diff --git a/emu/Makefile b/emu/Makefile
index 2445267..eac4f5c 100644
--- a/emu/Makefile
+++ b/emu/Makefile
@@ -3,12 +3,14 @@ H=pdp6.h ../tools/pdp6common.h
 # clang
 #CFLAGS= -Wno-shift-op-parentheses -Wno-logical-op-parentheses \
 #        -Wno-bitwise-op-parentheses
-CFLAGS=  -g -Wall -Wno-parentheses -fno-diagnostics-show-caret \
-	`sdl-config --cflags` `pkg-config SDL_image --cflags`
+CFLAGS=  -g -Wall -Wno-parentheses -fno-diagnostics-show-caret
+SDLFLAGS=`sdl-config --cflags` `pkg-config SDL_image --cflags`
 
-LIBS=	`sdl-config --libs` `pkg-config SDL_image --libs` -lpthread
+LIBS=`sdl-config --libs` `pkg-config SDL_image --libs` -lpthread
 
 
 pdp6: $(SRC) $(H)
-	$(CC) $(CFLAGS) $(SRC) $(LIBS) -o pdp6
+	$(CC) -o $@ $(CFLAGS) $(SDLFLAGS) $(SRC) $(LIBS)
 
+test_dc: test_dc.c dc.c $(H)
+	$(CC) -o $@ $(CFLAGS) test_dc.c dc.c
diff --git a/emu/dc.c b/emu/dc.c
index a25e9d3..4350fd7 100644
--- a/emu/dc.c
+++ b/emu/dc.c
@@ -9,13 +9,17 @@ char *dc_ident = DC_IDENT;
  *            18      1 1                3               2
  *            36      0 1                0               1
  */
-#define NSHIFT ((0x70 >> (dc->ch_mode*2)) & 03)
+#define NSHIFT ((0xE0 >> (dc->ch_mode*2)) & 03)
 #define NCHARS ((02316 >> (dc->ch_mode*3)) & 07)
 #define DC_SCT_DONE (dc->sct == NSHIFT)
 #define DC_CCT_DONE (dc->cct == NCHARS-1)
 //#define DC_CCT_CONT (DC_SCT_DONE && !(DC_CCT_DONE))
 //#define DC_SCT_CONT (!(DC_SCT_DONE) && !(DC_CCT_DONE))
 
+#define DB_JM_DA (dc->db = dc->da), dc->da = 0
+#define DA_JM_DB (dc->da = dc->db)
+#define DB_CLR dc->db = dc->db_rq = 0
+
 static void
 recalc_dc_req(Dc136 *dc)
 {
@@ -23,18 +27,38 @@ recalc_dc_req(Dc136 *dc)
 }
 
 static void
-dbdamove(Dc136 *dc)
+db_da_swap(Dc136 *dc)
 {
 	dc->da_rq = 0;
 	dc->dbda_move = 0;
 	dc->db_rq = 1;
 
 	if(dc->inout)
-		dc->da = dc->db;	// out
-	else{
-		dc->db = dc->da;	// in
-		dc->da = 0;
-	}
+		DA_JM_DB;	// out
+	else
+		DB_JM_DA;	// in
+}
+
+/* TODO: maybe make these two a bit nicer?? */
+static void
+set_da_rq(Dc136 *dc)
+{
+	int prev;
+
+	prev = dc->da_rq && dc->dbda_move;
+	dc->da_rq = 1;
+	if(!prev && dc->da_rq && dc->dbda_move)
+		db_da_swap(dc);
+}
+static void
+set_dbda_move(Dc136 *dc)
+{
+	int prev;
+
+	prev = dc->da_rq && dc->dbda_move;
+	dc->dbda_move = 1;
+	if(!prev && dc->da_rq && dc->dbda_move)
+		db_da_swap(dc);
 }
 
 static void
@@ -45,39 +69,37 @@ cp0:
 	if(dc->da_rq)
 		dc->data_clbd = 1;
 
+	// 1ms delay
+
 	// DASH CP1
 	if(DC_CCT_DONE){
 //printf("Full DA: %012lo\n", dc->da);
 		dc->cct = 0;
-		if(dc->da_rq == 0){
-			dc->da_rq = 1;
-			if(dc->dbda_move)
-				dbdamove(dc);
-		}else
-			dc->da_rq = 1;
+		set_da_rq(dc);
+	}else if(DC_SCT_DONE){
+		// CCT CONT
+		dc->sct = 0;
+		dc->cct = (dc->cct+1) & 7;
 	}else{
-		if(DC_SCT_DONE){
-			// CCT CONT
-			dc->cct++;
-			// this shouldn't be under the !CCT_DONE branch
-			// but the flow diagram FD1 does it too
-			dc->sct = 0;
-		}else{
-			// SCT CONT
-			dc->sct++;
+		// SCT CONT
+		dc->sct = (dc->sct+1) & 3;
 
