XStream-PKG-R™ RS-232/485 RF Modem – Product Manual v5.x00
3 .1 .3 . Flow Con t r ol
Figure 3.2.
Internal Data Flow Diagram
(The five most commonly-used pin signals are shown.)
D I ( D a t a I n ) Bu ffe r a n d Flow Cont r ol
When ser ial dat a ent er s t he XSt r eam Modem t hr ough t he DI Pin, t hen t he dat a is st or ed in t he DI
Buffer unt il it can be t r ansm it t ed.
When t he RO par am et er t hr eshold is sat isfied ( refer t o Tr ansm it Mode ( p14) and Com m and
Descr ipt ions ( p25) sect ions for m ore infor m at ion) , t he m odem at t em pt s t o init ialize an RF
connect ion. I f t he m odem is alr eady r eceiv ing RF dat a, t he ser ial dat a is stored in t he m odem ’s
DI Buffer . I f t he DI buffer becom es full, hardw ar e or softw ar e flow cont rol m ust be im plem ent ed
in order t o pr ev ent ov erflow ( loss of dat a bet w een t he host and XSt r eam RF Modem ) .
H ow t o e lim in a t e t h e n e e d for flow con t rol:
1. Send m essages t hat are sm aller t han t he DI buffer size. The size of t he DI buffer v ar ies
accor ding t o t he pack et size and par it y set t ing used.
2. I nt erface at a lower baud r at e ( BD Com m and) t han t he fix ed RF dat a r at e.
Tw o ca ses in w h ich t h e D I Bu ffe r m a y be com e fu ll a nd possibly ov e r flow :
1. I f t he ser ial int er face dat a r ate is set higher t han t he RF data r at e of t he m odem , t he m odem
w ill r eceiv e dat a from t he host fast er t han it can t r ansm it t he dat a ov er - t he- air .
2. I f t he m odem is r eceiv ing a cont inuous st r eam of RF dat a or if t he m odem is m onit or ing dat a
on a net w or k , any ser ial dat a t hat ar r iv es on t he DI pin is placed in t he DI Buffer . The dat a in
t he DI buffer w ill be t r ansm it t ed over - t he- air w hen t he m odem no longer det ect s RF dat a in
t he net w or k .
H a r dw a r e Flow Cont r ol (
) .
When t he DI buffer is 17 by t es aw ay from being full; by
default , t he m odem de- asser ts ( high)
t o signal to t he host dev ice t o st op sending dat a ( refer
t o t he FT ( Flow Cont rol Thr eshold) and CS ( DO2 Configur ation) com m ands) .
is r e- asser ted
aft er t he DI Buffer has 34 by tes of m em ory av ailable.
Soft w a r e Flow Con t r ol ( XON ) .
XON/ XOFF soft w are flow cont r ol can be enabled using t he FL
( Soft w ar e Flow Cont r ol) Com m and. This opt ion only w or ks w it h ASCI I dat a.
D O ( D a t a Out ) Bu ffe r a n d Flow Con t rol
When RF dat a is r eceived, t he dat a ent er s t he DO buffer and is t hen sent out t he ser ial por t to a
host dev ice. Once t he DO Buffer r eaches capacit y , any additional incom ing RF dat a is lost .
Tw o ca ses in w h ich t h e D O Bu ffe r m a y be com e fu ll a nd possibly ov e r flow :
1. I f t he RF dat a r at e is set higher t han t he int er face dat a r at e of t he m odem , t he m odem w ill
r eceiv e dat a from t he t r ansm it t ing m odem fast er t han it can send t he dat a t o t he host .
2.
I f t he host does not allow t he m odem t o t r ansm it dat a out fr om t he DO buffer because of
being held off by hardw ar e or softw ar e flow contr ol.
H a r dw a r e Flow Cont r ol (
) .
I f
is enabled for flow contr ol ( RT Par am et er = 2) , dat a w ill
not be sent out t he DO Buffer as long as
is de- asser t ed.
Soft w a r e Flow Con t r ol ( XOFF) .
XON/ XOFF softw ar e flow cont r ol can be enabled using t he FL
( Soft w ar e Flow Cont r ol) Com m and. This opt ion only w or ks w it h ASCI I dat a.
© 2014 Digi International Inc.
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