Printing Red Text via Siemens CP340 P_Print: Configuration Guide

1. Overview of CP340 Red-Color Printing

The Siemens CP 340 point-to-point communication processor is widely deployed on S7-300 stations to drive serial printers, barcode readers, and weighing terminals. The standard function block FB4 "P_Print" (part of the SIMATIC S7 PtP library) ships message text either from a CP340-resident message text list or from a user-defined data block, and forwards it to the printer through the configured protocol (3964R, ASCII, or RK512).

To colorize a section of a printed line - for example, marking a "Filling not ok" condition in red - the application must inject a printer-specific escape sequence into the data stream at the exact point where the color change is required. The CP340 supports this through a control character translation table in the PtP parameter assignment, where a printable ASCII character (commonly 'R') is mapped to the printer's vendor escape string (for example, ESC r 0 / ESC r 1 in Epson-compatible devices).

This document describes the engineering workflow that maps a logical "red" tag (M9.1 = "Filling not ok") onto a correctly framed ASCII stream, the CP340 parameter assignment, and the FB4 P_Print interface, including verification and troubleshooting.

2. Prerequisites

3. CP340 Architecture and P_Print Function Block

The CP340 runs the physical layer, frame handling (start/stop characters, XON/XOFF, 3964R control characters) and the message text list internally. The CPU-side interface is a dual-port mailbox accessed by the standard point-to-point function blocks. For printing, FB4 P_Print is used.

3.1 FB4 P_Print - Calling Interface

3.2 P_Print Data Block Layout

The pointer DB specified by DB_NO / DBB_NO contains the raw byte stream that will be transmitted. The byte stream is built string-fashion; a typical 80-column line looks like:

DB100 
  DBB  0  : 'Line 01: Filling = '           (22 bytes)
  DBB 22  : 27 dec (ESC)                       
  DBB 23  : 114 dec ('r')                      
  DBB 24  : 49 dec ('1')  -- enter red ribbon position 
  DBB 25  : '123.45 kg'                       (9 bytes)
  DBB 34  : 27 dec (ESC)
  DBB 35  : 114 dec ('r')
  DBB 36  : 48 dec ('0')  -- return to black
  DBB 37  : 16#0A (LF)

The two preceding bytes - 27 dec (ESC) and 114 dec ('r') - form the printer's red/black ribbon select escape. The third byte (48 dec = '0' or 49 dec = '1') selects the ribbon half: '0' for black, '1' for red on Epson ESC/POS / ESC/P ribbon-shift printers. The MOVE instructions in the user's program are simply writing either '0' or '1' into byte 24 or 36 of the message DB, controlled by the boolean tag M9.1 ("Filling not ok").

4. ASCII Escape Sequence Theory for Printer Color Control

An escape sequence is a multi-byte in-band command that begins with the ASCII control character ESC (27 dec / 0x1B). The CP340 must be told to forward the ESC byte literally to the printer (it is normally filtered as a non-printable control character). The mechanism is the control character translation table in the PtP parameter assignment.

4.1 ASCII Code Reference

4.2 Two Methods to Inject Color Sequences

1. Method A - String-internal escape (recommended for variable data): Build the message DB byte-by-byte using MOVE / BLKMOV instructions. The user's program is the source of the escape bytes; CP340 is left at its default configuration. This is the technique observed in the original program (FC40 / FC41 from the source project).
2. Method B - Symbolic control characters: Configure the CP340 control-character table so that a printable character such as 'R' is replaced at transmission time with the actual red-select sequence. The CPU program only contains \R text\-R tokens; the CP340 expands them. This is cleaner but requires HW Config parameter assignment and is limited to fixed escape strings per color.

5. Step-by-Step: Configuring CP340 Control Characters (Method B)

This section implements Method B for completeness. Skip to Section 6 if you intend to use Method A (string-internal escape), which is what the source program uses.

1. Open the S7 project in SIMATIC Manager and double-click the CP340 in the S7-300 station.
2. In HW Config, right-click the CP340 and choose Object Properties > Parameter > PtP.
3. Open the Control characters tab (chapter 2.7.5 of the CP340 manual). The default table contains 7 ASCII control chars (0x00..0x06, 0x09..0x0D, 0x1B).
4. Click New and add a symbolic control character. Recommended mapping:

   Symbol	ASCII	On-sequence (hex)	Off-sequence (hex)
   R	0x52	1B 72 31	1B 72 30
   B	0x42	1B 72 30	1B 72 31

5. Save, compile, and download the HW Config to the CPU.
6. In the CPU program, write the message text using STEP 7 STRING syntax:

   // String constant example
   "\R FILLING NOT OK \-R  Qty = 123.45 kg"
   

   The \R is replaced by the CP340 with ESC r 1, the \-R with ESC r 0.

6. Step-by-Step: Programming the P_Print FB for Colored Output (Method A)

This is the technique the user's program implements, and the one most likely to be the correct approach for variable-driven color decisions (e.g. M9.1 "Filling not ok").

