A direct serial cable or back-to-back RS-485 converters cannot replace the modem behavior expected by the documented PAC-family PPP path. The available evidence states that PPP on S-series and R-series equipment requires dial-up modems and their AT-command handling. If the site must retain RS-485 wiring, use a transport bridge or a separately implemented serial transport instead of assuming that electrical conversion alone will establish PPP.
Why the direct PPP connection does not start
PPP provides network-layer transport over a point-to-point link, but the implementation described here includes modem setup before PPP negotiation. The connection therefore depends on responses to modem commands; a passive serial cable or RS-485 converter changes the physical interface but does not generate those responses.
The evidence does not identify the exact command sequence, timeout, Windows driver behavior, or controller state transition. Do not diagnose a failed link as a PPP authentication or addressing problem until the modem-command prerequisite has been satisfied or removed by a supported transport.
Affected PAC-family configurations
| Configuration | Evidence-based status | Engineering decision |
|---|---|---|
| R-series PPP through dial-up modems | PPP requires modem AT-command handling. | Retain compatible modems if PPP is mandatory. |
| R-series direct serial or back-to-back RS-485 conversion | No modem responses are provided, so this does not satisfy the stated PPP requirement. | Do not treat converters as modem substitutes. |
| S-series or R-series serial transport without PPP modem commands | A requested Opto serial transport was discussed, requiring both master and slave portions on R-series controllers. | Confirm current product support before designing around it. |
| EB-series controller-side serial transport | Excluded from the cited request because EB brains lack a serial port. | Select another endpoint or network path. |
The reported test used software version 9.1b with an R2, while the initial description also refers to an R1. This evidence does not establish whether 9.1b is uniquely affected or whether R1 and R2 behavior differs.
Choose a workable transport
- If PPP is required, provide the dial-up modem behavior expected by both endpoints, including the required AT-command responses.
- If the installed RS-485 path must remain, evaluate Ethernet-to-RS-485 serial bridges so the application uses Ethernet across an explicitly bridged serial segment rather than unsupported modemless PPP.
- If new infrastructure is acceptable, use an Ethernet-capable wireless link; the reported configuration connected a Wi-Fi radio to a tablet running PAC Display successfully.
- If direct controller-to-controller serial networking is required, verify whether the requested Opto serial transport has since been implemented and whether both endpoint roles are supported.
Verification criteria
Verify the design at the transport boundary. For PPP, confirm that modem commands receive valid responses before expecting PPP negotiation. For a serial bridge or wireless design, confirm end-to-end Ethernet communication and PAC Display connectivity rather than looking for a PPP session.
The evidence alternates between the acronyms “OST” and “OTS” for the requested serial transport. Treat the feature name as ambiguous and identify it by function—serial transport without PPP modem commands—when checking current documentation or contacting the manufacturer through official support channels.
FAQ
Can an R-series controller run PPP over a direct serial cable without modems?
Not with the implementation described in the evidence. Its PPP path requires dial-up modem handling and AT-command responses before the PPP connection can proceed.
Will two RS-485 converters back to back make PPP work?
No. The converters can adapt the electrical serial path, but the evidence does not show that they generate the modem-command responses required by this PPP implementation.
What can replace modem-based PPP for PAC Display connectivity?
Evaluate Ethernet-to-RS-485 bridges when existing serial wiring must remain, or use an Ethernet-capable wireless link. Verify end-to-end Ethernet and PAC Display communication after installation.