Every RTU, TCP, and TLS profile field and its diagnostic meaning.
18 · TRANSPORT REFERENCE
Connection field reference
A connection profile is portable project configuration. It stores the endpoint and protocol settings required to reproduce a route. The currently selected profile, open connection, accepted TLS pin, and credentials outside referenced certificate files remain local runtime state.
Fields shared by every transport
Field
Meaning
Diagnostic consequence
Name
Human label used in menus, automation launches, CLI selection, and evidence context
Duplicate As creates a separate profile; the label shown in the picker is the name
Unit ID
Modbus server/slave identifier, 0–247
On RTU, 0 is broadcast and normally has no response; on TCP/TLS it may select a downstream gateway device
Response timeout
Maximum wait for one compliant response before the attempt fails
Too short creates false timeouts; too long makes wrong configuration and scans slow to diagnose
Retries
Additional attempts after a failed request
Useful for intermittent loss, but multiplies the time spent on persistent failures
Read-only
Native write guard for the profile
Blocks manual writes, FC22, automation, CLI, and MCP at the shared connection boundary
RTU serial fields
Field
What to enter
Common failure
Serial port
Discovered system port or an exact custom device path
Adapter moved, permission denied, another process owns it, or stale custom path
Baud rate
Device rate in bits per second
Every Unit ID appears silent or CRC/framing failures dominate
Data bits
Usually 8; match the device manual
Frames cannot be decoded consistently
Parity
None, even, or odd as specified
Silence, corrupted responses, or CRC errors
Stop bits
One or two as specified
Intermittent or complete framing failure
Serial port discovery only proves that the operating system exposes an adapter. RTU Unit-ID discovery proves that a Modbus participant responded with data or a valid exception under the selected framing. It does not prove that the discovered device owns the register map you expect.
TCP fields
Field
What to enter
Common failure
Host
IPv4 address, IPv6 address, or resolvable host name accepted by the profile
DNS, route, VLAN, VPN, firewall, or a stale address
Port
Actual listener port from device configuration or discovery
502 is conventional only; a device may listen on any TCP port
Unit ID
Direct device ID or downstream gateway address
TCP connects but requests time out or return a gateway exception
Network discovery first finds active hosts in a bounded IPv4 subnet and then validates Modbus responses across their TCP ports. Results are explicit IP:port endpoints. Choosing an endpoint updates the profile draft; confirm or save the settings before expecting another page to use the new route.
TLS fields
Field
Purpose
Where it lives
CA certificate path
Trust anchor used to verify the server certificate
Project profile; relative paths resolve from the project file
Client certificate path
Client identity for mutual TLS when required
Project profile; contains a path, never the certificate bytes
Private key path
Key corresponding to the client certificate
Project profile; the key remains an external file
Skip verification
Disables normal server certificate verification for deliberate development setups
Project profile and therefore visible in review
Accepted certificate pin
Machine-local SHA-256 identity accepted by this installation
Local state; never written to the project
Interpret connection outcomes
Visible outcome
What it proves
Next check
Port opened / TCP connected
The operating system created a transport
Send one bounded Modbus read
Valid exception response
A Modbus server parsed the request and rejected its function, address, or value
Inspect exception code and request fields
Timeout
No compliant response completed before the deadline
Route, Unit ID, serial framing, server load, then timeout
Malformed response
Bytes arrived but failed envelope or PDU validation
Raw frame, transaction ID, function, length, byte count, CRC
TLS verification failure
A TLS peer answered but identity/trust checks failed
Hostname, CA chain, validity dates, pin, client certificate role
