Troubleshooting PLC-2/30 Programming Over DH+ via 1785-KA3

Here is the fault. RSLinx lists the PLC-2/30 through both the 1784-PCMK and the 1770-KF2. AI 2 v6.25 on Windows 98 still won't go online. Clicking Who Active in AI 2 knocks every other node off the link.

The PLC-2 sits behind a 1785-KA3 at node 7. A ControlLogix DHRIO sits at node 5 and already exchanges data with it. A direct RS-422-to-USB connection to the processor works.

The processor is not the problem. The problem sits somewhere between the PC interface, the software driver, and what the KA3 actually presents on the wire. Work the checks below in order and skip none of them.

Map the panel symptoms to their causes first

Two fixes waste time. Re-flipping the KA3 protected/unprotected write DIP switches doesn't help. Rebooting Windows 98 into DOS mode by itself doesn't help either. The write switches gate data writes from messages. They don't control whether a PC node can hold the token.

Check 1: Read what the KA3 actually puts on the wire

Do this before touching software. A PLC-2 natively speaks the original Data Highway (DH), not DH+. The two networks use similar blue-hose cabling but run different protocols.

A PLC-2 only appears on DH+ through an adapter that performs the translation. The KA3 manual describes the module as a gateway between DH and DH+ (see page 2-2).

1. Identify which connector or channel faces DH+ and which faces the PLC-2 side. Confirm the cable to the PC and ControlLogix lands on the DH+ side.
2. Record the node address and link-rate switch settings on the adapter.

Outcome A: the link side is DH, not DH+. The PCMK and the ControlLogix DHRIO are DH+ devices, so a DH+ device will never hold a clean session with it. Put a DH-capable interface on that link, or add a DH-to-DH+ bridge module between the segments.

Outcome B: the link side is DH+ and RSLinx can browse it. The protocol layer works. Go to Check 2.

Keep the terminology straight while you do this. DHRIO is the ControlLogix module name, not a protocol. Each DHRIO channel is configured for either DH+ or Remote I/O. Open the DHRIO channel configuration in the ControlLogix project and confirm the channel wired to the KA3 is set to DH+. Note its baud rate and its node address (5).

Check 2: Isolate the KA3 from the ControlLogix trunk

This separates a network problem from a PC-interface problem in one step.

1. Disconnect the KA3 from the shared DH+ cable.
2. Connect the PCMK (or the KF2) point-to-point to the KA3. Terminate both ends of that short segment as the adapter manual specifies.
3. Try to go online from AI 2.

Same failure point-to-point: the ControlLogix, the trunk, and other nodes are cleared. The fault is in the PC interface configuration, the address plan, or AI 2's driver setup. Go to Check 3.

Works point-to-point, fails on the trunk: suspect a duplicate address, a missing or extra terminator, or a baud mismatch with the DHRIO channel. Go to Check 4.

Check 3: Stop browsing and go online with static addresses

Who Listen is passive. The PC card monitors token traffic and reports which addresses it hears. Who Active is different. The PC card enters the token rotation and solicits every address on the link.

If the card's station address collides with an existing node, the token gets corrupted. The same happens if the card's link timing doesn't match the other nodes, or if the KA3 can't keep up with a burst of diagnostic requests. Every node then drops and has to re-solicit. That is exactly the "everything falls off" behavior on the panel.

Don't use the browse to get online. Do this instead:

1. In AI 2, set the Computer Station Address to an unused address. Don't use 5 or 7.
2. Enter the PLC-2's station address (7) as the target directly.
3. Issue the go-online or attach command without running Who Active first.
4. Watch the ControlLogix DHRIO channel status and the PLC-2 messaging while you attach.

Online and other nodes stay up: the browse was the trigger. Stop using Who Active on this link and program with static addressing.

Attach fails and nodes still drop: the card itself disrupts the token whenever it goes active. Go to Check 4.

Attach fails but nodes stay up: the link is healthy and the software path is wrong. Go to Check 5.

Check 4: Rule out duplicate address, octal entry, and baud mismatch

Trying several different computer station addresses and getting the same result points away from a simple duplicate. It still leaves two traps.