6.1 Build the Message DB

Create a shared DB (e.g. DB100 "PrintBuffer") with the following initial values. The colored portion is parameterised by a flag byte at DBB24 / DBB36.

DATA_BLOCK DB100
TITLE = PrintBuffer
STRUCT
    Line1Red_On  : BYTE := B#16#1B;   // ESC  -- 27 dec
    Line1Red_Cmd : BYTE := B#16#72;   // 'r'  -- 114 dec
    Line1Red_Sel : BYTE := B#16#30;   // placeholder, set by program (48 or 49)
    Body1        : STRING[20] := ' Filling = ';
    Value1       : REAL := 0.000000e+000;
    Line1Red_Off : BYTE := B#16#1B;
    Line1Red_OffC: BYTE := B#16#72;
    Line1Red_OffS: BYTE := B#16#30;   // placeholder, set by program
    CrLf         : BYTE := B#16#0A;
END_STRUCT;
END_DATA_BLOCK

6.2 Conditional MOVE Logic (FC40 excerpt)

// In FC40, network 1: ribbon select = red when M9.1 = 1, else black
      A   M 9.1            // "Filling not ok"
      JCN NO_RED
      L   B#16#31          // '1' = red ribbon
      T   DB100.DBB  24    // Line1Red_Sel
      L   B#16#30          // '0' = black ribbon for the OFF token
      T   DB100.DBB  36    // Line1Red_OffS
      JU  CONT
NO_RED: L   B#16#30        // '0' = black ribbon for the ON token
      T   DB100.DBB  24
      L   B#16#30          // '0' = black ribbon for the OFF token (redundant but safe)
      T   DB100.DBB  36
CONT:  NOP 0

The exact byte offset depends on how the message DB is structured. The two pairs of MOVE instructions observed in the source project are populating the two positions reserved for the ribbon selector character (49 dec = '1' for red, 48 dec = '0' for black).

6.3 P_Print Call (FC41 excerpt)

// Triggered by I0.0 (manual print button) or print cycle flag
      A   I 0.0
      FP   M 10.0
      =    #print_trigger

      CALL FB   4, DB 5    // FB4 P_Print, instance DB5
        REQ   := #print_trigger
        RST   := FALSE
        PRINT := FALSE
        LEN   := 38        // total length in bytes
        DB_NO := 100
        DBB_NO:= 0
        LADDR := W#16#100
        DONE  := M 10.1
        ERROR := M 10.2
        STATUS:= MW 12

      AN   M 10.2         // no error
      JC   PRT_OK
      // handle error -- see Section 8
PRT_OK: NOP 0

7. P_Print Status Word and Error Reference

8. Troubleshooting Matrix

9. Verification Procedures

1. Offline verification - byte-level: Open the message DB online, force the M9.1 flag, and read DBB24 / DBB36. The byte at DBB24 must be 0x31 (red) and DBB36 must be 0x30 (black) when M9.1 = TRUE; both must be 0x30 when M9.1 = FALSE.
2. Online verification - FB4 status: After the print trigger, monitor MW 12. Expected transitions: 0x0F01 (job running) → 0x0F02 (job finishing) → 0x0000 (DONE set). Any 0x8xxx is a fault - consult Section 7.
3. Printer verification: Issue a print, visually confirm the red section aligns with the M9.1 condition. On Epson-compatible devices, also send a test string 1B 40 (ESC @) to reset the printer and ensure the escape parser is in a known state.
4. Cable and protocol verification: Use a serial line analyser or a passive RS-232 tap to capture the bytes on the wire. Confirm:
   • Bytes 0..2 of the stream are 0x1B 0x72 0x31 (or 0x30) when red (or black) is requested.
   • The 8-bit framing matches what the printer is configured for (commonly 8N1).
   • Handshake lines DTR/DSR toggle as expected.
5. Long-run verification: Print at least 1000 lines with alternating M9.1 states to confirm that the CP340 buffer never overruns and that the printer does not drop into an undefined color state.

10. Field-Commissioning Notes and Caveats

• DB initial vs. actual values: S7-300 retains DBs across power cycles only if the DB is non-volatile (set the "Retain" property). The CP340 control character table is in the CP's flash and survives power-cycle, but the message DB does not - always initialise the selector bytes in OB100.
• Know_How_protect on FB4: The original Siemens FB4 P_Print is delivered without know-how protection but with a comment header. A version that shows up as "cracked" in the cross-reference has been edited and is a red flag for code quality.
• Comment loss: The source program lost its comments in the upload. Re-importing FB4 from the original library will not affect the application blocks (FC40, FC41) but will reset FB4 internal documentation.
• Legacy CP340 vs CP341: If the project is migrated to a CP341, FB4 is replaced by FB8 P_Print, and the parameter assignment path moves under "CP341 > Properties > PtP". The escape-sequence mechanism is identical.
• Character set: The CP340 transmits only 7-bit or 8-bit ASCII; non-Latin characters (e.g. Cyrillic) require the printer to be set to a matching codepage (PC437, PC850, etc.). Otherwise the ESC byte (0x1B) is still forwarded correctly, but surrounding text will be illegible.

11. Frequently Asked Questions