Configuring ASCII Communications on Siemens CP 441-2 Modules

The CP 441-2 is the high-performance point-to-point (PtP) communications processor for the S7-300 and S7-400 families. Two pluggable interface submodules can be fitted, giving either two independent physical ports (RS232, RS422, RS485, TTY, or 20 mA current loop depending on module), or one port plus one redundant/independent backup. When configured for the ASCII driver, the CP presents the same programming interface as the ISO-on-TCP variants (CP 441-2 - S7-300, S7-400 reference), but with the underlying transport replaced by raw character I/O. This article consolidates the configuration steps, parameter assignments, SCL block wiring, status-code interpretation, and field troubleshooting required to bring up a stable ASCII link.

1. Hardware Identification and Slot Rules

The CP 441-2 occupies one slot in the S7-300 or S7-400 rack and requires one or two interface submodules plugged onto the front. Each submodule is independently addressable and each runs its own protocol stack. Without any submodule, no PtP port exists.

Download the PtP Parameter Assignment Tool from the Siemens support portal entry referenced in the CP 441-2 PtP configuration tool page (ID 27013524). The installer drops example projects into the STEP 7 example tree; these examples are the authoritative templates for ASCII link configuration and are the same examples referenced in the field report.

2. Driver Selection in HW Config

Right-click the CP 441-2 in HW Config, choose Properties > Interface > [submodule], then select the ASCII driver. The page exposes the following key parameters:

The driver choice is written to the CP at download. After changing driver parameters you must perform a full download of the station (PLC plus CP) and cycle power on the CP if the change is not picked up automatically.

3. NetPro Connection Configuration

Even for raw ASCII, NetPro must contain a configured PtP connection. The CP does not accept BSEND/BRECV calls until that connection is established and downloaded to the station. Without it, the connection remains in "not configured" state and BSEND returns status W#16#0001 indefinitely.

3.1 Creating the Connection

1. Open SIMATIC Manager > Options > NetPro with the S7 project open.
2. Drag from the CP 441-2 symbol to the same CP (the partner is the CP itself for ASCII). Alternatively, right-click the CP and select Insert New Connection; the partner object remains the same CP, type = PtP connection.
3. Assign a connection ID. The connection ID is the decimal number that appears as the ID input of BSEND/BRECV. It is also exposed as W#16# in the connection properties dialog.
4. Save and compile NetPro. The connection block is generated as a UDT attached to the CP.
5. Perform PLC > Download to Station on the S7 CPU. The CP downloads the connection independently when its slot configuration is reloaded.

3.2 Multiple Cards and Multiple Ports

Each physical port (submodule) is independent. Each port receives its own connection entry in NetPro. The CP distinguishes ports internally; the user selects a port by selecting the corresponding connection ID. There is no separate "port number" parameter exposed in BSEND/BRECV. If two CP 441-2 cards occupy two slots, each slot has its own logical address range, and you must create the connection against the correct card - confirmed by checking the LADDR displayed in NetPro matches the LADDR of the slot you intended.

4. Block Selection: BSEND/BRECV vs PtP-Specific FB

The CP 441-2 differs from CP 340 and CP 341 in block selection:

The CP 441-2 does not expose a dedicated P_SEND/P_RECV or P_SND_RK/P_RCV_RK variant for ASCII. The CP 441-2 manual - CP 441-2 manual entry (ID 1137419) - confirms that BSEND and BRECV are the standard blocks used against the ASCII driver. The application program treats BSEND/BRECV as the PtP send/receive interface; the CP translates the buffer payload to/from the ASCII frame.

4.1 Asymmetric Use

For ISO-on-TCP and TCP, BSEND requires a BRECV partner. For ASCII, BSEND can be used standalone: the CP discards the framing rules and simply transmits the buffer. Likewise, BRECV can be invoked without a peer BSEND if the partner device is sending freely. This asymmetric use is explicitly permitted for the ASCII driver and is the basis of "generic" PtP communication.

5. BSEND and BRECV Inputs (SCL)

5.1 BSEND (SFB 12) Signature

// Send data to the CP 441-2 ASCII port
// ID and R_ID are hex values matching the NetPro connection entry

bsend_instance(REQ := bSendRequest,           // edge-triggered send
               R   := bSendReset,             // abort job
               ID  := W#16#1001,              // connection ID from NetPro
               R_ID:= W#16#0,                 // must be zero for ASCII
               SD_1:= pSendData,              // ANY pointer to send buffer
               LEN := iSendLen,               // byte length, 1..1024 typically
               DONE=> bSendDone,
               ERROR=> bSendError,
               STATUS=> wSendStatus);

5.2 BRECV (SFB 13) Signature

// Receive data from the CP 441-2 ASCII port

brecv_instance(EN_R := bRecvEnable,           // receive enable, level
               ID   := W#16#1002,             // receive connection ID
               R_ID := W#16#0,                // zero for ASCII
               RD_1 := pRecvData,             // ANY pointer to receive buffer
               LEN  := iRecvMaxLen,           // buffer size
               NDR  => bRecvNewData,
               ERROR=> bRecvError,
               STATUS=> wRecvStatus,
               LEN  => iRecvLen);             // actual bytes received

6. ID and R_ID Parameter Semantics

Example projects shipped with the PtP Parameter Assignment Tool typically assign ID = W#16#1000 for BSEND and ID = W#16#1001 for BRECV. The choice of 0x1000 vs 0x1001 is arbitrary - the only rule is that the value matches the connection ID assigned in NetPro. The decimal-vs-hex mismatch (1001 decimal vs 0x1001 hex) is the single most frequent configuration error reported on this module.

