25	21	45	37	65	53
06	6	26	22	46	38	66	54
07	7	27	23	47	39	67	55
10	8	30	24	50	40	70	56
11	9	31	25	51	41	71	57
12	10	32	26	52	42	72	58
13	11	33	27	53	43	73	59
14	12	34	28	54	44	74	60
15	13	35	29	55	45	75	61
16	14	36	30	56	46	76	62
17	15	37	31	57	47	77	63

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