-			// DASH LT
-			dc->da <<= 6;
-			goto cp0;
-		}
+		// DASH LT
+		dc->da = (dc->da << 6) & FW;
+		goto cp0;
 	}
 }
 
+/* Give TO DC, this is the TK signal in the DC.
+ * NB: for dev 5/6 give only the lower N bits. */
 void
 dcgv(Dc136 *dc, int n, word c, int rev)
 {
 //printf("DC Taking %06lo from dev %d\n", c, n);
-	switch(n){
+
+	if(n != dc->device)
+		return;
+	switch(dc->device){
 	case 1: case 2:
 		if(rev)
 			dc->da = (dc->da >> 6 | (c&077)<<30) & FW;
@@ -97,11 +119,16 @@ dcgv(Dc136 *dc, int n, word c, int rev)
 	recalc_dc_req(dc);
 }
 
+/* Take FROM DC, this is the GV signal in the DC.
+ * NB: for dev 5/6 read only the upper N bits. */
 word
 dctk(Dc136 *dc, int n, int rev)
 {
 	word c;
-	switch(n){
+
+	if(n != dc->device)
+		return 0;
+	switch(dc->device){
 	case 1: case 2:
 		if(rev){
 			c = dc->da & 077;
@@ -127,36 +154,38 @@ dctk(Dc136 *dc, int n, int rev)
 	return c;
 }
 
+static void
+ic_clr(Dc136 *dc)
+{
+	DB_JM_DA;
+
+	dc->cct = 0;
+	dc->sct = 0;
+	dc->data_clbd = 0;
+	dc->dbda_move = 0;
+	dc->da_rq = 0;
+	dc->db_rq = 0;
+	dc->inout = 0;
+	dc->ch_mode = 0;
+	dc->device = 0;
+	dc->pia = 0;
+}
+
 static void
 wake_dc(void *dev)
 {
 	Dc136 *dc;
 	IOBus *bus;
-	int dbda_move_old, da_rq_old;
 
 	dc = dev;
 	bus = dc->bus;
 
 	if(IOB_RESET){
-		dc->db = 0;
-		dc->da = 0;
-
-		dc->cct = 0;
-		dc->sct = 0;
-		dc->data_clbd = 0;
-		dc->dbda_move = 0;
-		dc->da_rq = 0;
-		dc->db_rq = 0;
-		dc->inout = 0;
-		dc->ch_mode = 0;
-		dc->device = 0;
-		dc->pia = 0;
+		ic_clr(dc);
+		DB_CLR;
 	}
 
 	if(bus->devcode == DC){
-		dbda_move_old = dc->dbda_move;
-		da_rq_old = dc->da_rq;
-
 		if(IOB_STATUS){
 			bus->c12 |= (dc->cct&7) << 13;
 			if(dc->data_clbd) bus->c12 |= F23;
@@ -171,46 +200,27 @@ wake_dc(void *dev)
 		if(IOB_DATAI){
 			bus->c12 |= dc->db;
 			dc->db_rq = 0;
-			dc->dbda_move = 1;
-		}
-		if(IOB_CONO_CLEAR){
-			dc->db = dc->da;
-			dc->da = 0;
-
-			dc->cct = 0;
-			dc->sct = 0;
-			dc->data_clbd = 0;
-			dc->dbda_move = 0;
-			dc->da_rq = 0;
-			dc->db_rq = 0;
-			dc->inout = 0;
-			dc->ch_mode = 0;
-			dc->device = 0;
-			dc->pia = 0;
+			set_dbda_move(dc);
 		}
+		if(IOB_CONO_CLEAR)
+			ic_clr(dc);
 		if(IOB_CONO_SET){
 			if(bus->c12 & F23) dc->data_clbd = 1;
-			if(bus->c12 & F24) dc->dbda_move = 1;
-			if(bus->c12 & F25) dc->da_rq = 1;
+			if(bus->c12 & F24) set_dbda_move(dc);
+			if(bus->c12 & F25) set_da_rq(dc);
 			if(bus->c12 & F26) dc->db_rq = 1;
 			if(bus->c12 & F27) dc->inout = 1;
 			dc->ch_mode |= bus->c12>>6 & 03;
 			dc->device |= bus->c12>>3 & 07;
 			dc->pia |= bus->c12 & 07;
 		}
-		if(IOB_DATAO_CLEAR){
-			dc->db = 0;
-			dc->db_rq = 0;
-		}
+		if(IOB_DATAO_CLEAR)
+			DB_CLR;
 		if(IOB_DATAO_SET){
 			dc->db |= bus->c12;
-			dc->dbda_move = 1;
+			set_dbda_move(dc);
 		}
 
