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Hardware Errata

Transmitter (TXB)

TXB#1: i2c bug / transmit output level

Description

From hardware version 5.0 to 7.0, the TXB boards have a severe hardware bug associated with the wiring of the potentiometer to control the output transmit level (tx gain). This bug may cause the modem to become unresponsive (broken UART communication) or lead to dysfunctional receiver gain control (because receiver and transmitter share the same i2c bus).

Solution

  • This bug has been fixed in version 7.1 of TXB, which require firmware commit c1aa52ac or newer.
  • Affected boards (see below) must be manually fixed as described.

The bug can only be fixed by a small modification of TXB:

  1. The potentiometer POTI must be removed from the circuit.
  2. A resistor must be added to set a fixed gain. We suggest to use 10k, which results in maximum output level. You must not use a larger resistor; a smaller value may be used for decreased output level (this is not encouraged). There are two ways to insert the resistor (only use one of them!):
    1. Put a resistor on top of the capacitor C122 as indicated in the picture. Make sure that the resistor is connected on both sides properly.
    2. Replace resistor R122 and short pins 5 and 6 of the potentiometer. This is effectively shorting the path through the (removed) potentiometer, which is parallel to C122.

A transmitter board with fixed bug is shown below. The POTI IC has been removed and a 10k resistor has been stacked on top of C122.

Side Effects

  • After application of the bug fix, the modem no longer supports controlling the output level of acoustic transmissions (TX gain). When calling "txgain" from MoSh (or sending the corresponding control message to the modem through other means), it will send the i2c command and report the current gain level. However, this information is invalid.
  • When using a modem with TXB version 7.1 or above, a different potentiometer is being used. This requires a compatible firmware (commit c1aa52ac or newer).