--- a/FC-handset-spec	Fri Jun 11 08:04:44 2021 +0000
+++ b/FC-handset-spec	Fri Jun 11 18:29:06 2021 +0000
@@ -809,12 +809,12 @@
 USB UART
 DTE signal	Calypso signal
 ------------------------------
-TxD		RX_IRDA
-RxD		TX_IRDA
+TxD2		RX_IRDA
+RxD2		TX_IRDA
 
 However, as shown in the next section, these signals cannot be simply connected
-between Calypso and FT2232x, and a more complicated scheme needs to be
-implemented instead.
+between Calypso and FT2232x, instead a more complicated scheme needs to be
+implemented.
 
 1.12.2. USB and mobile power domains
 
@@ -916,3 +916,26 @@
 However, this current will be limited to a maximum of about 40 uA
 ((4.2-3.3 V) / 22 kOhm), and my feeling is that such small current won't hurt
 Nexperia LVC buffers.
+
+1.12.2.2.1. Signal pull-up/down directions
+
+Of the 4 UART signals going from FT2232x to Calypso (see section 1.12.1), TxD
+and TxD2 MUST be sensed high by Calypso when no USB host is connected, hence
+they will need the two chained LVC buffers with pull-ups to VBAT in between, as
+described above.  RTS and DTR are less critical in that either high or low level
+would be acceptable, as long as it is stable and not floating.  However, it is
+slightly preferable to have DTR pulled up like TxD (to be sensed as logically
+negated), and to have RTS (flow control signal going to CTS_MODEM) pulled down,
+to be sensed as logically asserted.
+
+The approach chosen by the Mother is to have TxD, TxD2 and DTR pulled up to VBAT
+as described above, and to have RTS pulled down to GND.  The value of the pull-
+down resistor to GND will be 47 kOhm, and the USB-side-powered LVC buffer can be
+skipped for this signal - that buffer is only needed in order to work correctly
+with the pull-up to VBAT.
+
+With only 3 FT2232x-to-Calypso signals needing to go through a USB-side-powered
+LVC buffer, and with 3 slots remaining available in the 74LVC541A buffer from
+section 1.12.2.1, that one 74LVC541A IC can serve both signal directions (5
+signals from Calypso to FT2232x and 3 signals going the other way), reducing the
+component count.