Add Epson TM-T20II receipt printing (title/body/QR, no ESC/POS knowledge needed)

Custom ReceiptBuilder instead of the escpos gem -- its diacritics handling
needed the same manual UTF-8->CP852 encode call anyway, based on earlier
experiments in ~/test/epson/.

app.rb

@@ -3,6 +3,7 @@ require 'sinatra'
 require "sinatra/reloader"
 require 'tmpdir'
 require 'net/http'
+require_relative 'lib/receipt_builder'
 
 set :protection, :except => :frame_options
 set :environment, :production
@@ -21,6 +22,8 @@ PRINTER_DPI = 203 # DatamaxONeil E4204B Mark III native head resolution
 PRINTER_DARKNESS = 8 # lower than factory default to stop thermal dot-gain from fusing barcode bars
 TEST_LABEL_PDF = File.join(File.dirname(__FILE__), 'test-label.pdf')
 
+PRINTER_DEVICE_EPSON = '/dev/usb/lp_epson' # see /etc/udev/rules.d/99-epson.rules
+
 BROTHER_PRINTERS = {
   'ql560'  => { port: 8560, label: 'Brother QL-560' },
   'ql700'  => { port: 8700, label: 'Brother QL-700' },
@@ -88,6 +91,19 @@ helpers do
     File.open(PRINTER_DEVICE, 'wb') { |f| f.write(data) }
   end
 
+  # Builds a simple receipt (bold centered title + left-aligned body,
+  # optional QR code) so callers don't need to know ESC/POS escape codes.
+  def print_receipt(title:, body:, qr: nil)
+    receipt = ReceiptBuilder.new
+    receipt.align(:center).bold.text("#{title}\n").bold(false) unless title.to_s.empty?
+    receipt.align(:left).text("#{body}\n") unless body.to_s.empty?
+    unless qr.to_s.empty?
+      receipt.align(:center).feed(1).qr(qr).feed(1)
+    end
+    receipt.feed(3).cut
+    receipt.print!(PRINTER_DEVICE_EPSON)
+  end
+
   # Transparently reverse-proxies the current request to one of the
   # brother_ql_web instances, so all three printers (web UI and API alike)
   # are reachable through this single sinatra port instead of their own
@@ -187,6 +203,20 @@ post '/test-print' do
   redirect '/upload'
 end
 
+get '/epson' do
+  erb :epson
+end
+
+post '/epson/print' do
+  begin
+    print_receipt(title: params[:title].to_s, body: params[:body].to_s, qr: params[:qr].to_s)
+    flash 'Sent to Epson printer', 'success'
+  rescue => e
+    flash "Epson print failed (#{e.message})", 'danger'
+  end
+  redirect '/epson'
+end
+
 get '/brother' do
   @printers = BROTHER_PRINTERS
   erb :brother_picker

lib/receipt_builder.rb

@@ -0,0 +1,106 @@
+# Minimal chainable ESC/POS builder for the Epson TM-T20II.
+#
+# Written instead of pulling in the `escpos` gem: the gem's diacritics
+# handling needed the same manual UTF-8->CP852 `.encode` call anyway, and a
+# couple of its helpers didn't behave as documented against this printer --
+# fewer moving parts doing it directly.
+class ReceiptBuilder
+  INIT       = "\x1B\x40".freeze
+  CP852      = "\x1B\x74\x12".freeze # code page for Slovak diacritics
+
+  ALIGN_LEFT = "\x1B\x61\x00".freeze
+  ALIGN_CENT = "\x1B\x61\x01".freeze
+  ALIGN_RIGH = "\x1B\x61\x02".freeze
+
+  BOLD_ON    = "\x1B\x45\x01".freeze
+  BOLD_OFF   = "\x1B\x45\x00".freeze
+  UNDER_ON   = "\x1B\x2D\x01".freeze
+  UNDER_OFF  = "\x1B\x2D\x00".freeze
+
+  TXT_NORM   = "\x1D\x21\x00".freeze
+  TXT_BIG    = "\x1D\x21\x11".freeze # double height and width
+  TXT_HIGH   = "\x1D\x21\x01".freeze # double height
+  TXT_WIDE   = "\x1D\x21\x10".freeze # double width
+
+  CUT        = "\x1D\x56\x00".freeze
+
+  def initialize
+    @buffer = "".b
+    init
+  end
+
+  def init
+    @buffer << INIT << CP852
+    self
+  end
+
+  def text(str)
+    @buffer << str.encode('CP852', invalid: :replace, undef: :replace, replace: '?')
+    self
+  end
+
+  def align(mode)
+    @buffer << case mode
+               when :center then ALIGN_CENT
+               when :right  then ALIGN_RIGH
+               else ALIGN_LEFT
+               end
+    self
+  end
+
+  def bold(enable = true)
+    @buffer << (enable ? BOLD_ON : BOLD_OFF)
+    self
+  end
+
+  def underline(enable = true)
+    @buffer << (enable ? UNDER_ON : UNDER_OFF)
+    self
+  end
+
+  def size(mode)
+    @buffer << case mode
+               when :big  then TXT_BIG
+               when :high then TXT_HIGH
+               when :wide then TXT_WIDE
+               else TXT_NORM
+               end
+    self
+  end
+
+  # Native ESC/POS QR code
+  def qr(data, size: 6)
+    qr_data = data.b
+    store_len = qr_data.bytesize + 3
+
+    @buffer << "\x1D\x28\x6B\x03\x00\x31\x43" << size.chr
+    @buffer << "\x1D\x28\x6B\x03\x00\x31\x45\x30" # error correction L
+    @buffer << "\x1D\x28\x6B" << [store_len % 256, store_len / 256].pack('CC') << "\x31\x50\x30" << qr_data
+    @buffer << "\x1D\x28\x6B\x03\x00\x31\x51\x30" # print
+    self
+  end
+
+  # Native ESC/POS EAN-13
+  def ean13(code)
+    @buffer << "\x1D\x6B\x02" << code.b << "\x00"
+    self
+  end
+
+  def feed(lines = 3)
+    @buffer << ("\n" * lines)
+    self
+  end
+
+  def cut
+    @buffer << CUT
+    self
+  end
+
+  def render
+    @buffer
+  end
+
+  def print!(device)
+    File.open(device, 'wb') { |f| f.write(@buffer) }
+  end
+end

views/_navbar.erb

@@ -12,6 +12,9 @@
         <li class="nav-item">
           <a class="nav-link" href="/upload">Shipping labels</a>
         </li>
+        <li class="nav-item">
+          <a class="nav-link" href="/epson">Receipts</a>
+        </li>
       </ul>
       <span class="navbar-text me-2">Brother:</span>
       <div class="d-flex gap-1">

views/epson.erb

@@ -0,0 +1,15 @@
+        <form action="/epson/print" method="POST">
+          <div class="mb-3">
+            <label for="title" class="form-label">Title (bold, centered)</label>
+            <input name="title" class="form-control" type="text" id="title" />
+          </div>
+          <div class="mb-3">
+            <label for="body" class="form-label">Body text</label>
+            <textarea name="body" class="form-control" id="body" rows="8"></textarea>
+          </div>
+          <div class="mb-3">
+            <label for="qr" class="form-label">QR code content (optional)</label>
+            <input name="qr" class="form-control" type="text" id="qr" />
+          </div>
+          <button class="btn btn-secondary" type="submit">Print</button>
+        </form>