Legacy Dish Network Switch

The SkyWalker-1 supports legacy Dish Network satellite switches (SW21, SW44) through vendor command 0x8F (SET_DN_SWITCH). This protocol predates DiSEqC and uses a simpler serial bit-bang scheme on a shared GPIO pin.

Protocol Overview

The legacy Dish Network switch protocol sends a 7-bit serial command via GPIO P0.4 (the same pin used for LNB voltage selection). The command is bit-banged LSB-first with fixed timing delays.

Note

The 8th bit (bit 7, value 0x80) of the original Dish Network switch command controls LNB voltage polarity and is not sent via this protocol. Instead, the kernel driver sends it separately via SET_LNB_VOLTAGE (command 0x8B). This split between data bits and voltage is handled by the send_legacy_dish_cmd callback in the kernel driver.

USB Command Format

USB SETUP:
  bmRequestType = 0x40 (vendor, host-to-device)
  bRequest      = 0x8F (SET_DN_SWITCH)
  wValue        = 7-bit switch command (bits 0-6)
  wIndex        = 0x0000
  wLength       = 0

Bit-Bang Sequence

The firmware transmits the 7-bit command using the following GPIO sequence:

1. Start pulse: Assert P0.4 HIGH, delay approximately 32 CPU cycles (~0.67 us at 48 MHz)

2. Gap: De-assert P0.4 LOW, delay approximately 8 CPU cycles (~0.17 us at 48 MHz)

3. Data bits: Shift out 7 bits LSB-first via P0.4, with approximately 8-cycle delays between each bit transition

4. End: P0.4 returns to its previous LNB voltage state

Switch Position Mapping

The legacy protocol supports the SW21 (2-input, 1-output) and SW44 (4-input, 4-output) switches. The 7-bit command encodes the desired input port and satellite selection:

Switch Type	Bit Pattern	Function
SW21	Port A 0bxxxxxxx with port bit = 0	Select satellite A input
SW21	Port B 0bxxxxxxx with port bit = 1	Select satellite B input
SW44	Multiple bit fields	Select one of 4 inputs to one of 4 outputs

The exact bit-field layout is proprietary to Dish Network. The kernel driver constructs the command value in dishnetwork_send_legacy_command() based on the requested switch position.

Timing Characteristics

The legacy switch protocol is significantly simpler and faster than DiSEqC:

Parameter	Value
Start pulse width	~0.67 us (32 cycles at 48 MHz)
Inter-bit gap	~0.17 us (8 cycles at 48 MHz)
Total command time	~5-10 us for 7 bits
GPIO pin	P0.4 (shared with LNB voltage)

Caution

Because P0.4 is shared between the legacy switch protocol and LNB voltage selection, a switch command briefly disrupts the LNB voltage level. The switch must complete before the voltage is returned to its correct state. The kernel driver handles this coordination automatically.

Kernel Driver Integration

The Linux kernel driver implements legacy Dish Network support through a dedicated callback:

Legacy Dish Network command callback

// Registered as .send_legacy_dish_cmd in dvb_usb_device_properties
static int gp8psk_send_legacy_dish_cmd(struct dvb_frontend *fe, unsigned long cmd)
{
    struct dvb_usb_adapter *adap = fe->dvb->priv;
    struct dvb_usb_device *d = adap->dev;

    // Bit 7 (0x80) controls voltage, sent separately
    gp8psk_usb_out_op(d, SET_LNB_VOLTAGE, (cmd >> 7) & 1, 0, NULL, 0);

    // Bits 0-6 sent via SET_DN_SWITCH
    gp8psk_usb_out_op(d, SET_DN_SWITCH, cmd & 0x7F, 0, NULL, 0);

    return 0;
}

Comparison with DiSEqC

Feature	Legacy Dish Protocol	DiSEqC 1.0
Standard	Proprietary (Dish Network)	EUTELSAT standard
Encoding	Serial bit-bang, LSB-first	Manchester encoding, MSB-first
Carrier	None (direct GPIO)	22 kHz modulated
Message length	7 bits	3-6 bytes (24-54 bits + parity)
Timing	~5-10 us total	~48-88 ms total
Switch support	SW21, SW44	DiSEqC 1.0/1.1/1.2 compatible switches
GPIO pin	P0.4 (shared with voltage)	P0.x (data) + P0.3 (carrier)
Vendor command	0x8F	0x8D

For modern satellite installations, DiSEqC is the preferred switch protocol. The legacy Dish Network protocol exists for backward compatibility with older SW21 and SW44 switches commonly found in North American installations. See the DiSEqC Protocol page for the full DiSEqC implementation.