-		if(dc->dbda_move && dc->da_rq &&
-		   !(dbda_move_old && da_rq_old))
-			dbdamove(dc);
-
 		recalc_dc_req(dc);
 	}
 }
diff --git a/emu/init.ini b/emu/init.ini
index c4fdae2..243443d 100644
--- a/emu/init.ini
+++ b/emu/init.ini
@@ -12,6 +12,7 @@ mkdev cmem1 cmem161C mem_1
 mkdev cmem2 cmem161C mem_2
 mkdev cmem3 cmem161C mem_3
 #mkdev netmem netmem its.pdp10.se 10006
+mkdev netmem netmem localhost 10006
 
 connectdev dc dt0
 connectdev dt0 dx0 8
diff --git a/emu/netmem.c b/emu/netmem.c
index 29f0d1c..c251fd2 100644
--- a/emu/netmem.c
+++ b/emu/netmem.c
@@ -120,6 +120,7 @@ makenetmem(int argc, char *argv[])
 	nm->fd = dial(host, port);
 	if(nm->fd < 0)
 		printf("couldn't connect\n");
+	printf("netmem fd: %d\n", nm->fd);
 
 	th = (Thread){ nil, netmemcycle, nm, 50, 0 };
 	addthread(th);
diff --git a/emu/test_dc.c b/emu/test_dc.c
new file mode 100644
index 0000000..1ae65fd
--- /dev/null
+++ b/emu/test_dc.c
@@ -0,0 +1,234 @@
+#include "pdp6.h"
+
+Dc136 *dc;
+IOBus iobus;
+IOBus *bus = &iobus;
+int pireq;
+
+void
+setreq(IOBus *bus, int dev, u8 pia)
+{
+	pireq = pia;
+}
+
+void
+wakedev(IOBus *bus, int d)
+{
+	Busdev *dev;
+	bus->devcode = d;
+	dev = &bus->dev[d];
+	if(dev->wake)
+		dev->wake(dev->dev);
+}
+
+void
+bus_reset(IOBus *bus)
+{
+	int d;
+	bus->c34 |= IOBUS_IOB_RESET;
+	for(d = 0; d < 128; d++)
+		wakedev(bus, d);
+	bus->c34 &= ~IOBUS_IOB_RESET;
+}
+
+void
+cono(IOBus *bus, int d, hword w)
+{
+	bus->c34 |= IOBUS_CONO_CLEAR;
+	wakedev(bus, d);
+	bus->c34 &= ~IOBUS_CONO_CLEAR;
+
+	bus->c34 |= IOBUS_CONO_SET;
+	bus->c12 = w;
+	wakedev(bus, d);
+	bus->c34 &= ~IOBUS_CONO_SET;
+	bus->c12 = 0;
+}
+
+void
+datao(IOBus *bus, int d, word w)
+{
+	bus->c34 |= IOBUS_DATAO_CLEAR;
+	wakedev(bus, d);
+	bus->c34 &= ~IOBUS_DATAO_CLEAR;
+
+	bus->c34 |= IOBUS_DATAO_SET;
+	bus->c12 = w;
+	wakedev(bus, d);
+	bus->c34 &= ~IOBUS_DATAO_SET;
+	bus->c12 = 0;
+}
+
+word
+coni(IOBus *bus, int d)
+{
+	word w;
+	bus->c34 |= IOBUS_IOB_STATUS;
+	wakedev(bus, d);
+	bus->c34 &= ~IOBUS_IOB_STATUS;
+	w = bus->c12;
+	bus->c12 = 0;
+	return w;
+}
+
+word
+datai(IOBus *bus, int d)
+{
+	word w;
+	bus->c34 |= IOBUS_IOB_DATAI;
+	wakedev(bus, d);
+	bus->c34 &= ~IOBUS_IOB_DATAI;
+	w = bus->c12;
+	bus->c12 = 0;
+	return w;
+}
+
+void
+printdc(Dc136 *dc)
+{
+	printf("DB/%012lo DA/%012lo CCT/%o SCT/%o CHMOD/%o\n",
+		dc->db, dc->da, dc->cct, dc->sct, dc->ch_mode);
+	printf(" CLBD/%o DBDAMOVE/%o DARQ/%o DBRQ/%o INOUT/%o DEV/%o PIA/%o\n",
+		dc->data_clbd, dc->dbda_move, dc->da_rq, dc->db_rq,
+		dc->inout, dc->device, dc->pia);
+}
+
+enum {
+	CLBD = 010000,
+
+	DBDAMOVE = 04000,
+	DARQ = 02000,
+	DBRQ = 01000,
+
+	IN = 0000,
+	OUT = 0400,