• Octal addressing. DH and DH+ station addresses are octal. An address entered as 10 is octal 10, which is decimal 8. Digits 8 and 9 are invalid.
  • Check whether each screen takes octal or decimal: AI 2, the PCMK configuration, the KF2 switches, and the DHRIO channel config.
  • If one tool displays decimal and another displays octal, two "different" addresses can land on the same node.
  • Write out all addresses in octal and confirm none of them collide: PC, KF2, KA3 (7), and DHRIO (5).
• Baud rate. Every node on a DH+ segment must run the same link rate.
  • Read the rate from the DHRIO channel configuration and the KA3 switch settings.
  • Set the PCMK driver and the KF2 link-rate switches to match.
  • A card set to a different rate hears nothing useful passively. When it goes active, it wrecks the token.
• KF2 node. The 1770-KF2 holds its own station address on the link, separate from whatever address AI 2 reports as the computer station. If the KF2 and the PCMK are both on the cable, both need unique addresses.

After a correction, repeat Check 3 before you go any further.

Check 5: Give AI 2 exclusive ownership of the PC interface

RSLinx seeing the PLC-2 proves the hardware path. It does not prove AI 2 can use that path. AI 2 v6.25 does not route through RSLinx. It drives the PCMK or talks to the KF2 with its own communication driver.

Two applications can't share one interface card. A driver that grabs a card the other application still holds gives exactly the "RSLinx works, AI 2 won't" split.

1. Delete or stop the RSLinx driver for the PCMK or KF2. Exit RSLinx completely and confirm it isn't loading at startup. Reboot.
2. Open AI 2's communication setup. Select the interface type that matches the hardware in use: PCMK card, or a serial KF2 path.
3. For the PCMK, enter the card resources exactly as the card configuration or PC card services report them. Don't guess these values.
4. If you boot to DOS mode, the Windows PC card services are gone. The PCMK must then be enabled by a DOS-level card enabler before AI 2 can open it.
   • Booting to DOS without that enabler leaves AI 2 with no card at all.
   • That explains why the DOS attempt changed nothing.
5. Attach to station 7 with static addressing, as in Check 3.

One more constraint: a PLC-2 on DH+ behind the KA3 cannot send unsolicited messages to RSLinx through Rockwell routers. Every exchange with the PC has to be initiated from the PC side. Don't design a data path that depends on the PLC-2 pushing data up to the PC.

Check 6: Switch to the 1770-KF2 or the direct RS-422 link

If the PCMK keeps disrupting the link after Checks 3 through 5, change the interface.

1. KF2 serial path.
   • Set the KF2 network-side switches for the correct network type (from Check 1), a unique octal station address, and the link rate that matches the segment.
   • Set the KF2 serial-side switches for the baud, parity, and error-check mode AI 2 is configured for.
   • Point AI 2's serial driver at the COM port. Target station 7.
   • The KF2 behaves like any other node on the token. A wrong address or rate here produces the same drop-out as the PCMK.
2. Direct RS-422. Your USB-to-RS-422 connection already works. Use it to make program changes while you resolve the network path. It bypasses the KA3 and the DH+ segment entirely.

Confirm the DH+ session holds under load

1. Go online from AI 2 to station 7 with static addressing. Stay online for several minutes.
2. Watch the ControlLogix DHRIO channel diagnostics while you are online. Node 5 and node 7 must stay present, with no rising error counts.
3. Confirm the ControlLogix-to-PLC-2 messages keep completing, with no errors or timeouts in the message instruction status.
4. Upload the program and compare it to the known-good copy taken over the RS-422 connection.
5. Make a harmless test edit, then reverse it. Confirm the processor accepts both.
6. Disconnect AI 2 and confirm the link settles with only nodes 5 and 7 active.
7. Record the final settings in the panel documentation:
   • Octal address of every node
   • Link rate
   • Which interface (PCMK or KF2) is approved for programming
   • A note not to run Who Active on this segment

FAQ

Escalate when the PCMK or KF2 still drops the link with the KA3 isolated point-to-point, unique octal addresses, and a verified matching link rate. At that point the adapter or interface hardware is suspect. Contact Rockwell Automation technical support with the KA3 catalog and series from the nameplate, the switch settings, and the DHRIO channel configuration.