diff --git a/src/devices/cpu/upd7810/upd7810.cpp b/src/devices/cpu/upd7810/upd7810.cpp
index 929070e61fc..1f899b0b547 100644
--- a/src/devices/cpu/upd7810/upd7810.cpp
+++ b/src/devices/cpu/upd7810/upd7810.cpp
@@ -1113,6 +1113,7 @@ void upd7810_device::upd7810_sio_input()
 				case 0x48:  /* 7bits, no parity, 1 stop bit */
 				case 0x68:  /* 7bits, no parity, 1 stop bit (parity select = 1 but parity is off) */
 					m_rxs >>= 16 - 9;
+					m_rxs |= ~0 << 9;
 					RXB = (m_rxs >> 1) & 0x7f;
 					if ((1 << 8) != (m_rxs & (1 | (1 << 8))))
 						IRR |= INTER;   /* framing error */
@@ -1120,12 +1121,14 @@ void upd7810_device::upd7810_sio_input()
 				case 0x4c:  /* 8bits, no parity, 1 stop bit */
 				case 0x6c:  /* 8bits, no parity, 1 stop bit (parity select = 1 but parity is off) */
 					m_rxs >>= 16 - 10;
+					m_rxs |= ~0 << 10;
 					RXB = (m_rxs >> 1) & 0xff;
 					if ((1 << 9) != (m_rxs & (1 | (1 << 9))))
 						IRR |= INTER;   /* framing error */
 					break;
 				case 0x58:  /* 7bits, odd parity, 1 stop bit */
 					m_rxs >>= 16 - 10;
+					m_rxs |= ~0 << 10;
 					RXB = (m_rxs >> 1) & 0x7f;
 					if ((1 << 9) != (m_rxs & (1 | (1 << 9))))
 						IRR |= INTER;   /* framing error */
@@ -1134,6 +1137,7 @@ void upd7810_device::upd7810_sio_input()
 					break;
 				case 0x5c:  /* 8bits, odd parity, 1 stop bit */
 					m_rxs >>= 16 - 11;
+					m_rxs |= ~0 << 11;
 					RXB = (m_rxs >> 1) & 0xff;
 					if ((1 << 10) != (m_rxs & (1 | (1 << 10))))
 						IRR |= INTER;   /* framing error */
@@ -1142,6 +1146,7 @@ void upd7810_device::upd7810_sio_input()
 					break;
 				case 0x78:  /* 7bits, even parity, 1 stop bit */
 					m_rxs >>= 16 - 10;
+					m_rxs |= ~0 << 10;
 					RXB = (m_rxs >> 1) & 0x7f;
 					if ((1 << 9) != (m_rxs & (1 | (1 << 9))))
 						IRR |= INTER;   /* framing error */
@@ -1150,6 +1155,7 @@ void upd7810_device::upd7810_sio_input()
 					break;
 				case 0x7c:  /* 8bits, even parity, 1 stop bit */
 					m_rxs >>= 16 - 11;
+					m_rxs |= ~0 << 11;
 					RXB = (m_rxs >> 1) & 0xff;
 					if ((1 << 10) != (m_rxs & (1 | (1 << 10))))
 						IRR |= INTER;   /* framing error */
@@ -1159,6 +1165,7 @@ void upd7810_device::upd7810_sio_input()
 				case 0xc8:  /* 7bits, no parity, 2 stop bits */
 				case 0xe8:  /* 7bits, no parity, 2 stop bits (parity select = 1 but parity is off) */
 					m_rxs >>= 16 - 10;
+					m_rxs |= ~0 << 10;
 					RXB = (m_rxs >> 1) & 0x7f;
 					if ((3 << 9) != (m_rxs & (1 | (3 << 9))))
 						IRR |= INTER;   /* framing error */
@@ -1168,6 +1175,7 @@ void upd7810_device::upd7810_sio_input()
 				case 0xcc:  /* 8bits, no parity, 2 stop bits */
 				case 0xec:  /* 8bits, no parity, 2 stop bits (parity select = 1 but parity is off) */
 					m_rxs >>= 16 - 11;
+					m_rxs |= ~0 << 11;
 					RXB = (m_rxs >> 1) & 0xff;
 					if ((3 << 10) != (m_rxs & (1 | (3 << 10))))
 						IRR |= INTER;   /* framing error */
@@ -1176,6 +1184,7 @@ void upd7810_device::upd7810_sio_input()
 					break;
 				case 0xd8:  /* 7bits, odd parity, 2 stop bits */
 					m_rxs >>= 16 - 11;
+					m_rxs |= ~0 << 11;
 					RXB = (m_rxs >> 1) & 0x7f;
 					if ((3 << 10) != (m_rxs & (1 | (3 << 10))))
 						IRR |= INTER;   /* framing error */
@@ -1184,6 +1193,7 @@ void upd7810_device::upd7810_sio_input()
 					break;
 				case 0xdc:  /* 8bits, odd parity, 2 stop bits */
 					m_rxs >>= 16 - 12;
+					m_rxs |= ~0 << 12;
 					RXB = (m_rxs >> 1) & 0xff;
 					if ((3 << 11) != (m_rxs & (1 | (3 << 11))))
 						IRR |= INTER;   /* framing error */
@@ -1192,6 +1202,7 @@ void upd7810_device::upd7810_sio_input()
 					break;
 				case 0xf8:  /* 7bits, even parity, 2 stop bits */
 					m_rxs >>= 16 - 11;
+					m_rxs |= ~0 << 11;
 					RXB = (m_rxs >> 1) & 0x7f;
 					if ((3 << 10) != (m_rxs & (1 | (3 << 10))))
 						IRR |= INTER;   /* framing error */
@@ -1200,6 +1211,7 @@ void upd7810_device::upd7810_sio_input()
 					break;
 				case 0xfc:  /* 8bits, even parity, 2 stop bits */
 					m_rxs >>= 16 - 12;
+					m_rxs |= ~0 << 12;
 					RXB = (m_rxs >> 1) & 0xff;
 					if ((3 << 11) != (m_rxs & (1 | (3 << 11))))
 						IRR |= INTER;   /* framing error */
@@ -1211,6 +1223,7 @@ void upd7810_device::upd7810_sio_input()
 			else
 			{
 				m_rxs >>= 16 - 8;
+				m_rxs |= ~0 << 8;
 				RXB = m_rxs;
 //              m_rxcnt = 8;
 			}
@@ -1870,7 +1883,7 @@ void upd7810_device::device_reset()
 	m_int2 = 1; // physical (inverted) INT2 line state
 
 	m_txs = 0;
-	m_rxs = 0;
+	m_rxs = ~0;
 	m_txcnt = 0;
 	m_rxcnt = 0;
 	m_txbuf = 0;