7. Common Status Codes and Root Cause Mapping

BSEND and BRECV expose STATUS (WORD) and ERROR (BOOL). The combination encodes the failure mode:

When STATUS is W#16#0001 after the very first BSEND call and never advances, the diagnostic ladder is:

1. Confirm NetPro > Connection > Status shows the connection icon green (established).
2. Confirm the connection has been downloaded to the station from NetPro (not just saved).
3. Confirm the ID entered into BSEND matches the connection ID in hex (NetPro shows it as W#16#1001; entering decimal 1001 by mistake routes to a non-existent connection).
4. Confirm the CP's slot address (LADDR) is not conflicting with another module.

8. Cable Wiring Reference

For RS232 to a laptop or PC running a terminal emulator, the source thread documents the pinout used during debugging. The standard Siemens-issued crossover is:

RS232 cable constraints for reliable ASCII communication:

• Length limit 15 m (50 ft) for RS232 per EIA/TIA-574.
• Shielded cable required for industrial environments; ground shield at CP end only.
• If RTS/CTS handshake is enabled in HW Config, both sides must drive RTS high when ready to receive; mismatched handshake will stall the transmitter indefinitely (TX LED never lights).
• If no handshake is selected, leave RTS/CTS lines disconnected at the partner device or wired back-to-back; do not tie them to a fixed level.

For RS422/RS485, terminate the bus at both ends with 120 ohm resistors and enable the failsafe biasing resistors built into the IF963-RS422/RS485 module through the HW Config > Properties > Interface tab. RS485 half-duplex applications must also configure the transmitter enable mode (auto via TXD echo or controlled via RTS).

9. LED Diagnostics on the CP 441-2

The source thread reports all other LEDs off except the green RUN with no TX activity - the canonical fingerprint of an ID mismatch between BSEND and NetPro.

10. SCL Template for a Generic ASCII Transceiver

FUNCTION_BLOCK FB_AsciiPtP
VAR
    bSendReq   : BOOL;
    bSendDone  : BOOL;
    bSendError : BOOL;
    wSendStatus: WORD;
    iSendLen   : INT;
    pSendData  : POINTER TO BYTE;

    bRecvEn    : BOOL := TRUE;
    bRecvNDR   : BOOL;
    bRecvError : BOOL;
    wRecvStatus: WORD;
    iRecvLen   : INT;
    pRecvData  : POINTER TO BYTE;
    iRecvMax   : INT := 1024;

    bInit      : BOOL := TRUE;
END_VAR

BEGIN
    // One-shot init of receive enable
    IF bInit THEN
        bRecvEn := TRUE;
        bInit := FALSE;
    END_IF;

    // Receive side: always enabled, BRECV completes when CP delivers bytes
    brecv_instance(EN_R := bRecvEn,
                   ID   := W#16#1002,
                   R_ID := W#16#0,
                   RD_1 := pRecvData,
                   LEN  := iRecvMax,
                   NDR  => bRecvNDR,
                   ERROR=> bRecvError,
                   STATUS=> wRecvStatus,
                   LEN  => iRecvLen);

    // Send side: trigger on rising edge of bSendReq, then poll
    bsend_instance(REQ := bSendReq,
                   ID  := W#16#1001,
                   R_ID:= W#16#0,
                   SD_1:= pSendData,
                   LEN := iSendLen,
                   DONE=> bSendDone,
                   ERROR=> bSendError,
                   STATUS=> wSendStatus);

    // Reset the trigger
    bSendReq := FALSE;

    // Handle BSEND DONE / ERROR transitions in calling logic
END_FUNCTION_BLOCK

The block uses the same connection ID twice (0x1001 / 0x1002) for clarity; in production create one NetPro connection per direction and reference the IDs in their respective BSEND/BRECV calls.

11. Commissioning Verification Procedure

1. Open NetPro and confirm both the BSEND and BRECV connections show the green established icon.
2. Open HW Config, verify the driver for the target port is set to ASCII, baud rate and frame match the partner.
3. Download the entire station (CPU + CP) and ensure the CP STOP/RUN transitions are clean.
4. Place a temporary breakpoint or flag monitoring wSendStatus and wRecvStatus in the online watch table.
5. Trigger a 10-byte ASCII write of known content (e.g., "TEST12345\n"). Observe the TX LED flicker on the CP.
6. Capture the partner side with a serial tap or port monitor. Confirm the byte stream matches what was sent.
7. Trigger an echo test if the partner supports it; confirm BRECV reports NDR=TRUE and the byte count equals the echoed length.
8. If TX never activates, immediately re-check ID format (hex vs decimal) and the connection download state.

12. Cross-Reference to Companion CP Manuals

• CP 441-2 Point-to-Point Communication, Installation and Parameter Assignment (ID 1137419)
• Data Transmission with the ASCII Driver, S7-300 / S7-400 (ID 1188835)
• PtP Parameter Assignment Tool download (ID 27013524)
• CP 441-2 configuration - TIA Documentation Cloud

All status code interpretations above should be cross-checked against these references when commissioning new firmware revisions. The CP 441-2 module underwent several firmware updates; some STATUS values were reclassified across revisions. Always note the firmware version reported in the module's diagnostic buffer before escalating an issue.