Printer sessions¶

Batch output on a mainframe does not come back on the display. The host binds a separate printer LU, sends it an SCS (LU1) or 3270 (LU3) datastream, and ends the job. The overnight balance report, the picking list, the audit extract: none of them appear on the screen the operator is looking at.

3270Web binds one of those printer LUs alongside a terminal session. There is no printer attached to the server and no reason to assume there should be, so each finished job lands as a file the session owns, and whoever is at the terminal downloads it.

What you need¶

A pr3287 binary. It ships with the x3270 suite alongside s3270 — the same project 3270Web already drives, and the same authors (see Acknowledgements). It speaks both printer datastreams, so 3270Web does not reimplement the protocol.

DockerDebian / UbuntumacOSWindows

Already installed. The image carries pr3287 beside s3270.

sudo apt-get install pr3287

brew install x3270

Install the x3270 suite and put wpr3287.exe beside 3270Web.exe, or point PR3287_PATH at it.

If the binary is somewhere of its own, name it:

PR3287_PATH=/opt/x3270/bin/pr3287

Without it the panel says so and the API answers 501 Not Implemented rather than failing at the moment somebody prints something.

Binding a printer¶

Session → Printer session in the terminal's menu bar.

There are two ways a host offers a printer, and the panel asks which one this host uses:

	When to use it
The associated printer	The host pairs a printer with each display LU. 3270Web asks the terminal what LU the host bound it as, and requests that LU's printer. Nothing to type.
A printer LU by name	The host has a named printer LU — `PRT001`, `LOCAL3287` — that your site allocated. Type the name.

If association is chosen and the host has not bound this session to a named LU, the panel says so and asks for a name instead. That is a real answer rather than an error: a host that binds no LU has no associated printer to offer.

Print options¶

Under Print options, and all optional:

Option	What it does
Line length (MPP)	Maximum presentation position for LU3 printing, 40–256. Default 132.
Code page	An alternate EBCDIC code page, if the printer LU differs from the display.
End-of-job timeout	End a job after this many idle seconds, for hosts that never send an end-of-job.
End lines with CR/LF	On by default, so a downloaded file opens correctly on Windows.
Keep blank lines	Keep the blank lines LU3 formatted mode otherwise suppresses.
Ignore a formfeed at the top of a page	Skip a formfeed that would otherwise produce a blank first page.
Pass formfeeds through	Emit the formfeed rather than interpreting it, in SCS mode.
Ignore the host's end-of-job	For hosts whose end-of-job arrives in the wrong place, or not at all.
Drop ASA carriage control	Strip the carriage-control character from the first column.

What is not yours to choose¶

A printer session always connects to the same host, port and TLS terms as the terminal session it belongs to. You pick the LU and the print options; nothing else is settable.

That is deliberate. An endpoint that accepted a hostname would be a way to make the server open connections wherever it can reach, from a request. And the real case is always the printer LU on the mainframe the operator is already signed in to. TLS is read from what the terminal actually negotiated, not from what its profile asked for, so the two halves of one connection cannot end up on different terms.

Collecting the output¶

Each finished job appears in the panel with the time it arrived and its size. Download saves it; Delete removes it.

A job is served as a file to save, never rendered in the browser. It is whatever the host sent, and this origin is not the place to run it as a document. What you get is the plain SCS or line-printer stream: open it in a text editor, or send it to a real printer with lpr.

Jobs go when the session does

Print jobs belong to the terminal session that collected them, and are deleted when it ends — a disconnect, a browser closed for long enough to be reaped, an administrator ending the session. Download anything you want to keep before then.

Stopping the printer does not remove them. That is something you do on purpose, often to rebind against a different LU, and losing the output you just collected would be a surprise.

Limits¶

Bound	Value	What happens past it
One job	8 MB	Kept, capped, and listed as truncated — a report that quietly stops two-thirds of the way down is worse than one that says where it stopped.
One session's spool	64 MB	The oldest jobs are dropped.
Jobs per session	200	The oldest jobs are dropped.

Over the API¶

Every one of these is under /api/v1/sessions/{id} and needs a bearer token — see REST API. The case for driving a printer without a browser is the ordinary one: a job runs overnight, the output goes to a printer LU, and something collects it in the morning.

# Bind the printer associated with this session's display LU
curl -X POST -H "Authorization: Bearer $API_TOKEN" \
     -H 'Content-Type: application/json' \
     -d '{"associate":true,"crlf":true}' \
     http://localhost:3270/api/v1/sessions/$SESSION/printer

# ...or a named LU, with a line length
curl -X POST -H "Authorization: Bearer $API_TOKEN" \
     -H 'Content-Type: application/json' \
     -d '{"lu":"PRT001","mpp":132}' \
     http://localhost:3270/api/v1/sessions/$SESSION/printer

# What is bound, and what has arrived
curl -H "Authorization: Bearer $API_TOKEN" \
     http://localhost:3270/api/v1/sessions/$SESSION/printer

# Take one job
curl -H "Authorization: Bearer $API_TOKEN" -OJ \
     "http://localhost:3270/api/v1/sessions/$SESSION/printer/jobs?name=$JOB"

# Let it go
curl -X DELETE -H "Authorization: Bearer $API_TOKEN" \
     http://localhost:3270/api/v1/sessions/$SESSION/printer

A status reply:

{
  "available": true,
  "running": true,
  "printer": {
    "session": "4f2c...",
    "host": "mainframe.example",
    "port": 992,
    "lu": "PRT001",
    "tls": true,
    "started_at": "2026-08-10T09:14:02Z",
    "running": true
  },
  "jobs": [
    {
      "name": "job-20260810T091455Z-3fa1c209.prt",
      "bytes": 18422,
      "received_at": "2026-08-10T09:14:55Z",
      "truncated": false
    }
  ]
}

If something goes wrong¶

"This installation has no pr3287 binary." Install it, or set PR3287_PATH. See What you need.

The printer stops immediately, naming the LU. The host refused the bind. Usually the LU name is wrong, or somebody else holds it. The panel shows what pr3287 said rather than a generic failure, because that sentence is normally the whole diagnosis.

The printer stops later, saying nothing. The host closed the printer session. 3270Web does not reconnect on its own: an LU that was refused would otherwise retry forever behind a status that says "running", and a printer session that stops should say so. Start it again.

Nothing arrives, but the status says running. The host has not printed anything to that LU yet, or it is printing to a different one. Check with the application; the Connection details panel shows what the terminal itself was bound as, which is the LU an associated printer hangs off.

A job arrives but ends early. Look for truncated beside it — see Limits.

A job is one long line, or has a stray character at the start of each line. That is carriage-control handling. Try Drop ASA carriage control, or adjust the line length.

What this is not¶

Printing to a physical printer on the operator's desk. The job arrives in the browser as a file, and what happens next is the operating system's business — which is the trade a browser-delivered terminal makes everywhere else too. A shop that wants paper can send the downloaded file to lpr, or run a pr3287 of its own beside the printer and let 3270Web handle the screens.

Where the pieces are¶


The panel	Session → Printer session
The API	`/api/v1/sessions/{id}/printer` — see REST API
The trail	`printer.started` and `printer.stopped` in the audit log, with the LU and the host
The setting	`PR3287_PATH`, when the binary is not on the path