+
+	CHMOD_6 = 0000,
+	CHMOD_36 = 0100,
+	CHMOD_12 = 0200,
+	CHMOD_18 = 0300,
+
+	DEV1 = 010,
+	DEV2 = 020,
+	DEV3 = 030,
+	DEV4 = 040,
+	DEV5 = 050,
+	DEV6 = 060,
+};
+
+/*
+ * output:
+ *  DARQ:	DA empty, need new data
+ *  DBRQ:	DB empty, need new data
+ *  DBDA MOVE:	DB waiting to send data to DA
+ * input:
+ *  DARQ:	DA full, need to empty
+ *  DBRQ:	DB full, need to empty
+ *  DBDA MOVE:	DB waiting to get data from DA
+ */
+
+int
+main()
+{
+	dc = (Dc136*)makedc(0, nil);
+	dc->dev.ioconnect((Device*)dc, bus);
+
+	bus_reset(bus);
+	assert(coni(bus, DC) == 0);
+
+	// output test
+	cono(bus, DC, DARQ|DBRQ|OUT|CHMOD_6|DEV1|7);	// 3417
+	assert(pireq == 7);
+	printdc(dc);
+	datao(bus, DC, 0123456000001);
+	assert(pireq == 7);
+	printdc(dc);
+	datao(bus, DC, 0123456000002);
+	assert(pireq == 0);
+	printdc(dc);
+	assert(dctk(dc, 1, 0) == 012);
+	assert(pireq == 0);
+	printdc(dc);
+	assert(dctk(dc, 1, 0) == 034);
+	assert(pireq == 0);
+	printdc(dc);
+	assert(dctk(dc, 1, 0) == 056);
+	assert(pireq == 0);
+	printdc(dc);
+	assert(dctk(dc, 1, 0) == 000);
+	assert(pireq == 0);
+	printdc(dc);
+	assert(dctk(dc, 1, 0) == 000);
+	assert(pireq == 0);
+	printdc(dc);
+	assert(dctk(dc, 1, 0) == 001);
+	assert(pireq == 7);
+	printdc(dc);
+
+	printf("\n\n");
+
+	cono(bus, DC, DARQ|DBRQ|OUT|CHMOD_12|DEV5|7);	// 3657
+	assert(pireq == 7);
+	printdc(dc);
+	datao(bus, DC, 0123456112233);
+	assert(pireq == 7);
+	printdc(dc);
+	datao(bus, DC, 0123456445566);
+	assert(pireq == 0);
+	printdc(dc);
+	assert((dctk(dc, 5, 0)&0777700000000) == 0123400000000);
+	assert(pireq == 0);
+	printdc(dc);
+	assert((dctk(dc, 5, 0)&0777700000000) == 0561100000000);
+	assert(pireq == 0);
+	printdc(dc);
+	assert((dctk(dc, 5, 0)&0777700000000) == 0223300000000);
+	assert(pireq == 7);
+	printdc(dc);
+
+	printf("\n\n");
+
+	// input test
+	cono(bus, DC, DBDAMOVE|IN|CHMOD_6|DEV1|7);	// 4017
+	assert(pireq == 0);
+	printdc(dc);
+	dcgv(dc, 1, 012, 0);
+	assert(pireq == 0);
+	printdc(dc);
+	dcgv(dc, 1, 034, 0);
+	assert(pireq == 0);
+	printdc(dc);
+	dcgv(dc, 1, 056, 0);
+	assert(pireq == 0);
+	printdc(dc);
+	dcgv(dc, 1, 011, 0);
+	assert(pireq == 0);
+	printdc(dc);
+	dcgv(dc, 1, 022, 0);
+	assert(pireq == 0);
+	printdc(dc);
+	dcgv(dc, 1, 033, 0);
+	assert(pireq == 7);
+	printdc(dc);
+	assert(datai(bus, DC) == 0123456112233);
+	printdc(dc);
+
+
+	printf("\n\n");
+
+
+	cono(bus, DC, DBDAMOVE|IN|CHMOD_12|DEV5|7);	// 4017
+	assert(pireq == 0);
+	printdc(dc);
+	dcgv(dc, 5, 01234, 0);
+	assert(pireq == 0);
+	printdc(dc);
+	dcgv(dc, 5, 05611, 0);
+	assert(pireq == 0);
+	printdc(dc);
+	dcgv(dc, 5, 02233, 0);
+	assert(pireq == 7);
+	printdc(dc);
+
+	return 0;
+}