diff --git a/sim_tmxr.c b/sim_tmxr.c
index 6efff478..75874755 100644
--- a/sim_tmxr.c
+++ b/sim_tmxr.c
@@ -439,6 +439,8 @@ static u_char mantra[] = {                  /* Telnet Option Negotiation Mantra
     TN_IAC, TN_DO, TN_BIN
     };
 
+#define TMXR_GUARD  ((int32)(lp->serport ? 0 : sizeof(mantra)))/* buffer guard */
+
 /* Local routines */
 
 static void tmxr_add_to_open_list (TMXR* mux);
@@ -2027,7 +2029,7 @@ if ((lp->conn == FALSE) &&                              /* no conn & not buffere
     return SCPE_LOST;
     }
 tmxr_debug_trace_line (lp, "tmxr_putc_ln()");
-#define TXBUF_AVAIL(lp) (lp->txbsz - tmxr_tqln (lp))
+#define TXBUF_AVAIL(lp) ((lp->serport ? 1: lp->txbsz) - tmxr_tqln (lp))
 #define TXBUF_CHAR(lp, c) {                               \
     lp->txb[lp->txbpi++] = (char)(c);                     \
     lp->txbpi %= lp->txbsz;                               \
diff --git a/sim_tmxr.h b/sim_tmxr.h
index b90b3f4e..6d84aa8e 100644
--- a/sim_tmxr.h
+++ b/sim_tmxr.h
@@ -65,7 +65,6 @@ typedef int SERHANDLE;
 #define TMXR_V_VALID    15
 #define TMXR_VALID      (1 << TMXR_V_VALID)
 #define TMXR_MAXBUF     256                             /* buffer size */
-#define TMXR_GUARD      12                              /* buffer guard */
 
 #define TMXR_DTR_DROP_TIME 500                          /* milliseconds to drop DTR for 'pseudo' modem control */
 #define TMXR_MODEM_RING_TIME 3                          /* seconds to wait for DTR for incoming connections */