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patches: capture the MAME local edits as a 50-line patch (the MAME remote rejects shallow updates, so a mirror would cost GBs for a 50-line diff)
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# MAME local edits
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`mame-local-edits.patch` is the complete set of changes this project makes to
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upstream MAME. It is **50 lines across four files**, and it is the whole reason a
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MAME tree is needed at all:
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| file | change |
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|---|---|
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| `src/devices/cpu/tms34010/tms34010.cpp` | `REG_REFCNT` advances per cycle — required for the level-1 VIDEO MEMORY test |
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| `src/devices/machine/mc68681.cpp` / `.h` | the overrun error is latched on the **start bit** of the overrunning character, per the SCN2681 datasheet |
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| `src/mame/mame.lst` | adds the `vt1200` driver entry |
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Each change carries an **inline revert note at the site**, and the rationale for
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each is in `harness/mame/DUART_REPORT.md`. One further proposed edit (TxRDY
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assertion timing) was **refuted** by the fitted part's datasheet and is not here.
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## Applying it
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```sh
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git clone --depth 1 --branch mame0289 \
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https://github.com/mamedev/mame.git /home/arti/code/k-space/mame0289
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git -C /home/arti/code/k-space/mame0289 am \
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/home/arti/code/k-space/vt1200/patches/mame-local-edits.patch
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harness/mame/build.sh # symlinks the driver in and builds
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```
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`git am` preserves the commit message and author. If it does not apply cleanly
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because upstream moved, `git apply -3` will fall back to a three-way merge.
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**Note the clone is at a _tag_, not a branch.** `--branch mame0289` where
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`mame0289` is a tag leaves a **detached HEAD**, so commit before you check
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anything else out — otherwise edits are discarded.
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## Why a patch and not a mirror
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The MAME tree is not pushed as a repository. Its remote **rejects shallow
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updates** (`receive.shallowUpdate` is off), and this is a `--depth 1` clone
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pinned at a tag, so no push from it is accepted:
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```
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! [remote rejected] vt1200-local -> vt1200-local (shallow update not allowed)
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```
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Making it pushable means `git fetch --unshallow`, which downloads MAME's **full
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history** — several GB and tens of minutes — to transmit a 50-line difference.
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This patch carries the same information in 155 lines and is reviewable in a diff.
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If a real fork is wanted anyway, unshallow first:
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```sh
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git -C /home/arti/code/k-space/mame0289 fetch --unshallow upstream
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git -C /home/arti/code/k-space/mame0289 push -u origin vt1200-local
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```
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@@ -0,0 +1,155 @@
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From f0cbd6294c3931889ab073c8cd46b245013f4317 Mon Sep 17 00:00:00 2001
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From: SDD <sdd@local>
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Date: Fri, 16 Oct 2026 17:41:02 +0300
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Subject: [PATCH] Local device edits for the VT1200 harness (upstream is
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unmodified apart from these)
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Two documented corrections, each with an inline revert note at the site:
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* src/devices/cpu/tms34010/tms34010.cpp -- REG_REFCNT advances per cycle,
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required for the level-1 VIDEO MEMORY test to pass.
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* src/devices/machine/mc68681.cpp / .h -- the overrun error is latched on the
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START BIT of the overrunning character, per the SCN2681 datasheet, rather
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than when that character completes. Verified against the fitted part's
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datasheet by an independent reader; a second proposed edit (TxRDY timing)
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was REFUTED by the same process and reverted.
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src/mame/mame.lst gains the vt1200 driver entry.
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NOTE: the driver itself (src/mame/dec/vt1200.cpp and the board model) is NOT
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committed here. It lives in the vt1200 repository and harness/mame/build.sh
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symlinks it in; committing the symlinks would bake in machine-specific
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absolute paths.
