From fac557150de39742126d9829d50c13df790d450e Mon Sep 17 00:00:00 2001
From: Bob Supnik <bob@supnik.org>
Date: Sun, 1 Mar 2020 18:23:41 -0800
Subject: [PATCH] PDP11, VAX: Fixed DZ race condition for multiple transmitters
 (Mark Pizzolato)

---
 PDP11/pdp11_dz.c | 28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/PDP11/pdp11_dz.c b/PDP11/pdp11_dz.c
index d146a895..4e51419e 100644
--- a/PDP11/pdp11_dz.c
+++ b/PDP11/pdp11_dz.c
@@ -1,6 +1,6 @@
 /* pdp11_dz.c: DZ11 terminal multiplexor simulator
 
-   Copyright (c) 2001-2008, Robert M Supnik
+   Copyright (c) 2001-2020, Robert M Supnik
 
    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the "Software"),
@@ -25,6 +25,7 @@
 
    dz           DZ11 terminal multiplexor
 
+   14-Feb-20    RMS     Fixed race condition for multiple transmitters (Mark Pizzolato)
    29-Dec-08    RMS     Added MTAB_NC to SET LOG command (Walter Mueller)
    19-Nov-08    RMS     Revised for common TMXR show routines
    18-Jun-07    RMS     Added UNIT_IDLE flag
@@ -373,7 +374,8 @@ switch ((PA >> 1) & 03) {                               /* case on PA<2:1> */
             if (c >= 0)                                 /* store char */
                 tmxr_putc_ln (lp, c);
             tmxr_poll_tx (&dz_desc);                    /* poll output */
-            dz_update_xmti ();                          /* update int */
+            dz_csr[dz] &= ~CSR_TRDY;                    /* clear TRDY for now */
+            dz_update_xmti();                           /* update int */
             }
         break;
         }
@@ -474,16 +476,18 @@ void dz_update_xmti (void)
 int32 dz, linemask, i, j, line;
 
 for (dz = 0; dz < DZ_MUXES; dz++) {                     /* loop thru muxes */
-    linemask = dz_tcr[dz] & DZ_LMASK;                   /* enabled lines */
-    dz_csr[dz] &= ~CSR_TRDY;                            /* assume not rdy */
-    j = CSR_GETTL (dz_csr[dz]);                         /* start at current */
-    for (i = 0; i < DZ_LINES; i++) {                    /* loop thru lines */
-        j = (j + 1) & DZ_LNOMASK;                       /* next line */
-        line = (dz * DZ_LINES) + j;                     /* get line num */
-        if ((linemask & (1 << j)) && dz_ldsc[line].xmte) {
-            CSR_PUTTL (dz_csr[dz], j);                  /* put ln in csr */
-            dz_csr[dz] |= CSR_TRDY;                     /* set xmt rdy */
-            break;
+    if ((dz_csr[dz] & CSR_TRDY) == 0) {                 /* xmiting? */
+        linemask = dz_tcr[dz] & DZ_LMASK;               /* enabled lines */
+        dz_csr[dz] &= ~CSR_TRDY;                        /* assume not rdy */
+        j = CSR_GETTL (dz_csr[dz]);                     /* start at current */
+        for (i = 0; i < DZ_LINES; i++) {                /* loop thru lines */
+            j = (j + 1) & DZ_LNOMASK;                   /* next line */
+            line = (dz * DZ_LINES) + j;                 /* get line num */
+            if ((linemask & (1 << j)) && dz_ldsc[line].xmte) {
+                CSR_PUTTL(dz_csr[dz], j);               /* put ln in csr */
+                dz_csr[dz] |= CSR_TRDY;                 /* set xmt rdy */
+                break;
+                }
             }
         }
     if ((dz_csr[dz] & CSR_TIE) && (dz_csr[dz] & CSR_TRDY)) /* ready plus int? */