BK Precision 1788B - RS-232/TTL Protocol Documentation

Hardware Setup

Overview

The BK Precision 1788B (0-32VDC, 0-6A programmable DC power supply) has a TTL-level serial interface on a DB9 connector, NOT true RS-232 levels.

Critical Hardware Information

DB9 Pin Configuration (Measured and Verified)

Pin 1 (DCD):  +5V    Output  - "Device present" signal
Pin 2 (RX):   Float  Input   - Data IN to power supply
Pin 3 (TX):   +5V    Output  - Data OUT from power supply (Idle HIGH)
Pin 4 (DTR):  Float  Input   - Not required for basic communication
Pin 5 (GND):  0V     Ground
Pin 6 (DSR):  Float  Input   - Not used
Pin 7 (RTS):  Float  Input   - Not used
Pin 8 (CTS):  Float  Input   - Not used
Pin 9 (RI):   Float  Input   - Not used

Wiring for Direct TTL Connection

USB-TTL Adapter          BK 1788B (DB9)
-----------------        --------------
TX (MOSI)       ──────>  Pin 2 (RX)
RX (MISO)       <──────  Pin 3 (TX)
GND             ──────   Pin 5 (GND)

Note: The IT-E131 cable performs internal pin crossing. When using direct TTL, use straight-through connection as shown above.

Communication Parameters

Baudrate:  4800, 9600, 19200, or 38400 (default: 4800)
Data bits: 8
Parity:    None
Stop bits: 1
Format:    8N1
Handshake: None (RTS/CTS not required)

CRITICAL: Both power supply and computer must be set to the same baud rate. Check power supply settings via front panel MENU > BAUDRATE.

Protocol Specification

Packet Structure

All communication uses fixed 26-byte packets in both directions (command and response).

Byte 0:      0xAA           (Start marker - always this value)
Byte 1:      Address        (0x00-0xFE, default: 0x00)
Byte 2:      Command        (See command list below)
Bytes 3-24:  Data           (22 bytes, command-specific)
Byte 25:     Checksum       (Sum of bytes 0-24, modulo 256)

Checksum Calculation

checksum = sum(packet[0:25]) % 256

Example:

0xAA + 0x00 + 0x26 + (22 × 0x00) = 0xD0

Data Encoding

Command Set

0x20 - Set Remote Control Mode

Purpose: Switch between front-panel and remote (PC) control

Packet:

Byte 0:      0xAA
Byte 1:      0x00 (Address)
Byte 2:      0x20
Byte 3:      0x00 = Front panel mode
             0x01 = Remote control mode
Bytes 4-24:  0x00 (Reserved)
Byte 25:     Checksum

Example (Enable Remote):

AA 00 20 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 CB

Response: Status packet (0x12) with result code

0x21 - Set Output ON/OFF

Purpose: Enable or disable the power supply output

Packet:

Byte 0:      0xAA
Byte 1:      0x00 (Address)
Byte 2:      0x21
Byte 3:      0x00 = Output OFF
             0x01 = Output ON
Bytes 4-24:  0x00 (Reserved)
Byte 25:     Checksum

Example (Output ON):

AA 00 21 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 CC

Response: Status packet (0x12) with result code

Display Indicator: “ON” annunciator lights when output is enabled

0x23 - Set Output Voltage

Purpose: Program the output voltage setpoint

Packet:

Byte 0:      0xAA
Byte 1:      0x00 (Address)
Byte 2:      0x23
Bytes 3-6:   Voltage in mV (4-byte little-endian)
Bytes 7-24:  0x00 (Reserved)
Byte 25:     Checksum

Example (Set 10.00V):

Voltage: 10.000V = 10000 mV = 0x2710
Little-endian: [0x10, 0x27, 0x00, 0x00]

Packet:
AA 00 23 10 27 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 04

Response: Status packet (0x12) with result code

0x24 - Set Output Current

Purpose: Program the output current limit

Packet:

Byte 0:      0xAA
Byte 1:      0x00 (Address)
Byte 2:      0x24
Bytes 3-4:   Current in mA (2-byte little-endian)
Bytes 5-24:  0x00 (Reserved)
Byte 25:     Checksum

Example (Set 1.50A):

Current: 1.50A = 1500 mA = 0x05DC
Little-endian: [0xDC, 0x05]

Packet:
AA 00 24 DC 05 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 1B

Response: Status packet (0x12) with result code

0x26 - Read Status

Purpose: Query all current values, setpoints, and operating state

Packet:

AA 00 26 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 D0

Response Packet Structure:

Byte 0-2:    0xAA 0x00 0x26           (Header)
Bytes 3-4:   Actual current (mA, 2-byte LE)
Bytes 5-8:   Actual voltage (mV, 4-byte LE)
Byte 9:      Status byte (see below)
Bytes 10-11: Current setpoint (mA, 2-byte LE)
Bytes 12-15: Maximum voltage (mV, 4-byte LE)
Bytes 16-19: Voltage setpoint (mV, 4-byte LE)
Bytes 20-24: Reserved
Byte 25:     Checksum

Status Byte (Byte 9) Bit Definitions:

Bit 0: Output state (0=OFF, 1=ON)
Bit 1: Over-temperature protection (0=normal, 1=active)
Bit 2-3: Operating mode
         01 = CV (Constant Voltage)
         10 = CC (Constant Current)
         11 = Unreg
Bit 4-6: Fan speed (0=stopped, 5=maximum)
Bit 7: Control mode (0=Front panel, 1=Remote)

Example Response (5.00V, 0mA, Output ON, CV mode):

AA 00 26 00 00 88 13 00 00 05 28 00 E8 80 00 00 88 13 00 00 01 00 00 00 00 9C

Decoded:
- Actual current:   0x0000 = 0 mA
- Actual voltage:   0x1388 = 5000 mV = 5.00V
- Status:           0x05 = 0b00000101
                    Bit 0=1 (ON), Bit 2=1 (CV mode)
- Current setpoint: 0x0028 = 40 mA = 0.04A
- Voltage setpoint: 0x1388 = 5000 mV = 5.00V

0x12 - Status Response

Purpose: Generic response to commands that don’t return data

Packet:

Byte 0-2:    0xAA 0x00 0x12
Byte 3:      Result code:
             0x80 = Success
             0x90 = Checksum error
             0xA0 = Parameter error
             0xB0 = Unrecognized command
             0xC0 = Invalid command
Bytes 4-24:  0x00 (Reserved)
Byte 25:     Checksum

Tested Communication Sequence

1. Initial Setup

2. Query Initial Status

Send:

AA 00 26 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 D0

Receive: 26-byte status packet with current state

3. Enable Output

Send:

AA 00 21 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 CC

Receive: Status response (0x12) with 0x80 (success)

4. Query Status Again

Send status query (0x26) to see actual output voltage

Observed: Display shows “LINK” indicator during active communication

Debugging Notes from Field Testing

Issue: No Response from Power Supply

Symptoms:

Root Causes Found:

  1. Baud Rate Mismatch: Power supply was set to 9600, adapter to 4800
    • Solution: Verify both sides match (use MENU on power supply)
  2. Wrong Pin Assignment: Initially tried crossed pins (thinking null-modem)
    • Solution: Use straight-through: TX→Pin2, RX←Pin3
  3. Voltage Level Issues: Initial attempt with 3.3V logic
    • Solution: Use 5V TTL levels
  4. Floating RX Pin: Measured 1.4V on Pin 2 (turned out to be 50Hz noise pickup)
    • Solution: Proper grounding, shorter cables

Successful Configuration

Display Indicators

Python Implementation Example

Basic Packet Structure

def create_packet(address, command, data):
    """Create a 26-byte command packet"""
    packet = bytearray(26)
    packet[0] = 0xAA
    packet[1] = address
    packet[2] = command
    
    # Copy data bytes (up to 22 bytes)
    for i, byte in enumerate(data[:22]):
        packet[3 + i] = byte
    
    # Calculate checksum
    packet[25] = sum(packet[0:25]) % 256
    
    return packet

def parse_status(response):
    """Parse 0x26 status response"""
    if len(response) != 26 or response[0] != 0xAA:
        raise ValueError("Invalid packet")
    
    # Extract values (little-endian)
    actual_current = response[3] | (response[4] << 8)  # mA
    actual_voltage = response[5] | (response[6] << 8)  # mV (first 2 bytes)
    status_byte = response[9]
    current_setpoint = response[10] | (response[11] << 8)  # mA
    voltage_setpoint = response[16] | (response[17] << 8)  # mV
    
    # Decode status byte
    output_on = bool(status_byte & 0x01)
    mode = (status_byte >> 2) & 0x03  # 1=CV, 2=CC, 3=Unreg
    
    return {
        'actual_voltage': actual_voltage / 1000.0,  # Convert to V
        'actual_current': actual_current / 1000.0,  # Convert to A
        'voltage_setpoint': voltage_setpoint / 1000.0,
        'current_setpoint': current_setpoint / 1000.0,
        'output_on': output_on,
        'mode': ['Unknown', 'CV', 'CC', 'Unreg'][mode]
    }

Command Examples

import serial

# Open serial port
ser = serial.Serial('COM3', 4800, timeout=1)

# Query status
cmd = create_packet(0x00, 0x26, [0]*22)
ser.write(cmd)
response = ser.read(26)
status = parse_status(response)
print(f"Voltage: {status['actual_voltage']:.2f}V")
print(f"Current: {status['actual_current']:.3f}A")

# Set voltage to 12.00V
voltage_mv = int(12.0 * 1000)  # 12000 mV
data = [
    voltage_mv & 0xFF,
    (voltage_mv >> 8) & 0xFF,
    0, 0  # Upper bytes for 4-byte value
]
cmd = create_packet(0x00, 0x23, data + [0]*18)
ser.write(cmd)
response = ser.read(26)  # Should be status response (0x12)

# Enable output
cmd = create_packet(0x00, 0x21, [0x01] + [0]*21)
ser.write(cmd)
response = ser.read(26)

Testing Checklist

References

Document Version

Version: 1.0
Date: 2025-11-11
Based on: Field testing and protocol analysis with Bus Pirate 6XL
Hardware Tested: BK Precision 1788B (32V/6A model)
Status: Verified and working

Additional Notes

Known Limitations

Future Enhancements

Safety Notes