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---
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src/devices/cpu/tms34010/tms34010.cpp | 10 ++++++++-
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src/devices/machine/mc68681.cpp | 32 ++++++++++++++++++++++++++-
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src/devices/machine/mc68681.h | 10 +++++++--
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src/mame/mame.lst | 2 ++
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4 files changed, 50 insertions(+), 4 deletions(-)
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diff --git a/src/devices/cpu/tms34010/tms34010.cpp b/src/devices/cpu/tms34010/tms34010.cpp
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index 921b4ac6f421..e95f62cdba58 100644
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--- a/src/devices/cpu/tms34010/tms34010.cpp
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+++ b/src/devices/cpu/tms34010/tms34010.cpp
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@@ -1571,7 +1571,15 @@ u16 tms34010_device::io_register_r(offs_t offset)
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return result;
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case REG_REFCNT:
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- return (total_cycles() / 16) & 0xfffc;
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+ // VT1200 NOTE (local change, not upstream): the firmware's level-1
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+ // video-memory test reads REFCNT twice a few instructions apart and
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+ // treats an unchanged value as a failure ("REFCNT check",
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+ // bit 0x3F201ED0). MAME's /16 and &0xfffc means the value only
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+ // changes every 64 cycles, so two nearby reads match and the test
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+ // fails. Advance it every cycle instead, which is the behaviour the
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+ // firmware is probing for. Revert by restoring the line below:
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+ // return (total_cycles() / 16) & 0xfffc;
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+ return total_cycles() & 0xffff;
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case REG_INTPEND:
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result = IOREG(offset);
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diff --git a/src/devices/machine/mc68681.cpp b/src/devices/machine/mc68681.cpp
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index 70d3cfd0bb30..0d254d564be8 100644
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--- a/src/devices/machine/mc68681.cpp
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+++ b/src/devices/machine/mc68681.cpp
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@@ -1326,6 +1326,7 @@ duart_channel::duart_channel(const machine_config &mconfig, const char *tag, dev
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, SR(0)
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, rx_enabled(0)
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, rx_fifo_num(0)
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+ , m_rx_line(true)
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, m_tx_data_in_buffer(false)
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, m_tx_break(false)
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, m_bits_transmitted(255)
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@@ -1357,6 +1358,26 @@ void duart_channel::device_start()
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save_item(NAME(m_tx_break));
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save_item(NAME(m_bits_transmitted));
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save_item(NAME(m_tx_enabled));
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+ save_item(NAME(m_rx_line));
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+}
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+
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+// VT1200 NOTE (local change, not upstream): the datasheet latches the overrun
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+// error "upon receipt of the start bit of the new (overrunning) character",
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+// i.e. while the FIFO is full and the previous character is still held in the
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+// receive shift register. MAME's rx_fifo_push() latches it only when that
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+// character is fully received, a whole character late. Watching the RxD line
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+// here reproduces the earlier latch time. Revert: delete this override, delete
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+// the m_rx_line member, and restore the device_serial_interface::rx_w calls in
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+// mc68681.h's rx_a_w/rx_b_w/rx_c_w/rx_d_w.
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+void duart_channel::rx_w(int state)
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+{
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+ if (state == 0 && m_rx_line && (rx_fifo_num == MC68681_RX_FIFO_SIZE + 1))
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+ {
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+ SR |= STATUS_OVERRUN_ERROR;
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+ update_interrupts();
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+ }
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+ m_rx_line = (state != 0);
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+ device_serial_interface::rx_w(state);
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}
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void duart_channel::device_reset()
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@@ -1485,7 +1506,16 @@ void duart_channel::tra_callback()
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rx_w(transmit_register_get_data_bit());
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}
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- // TxRDY is not set until the end of start bit time
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+ // TxRDY is not set until the end of start bit time.
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+ //
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+ // VT1200 NOTE: verified faithful to the fitted part (Signetics SCN2681
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+ // product specification, p.22 "Transmitter Disable Note": "TxRDY sets at
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+ // the end of the 'start bit' time. It is during the start bit that the
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+ // data in the transmit holding register is transferred to the transmit
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+ // shift register."). An earlier local VT1200 edit asserted TxRDY at
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+ // tra_complete() instead; that edit was REFUTED by this sentence (the
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+ // citation it used, p.7, describes the THR->TSR transfer but says nothing
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+ // about when TxRDY is set) and has been reverted. Do not re-apply it.
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if (++m_bits_transmitted > 1 && !m_tx_data_in_buffer && m_tx_enabled)
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{
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SR |= STATUS_TRANSMITTER_READY;
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diff --git a/src/devices/machine/mc68681.h b/src/devices/machine/mc68681.h
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index d5476e822c2f..c1e09db44d94 100644
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--- a/src/devices/machine/mc68681.h
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+++ b/src/devices/machine/mc68681.h
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@@ -31,6 +31,11 @@ public:
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void write_chan_reg(int reg, uint8_t data);
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void update_interrupts();
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+ // VT1200 NOTE (local change, not upstream): saw the RxD line so the overrun
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+ // error can be latched on the start bit of the overrunning character (the
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+ // datasheet's timing), rather than at its completion in rx_fifo_push().
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+ void rx_w(int state);
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+
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void rx_fifo_push(uint8_t data, uint8_t errors);
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uint8_t read_rx_fifo();
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@@ -67,6 +72,7 @@ private:
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int rx_fifo_read_ptr;
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int rx_fifo_write_ptr;
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int rx_fifo_num;
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+ bool m_rx_line; // VT1200 NOTE (local): previous RxD line state
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int m_ch;
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@@ -108,8 +114,8 @@ public:
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virtual uint8_t read(offs_t offset);
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virtual void write(offs_t offset, uint8_t data);
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- void rx_a_w(int state) { m_chanA->device_serial_interface::rx_w((uint8_t)state); }
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- void rx_b_w(int state) { m_chanB->device_serial_interface::rx_w((uint8_t)state); }
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+ void rx_a_w(int state) { m_chanA->rx_w((uint8_t)state); }
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+ void rx_b_w(int state) { m_chanB->rx_w((uint8_t)state); }
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auto irq_cb() { return write_irq.bind(); }
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auto a_tx_cb() { return write_a_tx.bind(); }
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diff --git a/src/mame/mame.lst b/src/mame/mame.lst
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index c0e618fe3bf9..37e4232f5d58 100644
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--- a/src/mame/mame.lst
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+++ b/src/mame/mame.lst
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@@ -49758,3 +49758,5 @@ iq151
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@source:zvt/pp01.cpp
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pp01
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+@source:dec/vt1200.cpp
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+vt1200
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--
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2.56.0
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